The bike is a 1975 R75/6, single owner bike with 105,000 miles. The crankshaft rear oil seal and oil pump cover o-ring are original and 40 years old. It’s time to replace them as part of a series of projects I’m doing to “freshen up” the bike.

I already replaced the crankshaft front oil seal, sprocket, nose bearing, and the camshaft seal. You can read about that project here.

• 11 BMW R75/6 Replace Timing Chain, Crankshaft Sprocket, Nose Bearing

The engine is out of the frame, but this work can be done easily enough with the engine in the frame.

Parts List

The following parts are used for this project.

Part No.	Description	Qnty
11 11 1 338 342	SHAFT SEAL – 100X80X10	1
11 41 1 335 895	O-RING, OIL PUMP COVER	1
11 41 1 260 182	OIL PUMP COVER	1
07 11 9 904 357	HEX BOLT – M6X16-8.8-ZNS3	4
11 22 1 262 060	HEX BOLT – M11X1,5 (from 09/75)	5

The flywheel bolts come in two versions, a M10 x 1 and an M11 x 1.5 bolt. This bike has the 11 mm bolts. The bike was manufactured in 05/1975 and the note in the MAX BMW parts fiche indicates the 11 mm bolts went into service starting 09/1975, so in theory my flywheel bolts should be the 10 mm version. It goes to show that BMW cut in changes “approximately” around the published cut over date.

Although the 11 mm bolts can be reused, I prefer to replace them when I remove the flywheel. It’s low cost insurance against an expensive bolt failure.

I found the oil pump cover is the old style using Phillips head screws. The cover was redesigned to reduce potential oil leaks and uses more convenient Allen head screws so I replace the old style cover with the newer one. You don’t have to do that, but it makes sense to take advantage of improved parts when doing work on a 40 year old bike.

First the clutch is removed and then the flywheel to get to the crankshaft rear oil seal and the oil pump. I inspected the clutch parts and decided to have them refurbished due to uneven wear of the clutch plate and scoring of the pressure plate and compression ring. The diaphragm spring measurements show it has lost spring height and I’m replacing it with the stiffer version of the spring.

I’ll document the clutch inspection and installation in a separate write-up.

Other Parts

There are other parts that you will need when you remove the transmission; the four bolts that connect the transmission output shaft to the drive shaft and if you remove the rear drive from the swing arm, the paper gasket between the rear drive and the swing arm. There are two types of drive shaft bolts; the original version of the M8x1 bolt (26 11 1 230 414) that requires a lock washer (07 11 9 930 840), and the newer bolt (26 11 1 242 297) which is shorter and does not use a lock washer. I always use the newer version of the bolt when I replace the four drive shaft bolts.

Part No.	Description	Qnty
26 11 1 242 297	SCREW – M8X1 (from 09/80)	4
33 17 2 311 098	GASKET ASBESTOS FREE	1

Tools

There are some special tools that I use. I bought them from Cycle Works. They make removing the clutch and flywheel easy.

To remove the flywheel, I use three M8x1 35 mm or longer hardened (Grade 10.9) bolts with nuts and washers. These are about twice as long as the clutch bolts.

• Clutch Installation/Removal Bolts

To remove the flywheel, I need to hold it so it won’t turn. I use the Cycle Works tool that is also used to pull/push the flywheel off/on the crankshaft.

• Flywheel Puller/Holder 1970-1980 Airheads

To remove/install the crankshaft rear oil seal I use another tool from Cycle Works that makes this very simple.

• Rear Main Seal Installer/Puller All Airheads 1970-1995

I use an impact driver to remove the oil pump cover Allen head screws. You can get these at automotive parts stores, Harbour Freight, etc.

I use a torque wrench that measures INCH/Lbs, not foot/lbs, to torque the oil pump cover bolts. A FT/Lb torque wrench is needed for the flywheel and clutch bolts.

