31 BMW 1973 R75/5 Install and Align Front Forks

Earlier, I rebuilt the front forks, refinished the sliders and replaced the steering head bearings. I also polished the steering damper knob and added a modern BMW emblem to the center of the knob. You can read about how I did that here:

So, now I’m ready to install the front forks and align them.

Parts List

Here are the new parts I used.

Part # Description Qty
31 42 1 230 696 Upper rubber gasket, headlight bracket 2
31 42 2 000 385 Lower rubber gasket, headlight bracket 2
31 42 1 232 511 Upper Fork Cross Brace 1
31 42 2 002 115 Fork boots, 13 rib, pair 1

Resources

I relied on material published by Randy Glass, Duane Ausherman and Robert Fleischer. Duane’s page includes a link to an in-depth article about fork alignment published by Randy Glass. However, the link in Duane’s page to Randy’s article doesn’t work with all browsers. The link below works.

I use an 8 inch vernier caliper with 0.001 accuracy for the measurements.

8 inch Vernier Caliper

8 inch Vernier Caliper, 0.001 inch Accuracy

I also use a glass plate, 10 inch x 12 inch, and a 0.002 inch feeler gauge to align the forks.

10 x 12 inch Glass and Feeler Gauge

10 x 12 inch Glass Plate and Feeler Gauge

Procedure Overview

NOTE: I added this section after I first posted this write-up. Although all of this information is available in the references I mention above, I think providing this overview helps you understand why I did things in the order I did.

Before I go into how I did the work and the problems I encountered along the way, let me summarize how the fork construction affects alignment.

If you look at Duane’s material, he has a very nice conceptual diagram of the front fork assembly; two vertical steel tubes with four horizontal members that keep the tubes parallel. From the top, the horizontal members are the upper fork brace (upper brace), the lower fork yoke (yoke), the lower fork brace that goes on the lower sliders, and the front wheel axle.

If you look at the fork tubes, you notice that there are two planes in which they have to be parallel; the X-Plane in which the distance between the tubes is equal from top to bottom of the tubes, and the Y-Plane in which the forward/rearward rotation of the tubes when clamped only in the lower yoke is equal so one tube is not forward of the other.

Each of the four horizontal connections between the tubes is an opportunity for misalignment in both planes. And, the tubes themselves can be bent, but I didn’t check them as this bike has not had any front end collisions. So, I need to measure the tube alignment in the X and Y planes as I add each horizontal connection and then correct any misalignment I find.

The top of the tubes touch, but don’t penetrate, the holes in the upper brace. A large fork cap nut goes through the hole in the upper brace, threads into the top of the fork tube and connects it to the upper brace. The upper brace also connects to the steering stem using a large acorn shaped steering stem nut. The steering stem nut has a boss that goes through the hole in the upper brace and presses on the slotted nut used to adjust the pre-load on the steering stem bearings. I show a picture of this later on. Therefore, the upper brace is connected at three points. Any differences between the height of the fork tubes and the top of the slotted nut will bend the top brace affecting fork tube alignment.

NOTE: Bob Fleischer told me that there have been instances where the top triple plate has been worn, and the hole the steering stem nut boss goes into is too large, which you find out after the fact as there is no official dimension published for that hole. Any wear or elongation of the three holes in the upper brace will result in unstable alignment.  That  is one reason I bought a new upper brace. You may need a new steering stem acorn nut as well.  For these reasons, some folks install beefier third party upper braces machined to closer tolerances.  I choose not to do that on this project.

The lower U-shaped fork brace that mounts on the lower sliders can also be a problem as the brace can be too wide or too narrow, one of the legs can be bent ahead of the other, and the face that mounts on the lower slider can be uneven. Both Duane and Bob talk about how to handle this. Mine was fine.

One other adjustment to perform is the steering stem bearing pre-load. This too is an opportunity to tweak the upper brace and/or compress the tubes creating misalignment. The slotted nut puts pre-load on the bearings and the steering stem acorn nut presses on the top of the slotted nut and also adds some pre-load to the bearings. My bike is 1973 and 1/2 with long wheel base. It has the newer /6 series steering stem slotted nut for adjusting the bearing pre-load. Duane and Bob have details on how to adjust the pre-load with the earlier split ring. I’m glad this bike has the newer slotted nut.

So we have two fork tubes, four horizontal members and various nuts all interacting and potentially creating misalignment of the fork tubes.

Summary of Alignment Measurement Steps

Here is a summary of my procedure for checking the fork tube alignment.

  1. Install the fork tubes in the lower yoke so they are flush with the bottom of the upper brace.
  2. Tighten the yoke nuts to 25 FOOT/pounds.
  3. Check the axle for smooth fit in the holes in the lower fork sliders.
  4. Check the lower yoke for X and Y plane alignment and correct as needed.
  5. Check the upper brace for X and Y plane alignment and correct as needed.
  6. Check the axle for X and Y plane alignment and correct as needed.
  7. Check the lower fork brace for X and Y plane alignment and correct as needed.

Summary of Final Assembly Steps

Now it’s time to put everything together starting with the fork tubes in the lower yoke and the lower yoke nuts torqued to 25 FOOT/pounds.

