11 BMW 1973 R75/5 Install Engine Top End

At long last, I’m ready to install the top end (pistons, rings, cylinders and heads) on the engine. Previously, I bead blasted the cylinders, measured the bores for ovality and taper, ground the ends down on new 0.5 mm oversize rings to get the correct end gap and had the heads rebuilt by Randy Long.  The parts have been sitting in boxes patiently waiting while I got the engine and wiring back into the frame.

You can read about the earlier engine and electrical work I did in these write-ups.

Resources

My resources are primarily from Oak Okleshen and Bob Fleischer. Oak publishes “Manual 1: Boxer Top End Disassembly, 1970-1975” and “Manual 2: Boxer Top End Reassembly 1970-1975” in a single bound booklet.

NOTE:
Oak died in April 2017. Consequently I don’t know if this manual will continue to be available any longer or by what means. 

Bob has content on top end assembly and other details at his web site:

I also read through Anton Largiader’s material on rocker arms.

Parts List

Here are the new parts I used.

Part # Description Qty
11 12 1 338 426 Valve cover gasket 2
11 12 1 338 716 Head gasket 2
11 11 1 255 001 Base gasket, Normal 2
07 11 9 934 460 C-clip, Wrist spin 4
11 32 1 250 267 Push Rod Tube, seal 4

Tools

Here are the special tools and base gasket cement I used.

Tools

Tools – Gasket Cement, Ring Expander and Ring Compressor

NOTE: I chose to use Hylomar gasket cement. Bob and Oak no longer recommend it. They have various “recommendations” for cement, and recommendations have changed over time with Hylomar being recommended at one time. See Oak and Bob’s material for more details. I bought Hylomar for my Grey Ghost build so I’ll get to see how well it holds up on both bikes.

The other tools are a ring expander (left side) and ring compressor (right side). Neither are required. In the previous top end work I’ve done, I used my fingers to install the rings into the grooves in the pistons and the pistons into the cylinders. But for this project, I added some new tools to the tool box.

I also tried some Permatex Engine Assembly Lube on the cam, cam followers, push rod ends and wrist pins. In the past, I had just oiled them.

Permatex U;tra Slick Engine Assembly Lube

Permatex Ultra Slick Engine Assembly Lube

Getting Started

This is how the bike looks when I start.

Progress So Far

Progress So Far-Ready To Install Engine Top End

Find The Cylinder on Compression Stroke

Following Oak’s method, I determined which side is on compression when the engine is at top dead center and that’s the side I will assemble. Looking at the cam shaft I can see which side has the cam followers on the heel of the cam. The heal is the end of the cam lobe that is circular without the bump, or nose, which is on the other side of the cam lobe.

Right Side Cam on Compression

Cam Appearance on Compression Stoke

The shiny metal surfaces on the horizontal cam shaft are the cam lobes. The cylinders at 90 degrees to the cam shaft are cast sleeves the cam followers fit inside of: exhaust (left side) and intake (right side). The blurry bar in the middle is the connecting rod.

NOTE:  The cam follower is NOT centered on the cam lobe and that is the CORRECT orientation of the cam follower on the cam lobe. The cam shaft is installed correctly and is NOT out of alignment.

BMW locates the cam shaft so the cam follower rides on one side of the cam lobe. The friction between the rotating cam lobe and the cam follower riding on top of the lobe creates a torque on the cam follower causing it to spin. In turn, the spinning cam follower spins the push rod riding on top. This evens the wear on the cam follower and the ball ends of the push rod.

In the picture below, you can clearly see the end of the cam follower sticking our of the cast sleeve riding on the cam lobe is offset from the center line of the cam lobe.

Left Side Exhaust Cam and Follower

Exhaust Cam Showing Off-center Cam Follower

Install Piston Rings

Earlier, I ground the ends of the rings to set the end gap and stored the rings in the cylinders for safe keeping. Here is the bottom, or oil control ring, which is the first to go on the piston.