Resources

I did this same work on a 1973 R75/5 and the procedure is the same for the 1975 R75/6. You can read about the entire procedure here and some of the adventures I had on the R75/5 project:

• 11 BMW R75/5 Remove & Install Flywheel, Replace Rear Main Seal, Oil Pump Cover & Cover O-Ring

Other on-line resources that discuss this work, which I based my procedures on, include:

• Bob Fleischer: Flywheel Removal Replacement – WARNING!!!
• Bob Fleischer: The F, OT, S, Z Marks on Your Flywheel
• Bob Fleischer: Clutch
• Duane Auscherman: BMW Motorcycle Clutch Removal, Inspection and Repair

Remove Swing Arm, Rear Wheel & Transmission

I use the rear wheel to help loosen the four bolts holding the drive shaft to the transmission output shaft. I documented how I remove the swing arm here.

• Remove Swing Arm

Once I get the drive shaft off the transmission output shaft, I remove the rear wheel. Then I remove the rear drive from the swing arm but you don’t have to do this.It’s easier to install the swing arm without the weight of the rear drive. However, if you remove the rear drive you will need to get a new paper gasket that goes between the rear drive and the swing arm.

• Remove Rear Drive

Now I remove the transmission.

• Remove Transmission

I inspect the clutch throw out arm, bushings, throw out rod and bearings for wear. I plan to replace a number of these parts as the roller bearings were damaged. I’ll document the inspection and replacement of these parts in the a separate write-up on the clutch that you will find here:

Blocking Crankshaft From Moving Forward

WARNING:
Before beginning, I make sure the crankshaft is pushed to the rear of the engine and blocked in this position. There is a thrust washer inside the engine that adjusts the end play of the crankshaft. It rests on two small pins. Normally the crank is constrained from moving forward by the flywheel mounted on the rear of the crankshaft. Since it is being removed, the crankshaft can shift forward and that can allow the thrust washer inside the engine to come off the pins. When the flywheel is tightened on the rear of the crankshaft the thrust washer will jam the crankshaft damaging the thrust washer and likely damaging the crankshaft. That’s a very expensive new repair you will have.

I set the engine to Top Dead Center (TDC) by getting the “OT” mark centered in the timing window in the engine case.

The front engine cover is already removed, but if working on the engine in the frame, remove the battery ground cable, then the front engine cover to expose the alternator. I insert a short bolt that fits inside the Allen Allen head bolt securing the alternator rotor as shown in the center of the picture below. Be sure the bolt diameter is small enough so it won’t damage the Allen head slots.

I replace the front cover using only the two long Allen bolts on the sides but not the short bottom one. I finger tighten the bolts so they are just snug. I verify that the front cover is pressing on the head of the bolt in the alternator rotor Allen bolt by the small gap between the front cover and the engine case.

The front cover is now pressing the crankshaft toward the rear so it presses against the thrust washer inside the engine.

Remove Clutch

I did this work on an R75/5 and on this bike. Both procedures are the same. You can read about clutch removal here:

• 21 BMW R75/6 Remove, Refurbish, Install Clutch
• 21 BMW R75/5 Remove and Inspect Clutch

The primary difference is I replaced the entire R75/5 clutch with new parts which have the same design as found in the /6 model bikes. On the R75/6, I had the clutch parts refurbished by Southland Clutch at about one-half the cost of  the new parts.

Remove Flywheel

There are five bolts in the center of the flywheel that attach it to the crankshaft.  There are two types of bolts, a 10 mm and 11 mm. The 10 mm ARE NOT REUSABLE. In theory the 11 mm are reusable, but for piece of mind, I always replace these anyway.

Before I remove the flywheel, I put a paint mark next to one of the bolt holes and another on the nose of the crankshaft next to the hole it goes into.

I immobilize the flywheel using the Cycle works tool so it can’t rotate when I remove the flywheel bolts. I use the bronze colored bar and two long bolts to secure it to the face of the flywheel.

I used a propane torch to heat the bolt heads in case someone used locktite on them. I let them cool down and then removed them. It takes a bit of grunt to get them loose. I steaded the engine while my son did the grunt work.

I use the same bronze bar and two clutch bolts to secure it across the flywheel so the threaded hole in the middle is centered over the center of the flywheel.

I put some wheel bearing grease on the large bronze bolt nose and the threads, screw it in until it presses on the face of the crankshaft and then tighten it with a crescent wrench.

It comes off quite easily and I keep a hand on it so it won’t drop off the end of the flywheel.