  1. Install the headlight ears with turn signal wiring (I didn’t install the turn signal wires as I’m using a Windjammer Fairing).
  2. Install the upper fork brace and finger tighten the fork cap nuts and steering stem nut.
  3. Loosen the lower yoke nuts so they are finger tight.
  4. Set the pre-load on the steering stem bearings with the slotted nut and then torque the steering stem nut and the fork cap nuts.
  5. Check X and Y plane alignment.
  6. Torque the lower yoke nuts to 25 FOOT/pounds.
  7. Check X and Y plane alignment
  8. Install the lower sliders on the forks.
  9. Install the axle and tighten axle clamp bolt
  10. Check X and Y plane alignment.
  11. Install lower fork brace
  12. Check X and Y plane alignment.

At this point, the forks are installed and parallel in both planes and it’s time to install the front brake assembly and front wheel. I’ll cover that in a separate write up.

Things I Learned Along the Way

  • It’s important to keep checking X and Y plane alignment after you make any change to the fork assembly. That way, when things change, you can start with what you just did to find out what caused the change.
  • The upper headlight ear gasket caused the upper brace to be above the top of the slotted steering stem nut. I added a washer on top of the slotted nut to correct this. I learned from  Bob Fleischer that at one time BMW offered different thickness top gaskets for the headlight ears. I didn’t see any of the other part numbers when I ordered the one I show in the table above. I suggest that you check X and Y plane alignment of the upper brace with the headlight ears and gaskets installed and then remove the top gasket and see what happens.
  • BMW upper braces are notorious for being out of tolerance. Many folks use third party products for the upper brace. I installed a brand new polished brace from BMW and did not find it out of tolerance.
  • Patience is very important. It took me two weeks to complete this process. I had to wait for two days for some of the adjustments to the lower yoke to become permanent and then had to do some of the adjustments again.

Getting Started

Here is where I started. I have the front steering stem installed in the new bearings in the steering head.

Steering Head after New Bearings Installed

Steering Head with Steering Stem After New Bearings Installed

I have the lower sliders mounted on the fork tubes.

After Rebuild & Polishing

Front Forks Assembled

Note there is a left and right lower as shown here: top is right and bottom is left side slider.

Fork Sliders, Top is Right Side, Bottom is Left Side with Axle Clamp Allen Head Screw

Fork Sliders, Top is Right Side, Bottom is Left Side with Axle Clamp Allen Head Screw

Temporarily Install Upper Fork Brace

Here is the new upper fork cross brace. The dark areas in the cross brace are reflections in the polished surface of me taking the picture, not discoloration.

New Polished Top Fork Brace

New Polished Upper Fork Cross Brace

This is the lock nut and washer on the stud in the lower yoke. This secures the fork tube in the yoke.

Lower Yoke Lock Nut and Washer

Lower Fork Yoke Lock Nut and Washer

The steering stem top nut has a boss on the bottom that fits inside the center hole in the upper cross brace centering it on the steering stem.

Top Nut Detail Showing Land

Top Nut Detail Showing Boss

Top Nut Land Aligned in Upper Cross Brace

Top Nut Boss Aligned in Cross Brace

Here is the upper fork brace mounted on the steering stem using the stem top nut. I tightened the top nut just enough (not fully finger tight) to be sure the boss on the bottom is inside the hole in the top brace and the top nut is flush with the top of the brace.

Lower Yoke and Upper Chrome Cross Brace Installed

Lower Yoke and Upper Chrome Cross Brace Installed

Install Fork Tubes in Bottom Yoke

The top of the fork tube is threaded for the large fork cap nut that holds the fork spring inside the fork tube. The fork cap nut sits on top of a large steel washer and the top unthreaded section gets centered inside the holes in the upper cross brace.

Fork Tube Cap Nut and Washer

Fork Tube Cap Nut and Washer

I want the top of the fork tubes to touch the bottom of the upper cross brace as I slide them through the holes in the lower yoke, but not enough to deflect the cross brace.

Upper Cross Brace Resting on Top of Fork Tube

Upper Cross Brace Resting on Top of Fork Tube

I slide a fork tube into the lower yoke so it touches the bottom side of the cross brace, I tighten the nut on the lower yoke stud to hold it in place. Then I insert the fork cap nuts into the fork tubes finger tight to settle the cross brace on top of the fork tube. I make sure the fork cap nuts pass through the hole in the cross brace so the bottom of the nut rests on the washer with no gap. I keep both the steering stem top nut and each of the fork cap nuts loose until I get the fork cap nuts aligned and then finger tighten all three nuts.

Fork Tube Spring Cap Nut

Fork Tube Top Cap Nut and Washer

I measure the distance from the machined land on the lower yoke to the bottom of the cross brace. It should be close to 160 mm.

Fork Tube Measurement from Lower Yoke

Fork Tube Measurement from Lower Yoke to Bottom of Cross Brace

Check Front Axle

With the forks assembled in the lower yoke, I torque the lower yoke lock nuts on the studs to 25 FOOT/pounds.

The front axle should slide through the holes in the lower sliders without any resistance. If there are any nicks or burrs on the axle that prevent this, they should be cleaned up removing the minimum amount of steel necessary for smooth sliding.

Front Axle Assembly

Front Axle Assembly

My front axle slides very smoothly through the holes and I can push it completely through the hole in the left fork slider. It can not go all the way through the right slider hole as that hole is a smaller diameter.