Bottom Oil Control Ring

Bottom Oil Control Ring

I install the rings starting with the bottom ring ending with the top ring. Note that each ring is different as it is designed for a different job. Each is marked with “Top” to show the side of the ring that faces up.

Oil Control Ring "Top" Mark

Oil Control Ring “Top” Mark

In the past, I used my fingers to spread the ring ends to get them to slide down the piston into the correct groove. But, I have a new ring expander tool and I use it this time.  Here is how the ring fits in the jaws of the expander.

Ring Expander

Ring Expander

WARNING: Rings are very hard and that means they are brittle. They don’t like being bent. As I was “playing” with the expander and the oil control ring, I found out it is very easy to expand the ring too far and snap it in two :-(.

Too Much Expansion Gets You a Broken Oil Control Ring :-(

Too Much Expansion Gets You a Broken Oil Control Ring 🙁

When you put a ring on the piston, expand it just enough to slip down the piston and into the groove. Don’t give a mighty squeeze on the ring expander or you, like I, get to buy another set of rings. Lesson learned, and reinforced, with a current retail price of $65.00 for a ring set and a week and a half to wait for a new ring set.

Here is the piston with all three rings installed. I confirm that “Top” is visible on all three.

NOTE: The picture below is NOT the correct orientation of the rings and the ring gaps. Keep reading to see how they are oriented on the left and right pistons.

All Three Rings Installed with Top Of Ring Facing Top of Piston

All Three Rings Installed with Top Of Ring Facing Top of Piston

I rotate the rings so the gaps are about 120 degrees apart. In the picture below of the left piston, you can see the orientation of the top ring (4:30) and middle ring (10:30) while the bottom ring (not visible) is at 1:30.

Left Piston Showing Ring Gap Position for Top and Middle Ring

Left Piston Showing Ring Gap Position for Top and Middle Ring

Oak uses different orientations for the left side and right side to help get them placed correctly to keep the gaps away from the thrust side (top and bottom) of the piston. By the way, I have read that rings “flutter” and rotate in their grooves so orienting them so the gaps are 120 degrees apart doesn’t mean they stay that way. Nonetheless, they are oriented to avoid the gaps lining up so they should seal well when the engine first starts up.

Install Piston in Cylinder

Now that the rings are installed in the piston, I install the piston in the cylinder. I wash the cylinder in hot soapy water and the rinse and dry it. I use a couple drops of engine oil to wet my fingers and wipe the cylinder walls so they have a very light coating of oil. A light coat will provide some lubrication at first start and ensure the rings seat into the new cross-hatching in the cylinder wall for a good seal to minimize oil consumption.

Two Drops of Oil for Cylinder Bore

Two Drops of Oil for Cylinder Bore

Light Oil on Fingers

Light Oil on Fingers

Applying Light Oil Film on Inside of Cylinder Bore

Applying Light Oil Film on Inside of Cylinder Bore

Correct Piston Orientation

I’m working with the left cylinder and piston and I want to ensure the piston is facing in the correct direction in the cylinder. The top of the piston has an arrow and “VORN” stamped into it. The arrow points to the front of the bike, and Vorn is German which loosely translates to “This Way Stupid!!”, or more formally, “Forward” :-).

Left Cylinder and Piston On Cylinder Studs

Left Cylinder and Piston On Cylinder Studs

Correct Left Side Piston Orientation with "Vorn" Pointing to Front

Correct Left Side Piston Orientation with “Vorn” Pointing to Front

I use the ring compressor tool to compress the rings into the grooves so I can gently push the piston and rings into the cylinder from the top. My compressor is a band of thin gauge metal that can be contracted or expanded using a wrench so it fits a range of piston sizes. There is a small lever on the take up reel that loosens the band a bit so the piston is easier to slide out of the band into the cylinder. The bottom of the band has some dimples that help hold the band inside the cylinder while the top of the band is smooth.