Now I can see the oil pump cover. It’s the old style with Phillips head screws, so I order the newer cover and the Allen head screws that secure it. On top of the pump cover is pile of oily grunge. And there is debris on the bottom of the rear crankshaft seal. This makes me think that the rear crankshaft seal has begun to leak oil. So it’s certainly time to replace the original seal

When I look at the back of the flywheel, I can see some streaks of oil on it too, so it wouldn’t be long before the clutch got oiled. It wasn’t slipping yet, but it maybe time to replace the clutch plate just in case.

The old style crankshaft oil seal is rigid and eventually cuts a groove into the face of the flyhweel hub. In time oil seeps past the seal. You can see the groove where the pencil is pointing.

The new seal has a flexible lip that is larger so it will provide a seal even if there is a groove in the flywheel hub.

Remove Oil Pump Cover

I use an impact driver to loosen the Phillips head screws of the oil pump cover. These are installed with blue locktite so heating the heads of the Phillips head screws and letting them cool is helpful.

The impact driver can loosen or tighten screws. To loosen, twist the large shaft in the direction you twist a screw to loosen it (Righty Tighty, Lefty Loosey), then strike the end of the tool with a hammer or mallet. The impact causes the impact driver to twist and shock the screw which loosens it.

Warning
YOU WANT THE PHILLIPS SCREW BIT TO TIGHTLY FIT THE SCREW or you can strip the head off and then you have much more work to do to get the screw out.

The front of the cover has a beveled edge on the top edge. On the back, you can see the orientation of the two cavities towards the bottom of the cover. Also on the back of the cover is a groove for an o-ring. The o-ring is black (the older version of the o-ring is black and the new, improved version is red) and stared inside the groove in the engine block.

Remove the Crankshaft Rear Oil Seal

These are the Cycle Works parts for removing the rear oil seal.

I heat the engine case around the outside of the seal with a heat gun.

I center the black plastic disk using two flywheel bolts. Then I screw in the two sheet metal screws all the way flush to the plastic disk. The screws go through the outside edge of the seal securing it the plastic disk.

When both sheet metal screws are screwed down, I remove the two flywheel bolts and screw in the large bronze colored puller bolt in the threaded hole in the center of the plastic disk.

I tighten the puller bolt and out comes the seal.

Underneath the seal is a metal thrust washer. Its the same as the one inside the engine, although the thickness of the two thrust washers can vary. You can see the two small metal pins it hangs on. So it’s not hard for the thrust washer inside the engine to fall off the pins. That’s why I block the crank using the small bolt in the alternator Allen bolt that the front engine cover pushes on.

You can see three holes underneath the gold colored thrust washer. These go inside the engine, so you may want to stuff some cylinders of paper towel into them to keep stuff out of the engine and make sure there is plenty of the tube outside the hole so it won’t fall inside the engine.  If it does, you can pull the oil pan and fish it out.

Clean Flywheel Housing and Flywheel

I always clean the inside of the housing where the flywheel and clutch live. It’s usually a mess since the clutch is a source of dust.  I plug the three holes below the thrust washer and use engine cleaner, various brushes and tooth brushes until I get all the crud out of there. Since my engine is out of the frame, I put it inside a rectangular oil drain pan to catch the goop. I use a spray bottle with water to wash off the engine cleaner and most of the stuff stays in the drain pan.

After I the inside of the engine housing clean, I use some brake cleaner and spray it on the block cleaning up the last bits of oily bits using blue shop towels. I want it to sparkle so there won’t be any chance of grit or gunk getting on the oil pump cover sealing surface and the crankshaft rear seal housing. It would be a bummer to have them start leaking due to crud.

There is a bit of oil and clutch dust on the flywheel, so I use engine cleaner to get it clean. I pay particular attention the teeth on the circumference of the flywheel so they are clean and shiny.

I use some metal polish on the outside and inside face of the flywheel hub so they shine. The outside face has the crankshaft rear main seal rubbing on it and I want the interface to be nice and clean.

There are some places on the flywheel teeth where the starter motor has caused a burr, so I take a small file and remove the burr on the inside of the tooth. Note the the face of the teeth is cut at a 45 degree angle so the starter motor gear, which is mounted at 90 degrees to the edge of the flywheel, can mate smoothly with the teeth of the flywheel.