Axle Slides All the Way Through Lower Legs

Axle Slides All the Way Through Lower Legs

I inspect the fork tubes for any nicks or gouges (there are none) and clean the outside of the fork tubes to get any dirt and grit off them. Then I use the axle to push the lower sliders up the fork tubes to feel how much effort this takes. I want to compare the effort before and after I align the fork tubes.

I remove the lock nut on the damper rod at the bottom of the fork slider using a 13 mm socket clamped in my vice grips and an Allen head socket. I keep the axle installed in the lower sliders and put them aside.

13 mm socket and vice grip

13 mm Socket and Vice Grip

Removing bottom nut from fork pipe

Removing Bottom Nut From Fork Tube

Lower Sliders Removed with Axle

Lower Sliders Removed with Axle

Measuring Fork Alignment to Reduce Stiction

Here is where I started with the fork tubes mounted in the lower yoke with the lock nuts on the lower fork yoke torqued to 25 FOOT/pounds.

Forks Mounted and Lower Sliders Removed

Forks Mounted and Lower Sliders Removed

I removed the the steering stem top nut, the top brace and the two fork cap nuts so the fork tubes are only clamped by the lower yoke.

Forks Ready for Alignment

Forks Ready for Alignment

A primary cause of stiction, which is resistance that hinders smooth movement of the slider up and down the fork tubes, is the fork tubes not being parallel.

There are two planes, the X and Y, in which the fork tubes have to be parallel. The Y-Alignment is measured with a piece of glass placed across the front of the fork tubes and a 0.002 inch feeler gauge to ensure one tube is not in front of the other. The X-Alignment is measured by the distance between the fork tubes at the top and the bottom of the tubes with the vernier caliper. The distances should be the same if the tubes are parallel in the X-Plane.

Y-Plane Measurement and Alignment

I bought a 10 in X 12 in piece of glass from the local Ace Hardware store. The edges and corners are very sharp so I used some 600 grit wet/dry paper with a little water and rounded all the edges. Pay attention to the corners to avoid chipping them but make sure all the sharp edges of the corners have been smoothed.

Removing Sharp Edge of Glass Plate

Removing Sharp Edge of Glass Plate with 600 Grit Paper

Put some blankets or soft padding under the fork tubes to cushion the glass plate should it slip and fall. Then, with clean hands so the plate won’t slip out of your fingers, place the plate flat on the fork tubes with the 12 inch side along the tubes and the 10 inch side across the tubes. Slide the plate over so you can hold the left edge against the left fork tube (as you face the bike from the front) with your thumb in the middle of the glass on the front side and the fingers behind the fork tube.

Positioning Glass on Fork Tubes

Positioning Glass on Fork Tubes

It’s tight against the left tube but loose against the right tube. Now, with the index finger of your right hand, gently tap the top right corner of the glass and then the bottom right corner of the glass and listen for any “click”ing. You will likely hear a click either when tapping the top edge or the bottom edge, but not both. The click indicates the tubes are not parallel.  No click and you are aligned, but check with the feeler gauge to be sure.

Tapping Edge of Glass to Check for Click

Tapping Edge of Glass to Check for “Click” Sound

In my case, the lower right edge clicked. To verify there is a gap and estimate about how much it is, I use a 0.002 inch feeler gauge and very lightly slide it behind the glass and on top of the tube. At the top right edge, with very little pressure, it stops and does not go between the glass plate and the fork tube. I slide the feeler down the tube with the same light inward pressure and sure enough as I get lower down the right tube, the feeler starts to move further over the top of the tube until toward the bottom, it slides between the tube and the glass.

Y-Direction, Tio Right, Feeler Doesn't Slip Between Glass and Tube

Y-Direction, Top Right Edge, Feeler Doesn’t Slip Between Glass and Tube

Y-Direction Gap, Bottom

Y-Direction, Bottom Right Edge, Feeler Slips Between Glass and Tube

Now I slide the glass over so the right edge is along the right tube and repeat the tapping test on the upper left and lower left corners of the glass. This time the click is at the top left edge but not the lower left edge. I use the 0.002 inch feeler gauge and can see it slip between the glass and fork tube at the top of the left tube.

Okay, the tubes are not parallel in the Y-Direction. The fix for this is to put pressure on the tubes so the lower yoke, which is aluminum, deforms enough to make the tubes parallel.

I use a four foot piece of 2 x 4 and put it between the tubes and then pull on the long end to move one tube forward and the other backward. Based on Randy’s description of how to interpret the clicking, I conclude that the left tube is in front of the right tube, as viewed from the front of the bike. I need to push the left tube back and the right tube forward, so this is how I inserted the 2 x 4 between the fork tubes. I used a rag between the wood and the tube to protect the tubes from being scratched.

Fork Alignment in X-Direction

Aligning Forks in Y-Direction with 2 x 4 Between Fork Tubes

I loop a piece of 1/2 inch rope around the rear foot brake lever and the 2 x 4 and knot it. I use the handle of a hammer between the ropes and twist them until they are pretty tight and rest the end of the hammer against the engine so it won’t unwind.

Aligning Forks in X-Direction

Aligning Forks in Y-Direction Using Rope to Apply Pressure

I estimate the tubes are not far out of alignment as the 0.002 inch feeler pushed the plate up a bit when it slide between the glass and the fork tube. So I leave the pressure on the tubes for an hour and then check the alignment with the glass plate. I can’t hear any more clicking and the feeler gauge doesn’t go between the glass and the tubes. I will measure the Y-Plane alignment again after I adjust the X-Plane to be sure the stretching in the yoke is stable.