Opening Up Ring Compressor

Opening Up Ring Compressor

I open up the compressor and slide the piston and rings inside being careful not to catch a ring on the edge of the band. Then I use the wrench to tighten the band compressing the rings into the grooves.

Putting Ring Compressor Around Rings

Putting Ring Compressor Around Rings

Tightening Piston Ring Compressor

Tightening Piston Ring Compressor

I adjust the tightness so the piston just slides when I move the lever that loosens the band. I put the piston skirt into the cylinder and slide it down until the dimples on the bottom of the compressor band fit inside the cylinder.

Inserting Piston in Cylinder with Ring Compressor

Inserting Piston in Cylinder with Ring Compressor

Then I gently push the piston into the cylinder until the bottom ring touches the top of the cylinder. I gently rock the piston side to side with gentle pressure until the ring slides into the cylinder. The edge of the ring catches the edge of the cylinder a tiny bit and if I just force the piston into the cylinder I will break the ring. I continue this technique for the middle and top piston until I have the piston and rings inserted into the top of the cylinder. I remove the ring compressor and push the piston through the cylinder until the holes for the wrist pin are showing at the bottom of the cylinder.

Piston Wrist Pin Hole Exposed At Bottom of Cylinder

Piston Wrist Pin Hole Exposed At Bottom of Cylinder

Oiling Cams, Followers, Connecting Rod, Piston Skirt

I used the engine assembly lube and a toothbrush to get it on the cam lobes.

Engine Assembly Lube on Toothbrush

Engine Assembly Lube on Toothbrush

Toothbrush Application of Engine Assembly Lube on Cam Lobes

Toothbrush Application of Engine Assembly Lube on Cam Lobes

I put some on the cam followers and the dimple on the top of the follower where the push rod rides. I used it on the push rod ball ends.

I oiled the main journal bearings and I put two drops of oil on the piston skirt and spread it across the skirt with my finger

Oil on Bearing Journal

Two Drops of Oil Smeared on Piston Skirt

Apply Gasket Cement

I’m ready to apply the gasket cement on the base gasket, the cylinder base and the engine case. The idea is to apply a uniform, light layer of cement on these surfaces. I use Acetone to clean all these surfaces so the cement will stick well.

Acetone for Degreasing Surfaces

Acetone for Degreasing Surfaces

Cleaning Cylinder Base with Acetone

Cleaning Cylinder Base with Acetone

Cleaning Engine Case with Acetone

Cleaning Engine Case with Acetone

Clean Base Gasket with Acetone

Clean Base Gasket with Acetone

I heat the Hylomar in front of my workshop heater and then put it in glass of very hot water to keep it warm. This helps keep it fluid so I can get a thin even coat.

Keeping Hylomar Hot

Keeping Hylomar Hot

I start with the engine case and get a uniform coating including around the cylinder studs using my finger to[ to smooth out the cement.

Hylomar on Engine Case

Hylomar on Engine Case

I coat one side of the base gasket and install it with the coated side facing the engine case. I’ll coat the other side of the base gasket after I get the wrist pin installed on the piston.

Thin Hylomar Layer on One Side of Base Gasket

Thin Hylomar Layer on One Side of Base Gasket

Hylomar on Engine Case and One Facing Side of Base Gasket

Hylomar on Engine Case and One Facing Side of Base Gasket

I coat the base of the cylinder.

Hylomar on Base of Cylider

Hylomar on Base of Cylider

Install Push Rod Tube Rubber Gaskets

I put the push rod tube rubber gaskets on the push rod tubes on the bottom of the cylinders. The rubber gaskets have a top and bottom. The bottom has a vertical line from the mold and is wider than the top.

Push Rod Tube Rubber Seal (Bottom is Facing Up)

Push Rod Tube Rubber Seal (Bottom is Facing Up)

The ridges on the outside of the rubber gasket fit into the engine case as shown here.