X-Plane Measurement and Alignment

To measure the X-Plane alignment, I use an 8 inch vernier caliper with 0.001 inch resolution. I take two measurements, just under the bottom of the lower yoke aligning the knives of the caliper using the fork boot pins as a guide. It’s very hard to ensure I am holding the calipers perpendicular to the tubes. So I removed the lock screw on the right side sliding jaw of the caliper, and adjust them to fit between the tubes with the fixed knife edge on the left tube and the slider end of the caliper on the right just touching the tube and angled a little up hill of the fixed knife edge. This measurement is too large since the distance isn’t the shortest distance between the tubes as the caliper is not perpendicular to both tubes. Then very slowly and gently I rotate the right side slider knife edge down the tube and then back up the tube until I get a constant reading. This is the shortest distance between the tubes with the sliding knife edge very lightly touching the tubes so they aren’t being pushed apart by the caliper. I take multiple measurements and get good repeatability. I find a difference of +0.013 inches, meaning the tubes are wider at the bottom than the top.

Checking X-Plane Measuring Distance Under Lower Yoke

Checking X-Plane Measuring Distance Under Lower Yoke

X-Plane Measurement at Bottom of Tube

X-Plane Measurement at Bottom of Fork Tubes

To align the tubes in the X-Plane, I clamp then with a bar clamp under the lower yoke. I need to close the tubes under the yoke to decrease the distance between the bottom of the tubes since the bottom is wider than the top. If the bottom was closer than the top, I would clamp the tubes above the yoke to increase the distance at the bottom of the tubes.

Clamping Forks Below Yolk to Adjust X-Plane Alignment

Clamping Forks Below Yoke to Adjust X-Plane Alignment

The objective is to deform the aluminum yoke which is softer than the steel fork tubes, not to bend the fork tubes. You don’t want bent fork tubes and it would be hard to do that with the clamp anyway.

I measure the top and bottom distances after applying the clamp and now have a negative difference of -0.027 inches, so the bottom of the tube is narrower than the top. I leave them this way for a day, remove the clamp and measure the distance between the tubes again. The bottom has closed and is now +0.006 inches wider at the bottom. I have removed just about half the original difference.

At this point I add a second clamp to spread the tubes above the lower yoke while compressing them below the yoke.

Using Clamps to Move Fork Tubes in X-plane

Using Clamps to Move Fork Tubes in X-plane

This increased the bottom distance to a negative -0.256 inches closer at the bottom than the top. I leave the clamps on for another 24 hours, remove them and measure the distance between the forks again. This time they are within about 0.001 inch.  I let them sit for another 24 hours and then measure them again. I find that both the Y and X alignment are still within 0.001 inch.

Assemble Upper Brace, Steering Stem Top Nut & Fork Cap Nuts

I add the upper cross brace, steering stem nut and fork cap nuts. I fit each nut finger tight so the upper cross brace is aligned with the fork tubes. The nuts should tighten until flush with the cross brace without any resistance and the brace should be flush with the top of the tubes and the top of the slotted steering stem nut under the upper brace.

Upper Brace Install with Stem Nut and Fork Tube Cap Nuts

Upper Brace Install with Stem Nut and Fork Tube Cap Nuts

Then I measure the Y-Plane and X-Plane to be sure they were still parallel. The Y-Plane was but the X-Plane wasn’t. The tubes moved about +0.006 inches closer at the bottom than the top. 🙁 The tube cap nuts went in very smooth with no binding so I don’t think there was any pressure on the tubes. Curious.

I install my wood clamp as a spreader between the tubes below the yolk for 24 hours. I measured the X-Plane and Y-Plane again. Both are parallel again.

Add Fork Brace & Lower Fork Slider, 1st Time

I fit the lower sliders on the fork tubes so I can mount the lower fork brace on the sliders to be sure the tubes remain parallel in the X-Plane. I have to fish the damper rods out of the holes in the bottom of the fork tubes with a small screw driver.

Coaxing Damper Rod Through Hole in Lower Slider

Coaxing Damper Rod Through Hole in Lower Slider

I insert the axle and made sure it still slides smoothly in the holes and then slide the lowers up and down the fork tubes to help center the lowers on the axle and eliminate any X-plane binding of the lower sliders. I measure the X-plane distance between the tubes to be sure they are still parallel, and they are.

Centering Lower Sliders on Axle

Centering Lower Sliders on Axle

Then I add the fork brace to lower sliders and finger tighten the four lock nuts then torque them in a cross wise pattern to 12 FOOT/pounds. I kept the forks centered with my hand on the axle while I torque the lock nuts. I check the X-Plane distance between the fork tubes again and the measurement hasn’t changed.

Tighten Steering Stem Nut and Fork Cap Nuts

It’s time to put the headlight ears and turn signal stalks on the forks and tighten the steering stem nut and fork cap nuts to the final torque. I remove the finger tightened steering stem nut and fork tube cap nuts along with the upper brace so I can slide the turn signal stalks and headlight ears on the top of the fork tubes. Here are the parts for the headlight ears, turn signal stalks and the chrome decorative caps that go on top of the fork tube cap nuts.

Headlight Ear and Turn Signal Assembly

Headlight Ear and Turn Signal Assembly

The turn signal stalk mounts over the locating pin on the lower fork yoke. These stalks are cut down to fit inside the Windjammer fairing so they don’t bind on the fairing.

Turn Signal Stalk (Cut Down) Installed on Lower Yoke Pin

Turn Signal Stalk (Cut Down) Installed on Lower Yoke Pin

I install the rubber gaskets on the headlight ears. and then install the ears over the top of the fork tubes. The bottom gasket has a hole that fits over the same locating pin on the lower yolk.

Rubber Gaskets Installed in Headlight Ear

Rubber Gaskets Installed in Headlight Ear

Bottom Gasket of Headlight Ear Goes on Lower Yoke Pin

Bottom Gasket of Headlight Ear Goes on Lower Yoke Pin

The top rubber gasket is a little bit proud of the top of the fork tube, so it will be compressed by the upper brace when the tube cap nut is tightened.

NOTE: This turns out to be a the cause of a problem later on, but I didn’t know that yet.

Headlight Ear Upper Gasket is Proud of Top of Fork Tube

Headlight Ear Upper Gasket Rides is Proud of Top of Fork Tube

I install the upper brace on the steering stem, insert the new fork springs in the top of the fork tubes and finger tighten the steering stem nut and the fork cap nuts being sure they go down flush on top of the upper brace.

Upper Brace On Top of Upper Headlight Ear Gasket

Upper Brace On Top of Upper Headlight Ear Gasket

I install the fork sliders on the tubes with the lower fork brace and measure the X-Plane with the vernier caliper to be sure the tubes are still parallel.

Lowers with Fork Brace Installed

Lowers with Fork Brace Installed

I loosen the lower yoke nuts to finger tight. I adjust the steering stem bearings with the hook spanner so the bearings are tight enough that the forks don’t fall to one side when nudged from the center, but just start to fall when they are very close to the stops. Then I torque them to 25 FOOT/pounds.

I plan to use the old handlebar as an “anti-torque” tool to prevent tweaking the forks when I tighten the steering stem and fork cap nuts. I install the handlebar clamps with the pull backs my wife prefers and then lock one end of the handlebar in the clamps.

Handlebar Brackets with Pull Backs

Handlebar Brackets with Pull Backs

Old Handle Bar as Anti-Torque Tool

Old Handle Bar as Anti-Torque Tool

I torque the steering stem nut to 80 FOOT/pounds with a 36 mm socket using the anti-torque tool to keep the forks centered. I use the dog-bone wrench in the tool kit to tighten the fork cap nuts again using the handlebar to keep the forks centered. I snug the cap nuts up using a plastic hammer on the end of the dog bone wrench.

I pull the fork lowers off the tubes and measure the X-Plane and Y-Plane alignment. They both have moved. The X-Plane is too close at the bottom by about 0.013 inch. I am a bit bummed out by this. 🙁

I’m not sure if tightening the fork cap nuts and steering stem nut caused the tubes to walk or not. Dennis said that the /5 tubes are prone to walking when everything is tightened and Duane’s material talks about the problems shops had with brand new /5 bikes from Munich with forks made with out of spec parts.

So Why Did The Forks Suddenly Go Out of Alignment?

I remove the upper brace, steering stem nut and fork cap nuts. I loosen the nuts on the lower yolk studs. Then I install the upper brace, steering stem nut and tube cap nuts and torque them using the anti-torque handlebar to prevent tweaking them. I install the fork sliders with the lower fork brace and then I measure the X-Plane distance with the vernier caliper and find the tubes are within 0.001 inch. Next, I evenly and alternately tighten the lower yolk nuts and measure the X-Plane distance between the tubes as I go. As I get them close to tight, the X-Plane goes out of parallel by about 0.011 inch. Interesting

Now I loosen the steering stem nut and fork cap nuts and the forks move back to being close to parallel.  Hmm.  Something is pushing the tubes outward at the top and when I tighten the nuts on the yoke and that pushes the tubes inward at the bottom. When the yoke nuts are loose, the bowing at the top is absorbed by the slop in the yoke.

I take the upper brace off and the headlight ears. The rubber gasket on the ears is above the top of the fork tube which I noted when I installed the ears.

Upper Brace On Top of Upper Headlight Ear Gasket

Note Gap Between Top of Fork Tube and Top of Rubber Gasket

So, I loosen the yoke nuts and then reset the tubes so they touch the bottom of the upper brace when the steering stem nut and tube cap nuts are finger tight instead of leaving the rubber gasket proud of the top of the fork tubes. Then I tighten the yolk nuts to 25 FOOT/Pounds. Of course, the tubes move in the X-Plane again.

What is going on?

I remove the ears so I can measure what is happening in the X-Plane at the top of the fork tubes. I keep loosening and tightening the fork cap nuts and steering stem nut while measuring the distance between the top of the fork tubes with the caliper, and it looks like tightening the fork cap nuts pushes the forks outward when the steering stem nut is tight. But, the fork tubes stay put if I don’t tighten the steering stem nut.  As I look closely under the upper brace, the bottom of the brace is no longer resting on top of the slotted steering stem nut when the steering stem nut is loose. There is a gap. Here is a picture of the gap. The upper brace is polished so there is a mirror image of the steering stem slotted nut on top, but there is only one nut.