Orientation of Push Rod Tube Rubber Seal (Seam and Wide Part on Bottom)

Orientation of Push Rod Tube Rubber Seal (Seam and Wide Part on Bottom)

The gaskets need to slide on the push rod tubes which move as the tubes heat up. I put a light smear of silicone grease on the inside of the rubber gasket and then put them on the push rod tubes.

Silicone Grease for Inside of Push Rod Tube Rubber Gasket

Silicone Grease for Inside of Push Rod Tube Rubber Gasket

Light Smear of Silicone Grease on Inside of Push Rod Tube Rubber Rubber Gasket

Light Smear of Silicone Grease on Inside of Push Rod Tube Rubber Rubber Gasket

Then I oil the ridges on the outside of the push rod tubes.

Installing Cylinder and Wrist Pin

I’m ready to put the cylinder on the engine by sliding it on the four cylinder studs. To make it easy to slide the wrist pin through the piston and connecting rod bushing, I freeze the wrist pin for several hours. It is secured by two C-rings that go into grooves in the piston wrist pin holes.

Frozen Wrist Pin and C-rings

Frozen Wrist Pin and C-rings

I put rags in the engine case around the connecting rod so it is supported.  I slide the cylinder with the piston sticking out of the bottom on to the cylinder studs. Then I make sure the arrow on the top of the piston is pointing to the front of the bike and gently slide the cylinder and piston down the cylinder studs. I guide the connecting rod inside the piston and align the connecting rod bushing with the wrist pin holes in the piston. I oil the outside of the wrist pin and then I push the wrist pin into the wrist pin hole on one side of the piston, through the connecting rod bushing until it passes through the other piston wrist pin hole. I center the wrist pin on the connecting rod so it doesn’t cover the grooves on either end of the wrist pin.

Wrist Pin Inserted

Wrist Pin Inserted with C-ring Groove Exposed

I make sure rags fill the hole in the engine case because putting the C-ring into the groove will take several tries with the C-ring flying out of the wrist pin hole when I can’t quite get it to seat. I don’t want it fly into the engine 🙁

I put one end of the C-ring into the groove and with a pair of pliers I bend the other end of the ring down and toward the groove forcing more of the C-ring into the groove until I get to the end of the C-ring where I use the screw driver to push the end inside the wrist pin hole and then back inside the hole until it to seats into the groove. The 6th time, it seats correctly :-). Then I put the another C-ring into the other wrist pin hole.

Wrist Pin C-ring Installed in Groove

Wrist Pin C-ring Installed in Groove

Now I put the Hylomar on the top side of the base gasket and I’m ready to push the cylinder toward the engine.

Piston with Wrist Pin Inserted & Hylomar on Top Side of Base Gasket

Piston with Wrist Pin Inserted & Hylomar on Top Side of Base Gasket

I make sure the transmission is in 2nd gear so I don’t turn the crankshaft as the piston is pushed up the cylinder. I make sure the push rod tube rubber gaskets are aligned correctly.

Oil on Outside of Push Rod Tube Rubber Gasket

Oil on Outside of Push Rod Tube Rubber Gasket

Then I slide the cylinder toward engine and align the push rod tube rubber gaskets into the holes in the engine case. I want them to be aligned in the holes, but they won’t be fully seated yet.

Aligning Push Rod Tube Rubber Gasket in Engine Case

Aligning Push Rod Tube Rubber Gasket in Engine Case

Install the Head Gasket, Head and Rocker Arm Assembly

At this point the cylinder is still loose but close to the engine block with the base of the cylinder inside the hole in engine case. I install the head gasket being sure the printing and the blue rings around the holes face me. At the bottom with the holes for the push rods, I make sure the hole in the gasket lines up with the hole in the cylinder and does not obstruct any part of the hole. If the gasket is installed backwards, the push rod tube holes in the cylinder are obstructed by the edge of the holes in the head gasket.