Gap Between Steering Stem Slotted Nut & Bottom of Upper Brace

Gap Between Steering Stem Slotted Nut & Bottom of Upper Brace

It dawns on me this is due to the rubber gasket on top of headlight ears. When I moved the tubes up to touch on the bottom of the upper brace to get them flush with the bottom of the upper brace, the ends of the upper brace are now higher than the end of the upper brace around the steering stem creating the gap under the steering stem nut. When I tighten the steering stem nut, it bends the upper brace downward putting pressure on the top of the fork tubes knocking them out of alignment.

The Solution

I go over to the Ace Hardware store and find a 14 gauge and 18 gauge washer that fit over the threads on the steering stem. I find that adding the 18 gauge washer between the slotted steering stem nut and the bottom of the upper brace eliminates the gap.

Washer

Washer That Fits Over Steering Stem

The steering stem nut presses on the top of the slotted nut to load the steering stem bearings. A washer between the slotted nut and the bottom of the upper brace won’t change that since steering stem nut sits fully on the washer.

Steering Stem Slotted Nut

Steering Stem Slotted Nut

18 Gauge Washer on Top of Steering Stem Slotted Nut

18 Gauge Washer on Top of Steering Stem Slotted Nut

Now when I finger tighten the steering stem nut and the fork cap nuts, the top brace is flush with the steering stem slotted nut. In the picture below, the polished upper brace makes it appear there is a second washer and slotted nut on top, but it is just a reflection.

18 Gauge Washer Between Steering Stem Slotted Nut and Bottom of Upper Brace

18 Gauge Washer Between Steering Stem Slotted Nut and Bottom of Upper Brace

The tubes aren’t being pushed outward at the top any more. Next, I loosen the steering stem nut and remove the fork cap nuts to add the headlight ears. Then I torque the steering stem and fork cap nuts using the anti-torque handlebar. What do you know, the tubes are now parallel in the X-Plane and the Y-Plane. It doesn’t take much to prevent the tubes from being parallel.

Installing the Fork Boots, Lower Brace and Sliders, 2nd Time

Now I add the fork boots on the fork tubes. The top of the boots have a hole that locates the top of the boot on the pin in the lower yoke.

Hole in Top of Fork Boot

Hole in Top of Fork Boot

Locating Fork Boot on Yoke Pin

Locating Fork Boot on Yoke Pin

Then I add the lowers and the fork brace.

Lower Brace Studs with Lock Nuts

Lower Brace Studs with Lock Nuts

I snug up the nuts finger tight and measure the X-Plane distance. I adjust the brace so the alignment stays the same. Then torque the nuts on the lower fork brace to 12 FOOT/pounds while holding the axle to avoid twisting the fork tubes and measure the X-Plane distance between the tubes. The tubes get out of parallel and I loosen the nuts and get the tubes aligned and then tighten them until everything is within 0.001 inch.

Torquing Lower Fork Brace Nuts

Torquing Lower Fork Brace Nuts

Here is the final assembled and aligned fork tubes with fork boots.

Forks Installed and Aligned

Forks Installed and Aligned

Install Steering Damper Assembly

Here are the damper friction assembly parts.

Damper Friction Plate Assembly

Damper Friction Plate Assembly

The two plates mount under the steering stem. The one on the left threads on to the steering damper rod and pushes against the friction plate on the right which is attached to the frame via the Allen head bolt.

Here is the friction plate installed at the bottom of the steering stem with the threaded end of the damper rod in the center.

Install Damper Friction Pressure Plate

Install Damper Friction Pressure Plate

The damper friction bracket installs on top of the friction plate and the threaded damper rod screws into it.

Install Damper Friction Bracket

Install Damper Friction Bracket

Final Product

Here is the condition I started with for the upper fork brace and lower fork tubes.

Steering Head Grunge & Damper

Original Steering Head Grunge & Damper Scratches

Fork Slider Details

Original Fork Slider Condition

Earlier, I refinished the steering damper knob to remove the scratches and to add a modern BMW emblem to the center of the knob.

Polished Damper Knob, Emblem and Polished Upper Brace

Polished Damper Knob, Emblem and Polished Upper Brace

I think the effect is very nice and will pull your eye to this small detail.  It certainly is a major improvement over the original state of affairs.

Forks Installed and Aligned

Forks Installed and Aligned

Finished Upper Fork Brace with Damper and Handlebar Mounts

Finished Upper Fork Brace with Damper and Handlebar Mounts

Finished Upper Fork Brace with Damper and Handlebar Mounts

Finished Upper Fork Brace with Damper and Handlebar Mounts

28 thoughts on “31 BMW 1973 R75/5 Install and Align Front Forks

  1. Pingback: 1973 BMW R75/5 Rebuild: Install and Align Front Forks | Motorcycles & Other Musings

  2. Do yourself a favour and order one of the special machined top plate braces from toastertan via the airhead list, it will really make a difference. I’m actually surprised your top plate has the holes in the right spots…

    • Hi Spokenoise,

      Thanks for visiting and the mention of the toaster tan products (http://www.pbase.com/toastertan/top_braces). It’s on my list of items to try out as projects permit. This project is essentially a stock build, so I’ve kept close to the original spec parts. I may get one for the R75/6 Grey Ghost when I do the “Grey Ghost II” build in the near future.