Head Gasket Correct Orientation (Print & Blue Rings Face You)

Head Gasket Correct Orientation (Print Faces You)

When I removed the rocker arms and push rods, I labeled them so they I know which are the left and right side and which are exhaust and intake.

Left Side Rocker Arm Assembly with Push Rods

Left Side Rocker Arm Assembly with Push Rods

There four dark colored nuts with a boss on one side secure the rocker arm assembly on the cylinder studs in the head and also to tighten the head and cylinder onto the engine block. The boss goes against the rocker block.

Rocker Arm Head Nuts with Boss on the Bottom

Rocker Arm Head Nuts with Boss on the Bottom

There are two thick washers with nuts that are used to secure the head to the cylinder using the short studs in the cylinder at the 12:00 and 6:00 positions.

The head has a mark indicating if it is left or right (shown) and of course, the spark plug hole goes on top and the threaded spigot for the exhaust pipe is on the front. I use the left head as I’m assembling the left side first.

Right Head Marking. The Left Side has "L"

Right Head Marking. The Left Side has “LS”

Left Cylinder and Head Installed on Cylinder Studs

Left Cylinder and Head Installed on Cylinder Studs

Some heads use an O-ring under the rocker block and some don’t. Bob’s material has a picture of both style heads. My heads have the passage for the cylinder studs enclosed in the aluminum casting, so I DON’t need the O-rings. You can see the cast aluminum cylinder behind the metal pin through the cooling fins in the picture below.

Enclosed Casting Around Cylinder Studs Means NO Rocker Block O-Rings

Enclosed Casting Around Cylinder Studs Means NO Rocker Block O-Rings

I insert the push rods into the tubes and make sure they are in the indentation in the top of the cam follower.

Inserting Push Rods

Inserting Push Rods

I use the rocker arm nuts to help draw the cylinder and head down to the engine case but I don’t install the rocker arms yet.

Using Rocker Nuts to Draw Cylinder and Head to Engine Block

Using Rocker Nuts to Draw Cylinder and Head to Engine Block

I want the cylinder to be drawn evenly into the engine and not get it cocked to one side. I tighten each of the four nuts in a cross wise pattern about 3/4 of a turn on the bottom nuts and 1/2 turn on the top watching the base of the cylinder and adjusting how much I turn each nut so the cylinder is drawn into the engine case evenly. There is resistance from the push rod tube rubber gaskets so the cylinder tends to get cocked if you aren’t paying attention.

I keep an eye on the push rod rubber gaskets and when they are fully seated I stop.

Push Rod Tube Drawn Tight to Engine Casing

Push Rod Tube Rubber Drawn Tight to Engine Casing

I see that the cylinder is uniformly snug against the engine case with a small amount of gasket cement squeezed out around the base gasket.

Cylinder Drawn Even and Tight to Engine Case

Cylinder Drawn Even and Tight to Engine Case

I remove the rocker arm nuts from the cylinder studs so I can install the rocker arms. Each of the arms has a block at the top and bottom. The top block has a disk in the center while the bottom block doesn’t. Each block is split with a cut on one side. The cut faces toward the outside of the head.

Top Block of Rocker Shaft

Top Block of Rocker Shaft

Bottom Block of Rocker Shaft

Bottom Block of Rocker Shaft

It’s important to install the rocker arms with the top block at the top and the split in the blocks facing to the outside of the head as shown below.

Rocker Block Correctly Installed with Top Blocks on Top

Rocker Blocks Correctly Installed with Top Blocks on Top

I put the four rocker block nuts on the cylinder studs and tighten them to just snug (maybe 5 FOOT/pounds) while pinching the top and bottom blocks of the rocker arm assembly together. I want the rocker arm to rotate easily but I don’t want the arm to be able to move up and down. Based on Oak’s guide, I want the push rods to be vertically just above the center line of the push rod tube holes and horizontally in the center of the hole to prevent the rods rubbing on the inside of the tubes. Oak provides an extensive and detailed description of precisely aligning the rocker arms and push rods in an addendum included at the back of his top end reassembly guide.