  3. ABSOLUTELY FANTASTIC!
    Thank you tremendously, for this detailed account of all the descriptions complied on the “interwebz”.
    I NOW have a much better understanding of all the “gobbledigook”.
    My 1973 LWB R75/5 Toaster will NEVER look like what you have done…..I am in awe.
    Congratulations on a super rebuild.
    LOVELY.
    Once again, thanks much for all of these blog postings.
    Ride Fast and Safe!

    spo

    • Hi Dan,

      It’s the stock brace for a /5 model. On my /6 model, the brace was from San Jose BMW who had them back in the 1980s.

      Thanks for stopping by.

      Best.
      Brook

  4. Brook – great documentation and rebuild…Nice job! I have a 1973 BMW R60/5 I did a similar restoration on (though, NOT as thorough as yours) – complete dis-assembly, powder coated frame/sub-frame/final drive, painted fenders/tank/Wixom side bags, etc…still LOTS of detail work left…which, as you know, is the fun part. Question on something you did:

    “Earlier, I refinished the steering damper knob to remove the scratches and to add a modern BMW emblem to the center of the knob.”

    Looks AWESOME! I’ve GOT to know where you found that size BMW roundel/badge/emblem (whatever you want to call it) for the steering damper knob…searching eBay, etc…no luck. Adhesive mount or glue? Did you inlay it (can’t tell from the photos)? I did something similar on the key with a BMW key fab slicker/logo, but I’d like to inlay it. I thought about going to a polished /2 “metal” damper knob, but after seeing yours, I’m reconsidering.

    Thanks much,
    Todd

  5. Dan,

    I recently purchased a used fork brace from ebay and wanted to verify it is not bent. Do you have any dimensions for the brace? With the mounting surfaces pressed flat to a table there is a slight rock. The original is badly bent so it is of no use for evaluation purposes.

    Mike

    • Hi Mike,

      Hmmm … it’s not surprising that many fork braces are not true, but deformed. They experience a lot of force.

      The best approach I think, is to remove the wheel, fork springs and fork oil. Then feel for “stiction” in the fork with the axle inserted into the sliders, but not with the axle nut tightened. They should move smoothly throughout the travel. If not, the alignment procedure in this post helps get them that way. Then, install the fork brace with the bolts loose and not tighted. Move the sliders again and see if it’s harder with the brace than without. If harder, then the brace is causing the tubes to bend. If not harder, tighten up the bolts to correct torque. And test again. If the forks are harder to move up and down when you tighten the bolts, then the brace is not correctly aligned to the fork slider. Loosen the bolts and carefully look at where the brace mounts to the slider. Likely you will see a gap at one, or more, bolt hole while one or more of the others are flush with the mounting hole in the slider. Try some metal shims to remove the gap and retighten the bolts. This often eliminates binding in the fork tubes due to small deformities in the brace.

      I hope this helps.

      Best.
      Brook.

  6. Thank you Brook! I am rebuilding the whole front suspension due to a bent fork tube and was just concerned about not having a great way to measure the dimensional accuracy of that part but feel a lot better now.

    I was able to pick up a set of new fork tubes from Fast from the Past in ALT for 168.00 ea. He stocks sizes for most models if anyone needs one quick.

  7. Hello Brook:

    Thank you for your amazing site, which I have just discovered. I have recently purchased my first airhead – a 1975 R90/6, and like you, I reside in the Denver metro area.

    I’d like to have a complete fork service done and would like your recommendation on who in the Denver area I should contact.

    Thanks very much, again, for the wealth of information.

    Jim

  8. Thanks again for the great info. I just finished putting my forks an front wheel on my 75/5. I thought I was all good till I noticed a small gap between the top brace an fork spring nuts. Is this normal? Doesn’t look like you had a gap in your photos. I have a photo but am not sure how to include it in comment. Any advice would be very appreciated.

  9. Hi Brook,

    Aren’t the 70/71 steering adjusters better in this regard? They are constant-height. The nut is variable height and you’ll theoretically need a different size of washer every time you adjust the steering bearings.

    It seems to me that the forks should be set first from the steering nut, the fork caps, and then the lower clamp. The nut sets the “fork height”. The bending occurs when the ears + rubbers extrude further than the height that the steering is set to. If the ears and rubbers were too short relative, they would fit loosely, but the forks would not be set out of plane from that.

    Does that seem correct, or am I missing something? Great write up, thank you for posting it!

    • Hi Teran,

      It seems to me that the headlight ear rubber grommets (top and bottom) and the turn signal bracket end up being a variable. I believe that the new headlight ear rubbers are thicker than the original ones and that raises the top plate a bit. So, the height of the fork tube above the lower triple clamp isn’t really pushing the top plate up, it’s the combined thickness of the rubber grommets that seem to be the cause.

      I’ve also seen differences in turn signal stalk thickness cause the headlight ear to be a bit higher than normal.

      If I see more than 0.004-0.005″ deviation in distance between the fork tubes top to bottom, I usually can see a gap between the top plate and the bottom of the steering stem bearing nut. The simple solution is using the 14 or 18 gauge washers to remove the gap.

      Thanks for you note. I appreciate it.

      Best.
      Brook.

  10. I just need to replace the gaiters on the front forks on My very tidy BMW R75/5. I assume that I will need to drop the forks down to replace?