Rocker Assembly Installed with Snug Nuts - 5 FOOT/pounds

Rocker Assembly Installed with Snug Nuts – 5 FOOT/pounds

Here is the proper orientation of the split in the block and the rocker block nuts as seen from the front. The split faces me and the boss on the nut is against the face of the rocker block.

Proper Orientation of Rocker Block (Slot Facing Outside of Head) and Nut Boss on Face of Block

Proper Orientation of Rocker Block (Slot Facing Outside of Head) and Nut Boss on Face of Block

Then I install the large washers on the short cylinder studs at the 12:00 and 6:00 positions. The washers have a beveled edge on one side that faces you.

Short Cylinder Stud Washer Showing Bevel Edge That Faces You

Short Cylinder Stud Washer Showing Bevel Edge That Faces You

The holes are deeply recessed in the head. I use a magnetic telecoping rod to push the washer into the hole on the stud and then hold it with a finger as I remove the magnet.

Using Magnetic Holder to Insert Bottom Washer On Bottom Short Stud

Using Magnetic Holder to Insert Bottom Washer On Bottom Short Stud

To get the nut to start on the stud, I use the magnet to hold it in place, put a finger on the nut and remove the magnet and then with the tip of the finger on the face of the nut, I spin it a 1/2 turn or so to get it threaded. I use a socket with extension to get it snug.

Starting Bottom Nut on Head with My Finger

Starting Bottom Nut on Head with My Finger

Oak provides measurements to help align the rocker blocks. These are based on the thickness of the blocks which can vary. The “standard” width is 22 mm which is what I have went I measure mine.

Measuring Width of Rocker Block

Measuring Width of Rocker Block

Oak provides a table for the distance between the inside and outside faces of the rocker blocks. I adjust the blocks with the nuts snugged up lightly tapping on the blocks with a rubber mallet. For the 22 mm wide blocks, I want 71 mm distance between the inside faces of the blocks while I ensure the push rods stay in the correct orientation.

Measuring Distance Between Rocker Blocks

Measuring Distance Between Rocker Blocks

Torquing the Head

Their are six nuts to torque: the four rocker arm nuts and the two nuts on the short cylinder studs at 12:00 and 6:00. The head is torqued down in stages and in a cross wise pattern. Facing the head, the six nuts layout in a 12:00, 2:00, 4:00, 6:00, 8:00 and 10:00 position. The sequence I tighten the nuts in is 2:00, 8:00, 4:00, 10:00, 6:00 and 12:00. The stages I tighten the nuts are 10 FOOT/pounds, 15 FOOT/pounds, 20 FOOT/pounds, 27 FOOT/Pounds.

As I increase the torque, I make sure the push rod is not moving in the tube and there is no vertical movement of the rocker arm and the rockers still rotate easily. I’ll let the head sit over night, check the torque again and then set the valve clearance.

When I’m done, I use some Acetone to clean up the little bit of gasket cement that has squeezed out around the base gasket and put an old spark plug into the hole and clean shop rags into the intake and exhaust spigots to keep anything from getting into the cylinder.

Here is the left side top end assembled. I put the valve cover on just to show how it will look, but since I need to set the valves after rechecking the torque the next day, I didn’t tighten the cover yet.

Left Top End Installed & Torqued Down

Left Top End Installed & Torqued Down

Left Side Installed

Left Side Installed

And, here is what I started with just about a year ago. Not a lot of difference except for the missing barn straw. 🙂

Left Side Before Rebuild

Left Side Before Rebuild

Installing the Other Side

Now, I carefully use the rear wheel to bump the engine until the “S” mark on the flywheel is again centered in the hole so the right side is on the compression stroke. And I repeat the procedure on the right side. Here is the right side installed.