  11. Brook, A couple observations on my attempts at this (72 R75/5). On the fork tubes, there are two holes. I recall reading somewhere that they are supposed to point towards the center (inside). When I try to apply torque to the nuts at the top, the tubes spin unless the lower yoke clamps are tight. So I tightened them by hand, fully assembled with ears on, then spin the tubes back around where the holes are pointed at each other. Then apply the 25 lbs to the lock nuts to keep them from turning, then torqued the upper nuts to spec. Then release the lower yolk nuts and let it sit for a few. Do you recall seeing why the holes were supposed to be pointed at each other? Also, if the top plate is not installed when making adjustments to the fork straightness with the glass plate etc., wouldn’t the forks be pulled back out of alignment when the top plate is installed since the top plate has a fixed distance between the tubes? Your thoughts, as always, appreciated.

    • Hi Jim,

      Fork tubes are usually bent, but slightly, due to the forces on them. An approach is to roll each tube on a formica counter top with the sun behind them and look at the edge that touches the counter. Roll the tube until you see some light coming through the gap between the tube and counter. Find the largest gap and mark the opposite edge of the tube, which is the high spot, with a sharpie near where the lower triple clamp attaches to it. Repeat for the other tube. When assembling orient the tube so the sharpie marks face forward. This ensures the slight bend in the forks are in the same plane so the lowers will slide over the tubes with the minimum stiction. In general, this will usually put the holes in the tubes facing each other.

      Many things will pull the fork tubes out of alignment. What I try to do is find out what part is causing the alignment to change. The approach I used lets me see what causes them to move out of alignment so I can correct the problem. That said, yes, the stock thin top plate is often bent and adding it usually causes the alignment to change. I test fit the plate when I have the forks clamped in the lower triple clamp and hand thread the top nuts into the top of the fork tube. If the top plate is square and flat, I can screw the top nuts in by hand until they are snug. If I can’t and the nuts bind, then the top plate, and or fork tubes are bent.

      I found the top rubber gaskets on the headlight ears cause bending. They do come in different thickness’ according to Bob Fleischer’s site (http://bmwmotorcycletech.info/hardware.htm).

      31-42-1-232-527 is 3 mm thick.
      31-42-1-230-696 is 4 mm thick. <– STANDARD
      31-42-1-230-697 is 5 mm thick.
      31-42-1-230-698 is 6 mm thick.

      Getting the top of the fork tube exactly flat with the top of the headlight ear gasket is necessary of tightening the top fork nut will bend the ends of the top plate down and push the tubes out of alignment. I show a way to over come some of this using washers rather than different thickness top gaskets to remove slack under the top plate.

      The thin top plate is a weakness in the design. I'm going to use Toaster Tan top clamps on the 1977 R100RS rebuild since I don't have to cut down the headlight ears as I would have to on the R75/6.

      I hope this is helpful.

      Best.
      Brook.

  12. I would love to be a part of your blog readers. I have a 1964 R60/2 that I bought new. I have a 1972 R60/5 that was wrecked an I am trying to restore it. I’ll be needing a lot of advice in doing so. I will reading this blog a lot. I am glad I found it.. THANKS!

    • Jim,

      Thanks for dropping by. I see you subscribed to this blog so you should receive notifications of new blog posts. I always post a short blog when I add a new write-up since Word Press only sends Email notifications for blogs, not write-ups.

      Best of success on the 1973 R60/5 project. It’s good to know another airhead will get a new lease on life.

      Best.
      Brook.

  13. Thank you so much for this extensive step-by-step explanatory site. I have all kinds of issues with my own 75/5 with a 90 engine and a /6 or /7 transmission, but in regards to these picture I have a question.

    Do you find it difficult to route wires to the front blinkers?
    I have a problem with my front right blinker shifting/rocking back and forth, somehow causing cable related issues resulting in the blinker rele going twice its normal speed and the blinker lights (all) arent glowing strong, or at all.
    Im considering pulling the cables (with a string attached) or pulling the front forks, since the right headlight ear is also movable. This is not something I would like to do since I could cause front fork misalignment. But it could be cool to check them out and perhaps install progressive front fork springs to aid in the further installment of a Gläser fairing I bought a month ago 🙂

    Any tips or suggestions are appreciated.

    -Christian

    • Hi Christian,

      With the top fork clamp off, the headlight ears are free to pull off the forks. You do have to remove the top fork nuts. I’ve not tried to install the wiring any other way.

      If you have /5 switches, there is no ground wire to the front turn signals. If the turn signal stalk is not firm attached to the fork tube, since it is part of the ground path back to the battery, your lights will not work correctly.

      Inside the turn signal housing is a clamp the fits into a notch on the turn signal stalk. This has to be tight, the bent piece of steel on the clamp not broken off, or the signal will rotate on the stalk.

      The end of the stalk that fits over the top of the fork tube has a notch in it that slides over a roll pin pressed into the lower triple clamp. If the pin is missing, the turn signal stalk will not fit securely on the fork tube and you don’t have a reliable ground.

      BTW, the final path of the ground for the front turn signals is through the steering head bearings. On rare occasions, loose bearings with old grease could possible interfere with getting a good ground.

      You can run a separate brown ground wire to each bulb holder and route them back inside the headlight shell and connect them to a ground in side the headlight.

      I hope this helps.

      Best.
      Brook.

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