Left Side Done

Right Side Done

Here is the final result with the top end assembled with new spark plugs that I set to 0.025 inches of gap.

View From the Saddle

View From the Saddle

Top End Done

Top End Done

I’ll add the exhaust, carburetors and cables later and I’ll document that in another write-up.

Revisions

2017-04-30   Note about Oak Okleshen passing away.

13 thoughts on “11 BMW 1973 R75/5 Install Engine Top End

  1. Pingback: 1973 BMW R75/5 Rebuild: Install Engine Top End | Motorcycles & Other Musings

  2. I’m sure many, many of us appreciate your good documentation and pictures. Makes me BOLD ! Hey, I could do this too.

    Thanks, Steve

    • Hi Steve,

      The nice thing about airhead top ends is that they are pretty simple to work on. The only special tool that I would recommend is the vernier caliper to get the rocker arm spacing correct, but a decent one is less than $50. So, get BOLD 🙂

      Best.
      Brook

  3. Just want to drop a note and say your site is beyond helpful. I haven’t even started to really tear into my 78 r100/7 and 78 r80/7 and i already feel like i know the bikes based off your blog and photos. Thanks again!!!

    • Hi Brian,

      Thank you very much for the kind words. Your comment about what the content has done for you is exactly what I am trying to achieve.

      Best of success on your airhead project. It is always cool to hear from someone invested in getting or keeping one of these machines on the road.

      Best.
      Brook.

  4. Thank you so much for posting Brook. My 75/5 is just starting to smoke a little on the left side and I am planning on doing the cylinders and heads next winter. Your descriptions and pictures will be very helpful. Ben Zehnder, Orleans, MA

  5. Hi Brook, I am beginning to put my 74 R90S back together after stripping it down and painting the frame. Besides cleaning and new parts as nessasary I did the top end with 1st over rings, valve job and new pushrods and seals. I am reviewing your blog every step of the way. Your blog is excellent, and I thank you for the time and effort you put into it. Big Thanks, Richard

    • Hi Richard,

      I’m pleased to hear this material is helping you with your R90S project. Should you need an “independent road test in the Colorado mountains”, don’t hesitate to drop the bike off at my place :-).

      Best.
      Brook.

  6. Wow what a great article. I have become the owner of a 72 R75/5 with about 30,000 miles on it and has set for many years. It seems to be free. Do you think I should tear it down anyway? Any advise?

    • Hi Bob,

      Thank you for the kind words. To the question of “Do you think I should tear it down anyway?”, it depends. If the bike runs well, has good compression, passes a leak down test, idles well and smoothly accelerates, does not have oil leaks around the front cover (leaking front crankshaft seal), the rear of the engine where the transmission mounts (leaking rear crankshaft seal), the clutch works smoothly, the front forks aren’t leaking, and the steering head bearings are smooth when you move the handlebars lock-to-lock, then there’s nothing to worry about that routine maintenance won’t fix. If any of the above is evident, then you can correct whatever is wrong without doing a complete tear down.

      That said, if the idea of doing a tear down, a new paint job for the frame, tank, and fenders, and replacing engine seals, oil pump cover and seals, doing a carburetor rebuild, etc. appeal to you, then why not? Well, one answer is the cost of “preventive maintenance” all at once, and the time the bike will be unavailable for riding.

      I hope the above is helful.

      Best.
      Brook Reams.

  7. Thank you.

    I have no formal mechanic training, but have been able to follow your instruction to:

    set valve gap clearance
    replace front and rear brakes
    replace front brake cable
    rebuild both carbs

    Next on the agenda is to replace push rod gaskets, top end gaskets, oil pan gaskets. It is a 1970 R75/5.

    Thanks again,
    Andrew

    • Hi Andrew,

      Cool beans. You are taking on, and completing, a lot of the routine items necessary for the “care and feeding” of an airhead.

      Best.
      Brook.

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