Finally, I use three colors of paint to on the timing marks on the flywheel so they are easy to see when I set the valves and set the ignition timing. The red indicates “OT” or top dead center (small red dot at the middle of the OT). There are three marks near the “S” that I painted white; the top one is below the “OT” mark, the second one is just above the “S” and the third is just below the “S”. Below the white marks is a dot and the letter “F” below it that I painted yellow. The dot is the full ignition advance mark.

Dual Plug Ignition Timing Recommendation

This bike was modified for dual spark plugs back in 1981. Dual plugs create two flame fronts instead of the single front created when a single spark plug fires. Therefore, it takes less time for the peak pressure to develop in the cylinder. So ignition doesn’t need to occur as much in advance with dual plugs as it does with a single plug.

I originally installed an Accel ignition amplifier system which eventually died and I ran the dual plugs on the stock ignition for awhile. When I did the rebuild in 2010, I added a Dyna III electronic ignition. I set the ignition timing as I had for the Accel unit, but I wasn’t sure that setting was the best one.

I checked on the micapeak Airhead forum for advice on the proper ignition timing settings for dual plugs with Dyan  III electronic ignition.  Tom Cutter recommends ignition timing at idle should be about 5 degrees BTDC with full advance at about 24 – 25 degrees BTDC. This is different from what was recommended when I had the Accel ignition.

Determining Flywheel Rotation in Degrees

Ignition timing is measured in degrees of flywheel rotation. The rule of thumb is each 2 mm of distance is about 1 degree of rotation. I measured the diameter of the flywheel to the outer edge where the timing marks are: it is 8-3/8 inches. To compute the circumference of the flywheel, multiple the diameter by PI (3.14159) giving 26.31 inches.There are 25.4 mm per inch, so the circumference of the flywheel is 668.3 mm. The flywheel rotates 360 degrees in one revolution, so dividing the circumference by 360 gives the distance around the flywheel per degree of rotation, or about 1.85 mm per degree of rotation. This is close to the rule of thumb of 2 mm per degree of rotation (it  is 7.5% less).

To measure degrees of rotation on the flywheel, I use some graph paper I have with mm markings (the smallest markings). Every dark marking is 4 mm or about 2 degrees of rotation. I put “0”, “10”, “20” and “30” on the paper strip to indicate the approximate number of degrees before BTDC.

The middle of the “S” mark is used to set ignition timing at idle, and for this bike, that occurs at about 9 degrees before top dead center (BTDC). With dual plugs, ignition timing at idle moves to 5 degrees BTDC.  Using 1.85 mm per degree of rotation, the proper “S” mark for dual plugs is about 9 mm BTDC or 9 mm from the small dot next to the red “OT” mark. As shown below the top white mark is about 9 mm or very close to 5 degrees BTDC, so I’ll use that to set ignition timing at idle.

Ignition timing at full advance should be 5 degrees less with dual plugs as it is at idle. The dot above the “F” mark (yellow in the picture above) is at 29 degrees BTDC. The Dyna III electronic ignition uses the stock automatic timing unit (ATU). The ATU uses weights (curved arms in the picture below) that move outward due  to centrifugal force as the RPM increases. The arms rotate the shaft that includes the points cam to advance the timing. As shown in the picture below, there are two pins inside the two large holes on either side of the center hole with the flat along one edge. As the weights rotate, the pins move from the lower edge of the large holes to the top edge. That movement corresponds to about 29 degrees of points cam rotation. The small springs pull the pins back toward the bottom of the hole when the RPM slows until the pin is at the bottom of the hole at idle (approximately 1000 RPM). The strength of the springs determine the RPM for full advance. For this bike, full advance occurs at approximately 3,000 RPM.

The stock ATU advances the ignition timing 29 degrees which is too much for dual plugs. The motion of the small pins needs to be reduced by 5 degrees or about 17%. Tom Cutter puts an old set of the advance springs over the pins. The spring diameter is large enough to reduce the amount of full advance by 5 degrees. I plan to purchase new advance springs as they weaken with age and I’ll use the old ones to reduce full advance.

CAUTION:
The ATU has become very expensive, so don’t damage it when modifying them to reduce the timing advance.

Installing New Oil Pump Cover and O-ring

Here are the parts I used.

The new style cover uses hex bolts instead of the original Phillips head screws.

The red O-ring fits in the groove inside the cover. I use a couple dabs of grease to keep in place on the cover before installing it. The cut-outs on the inside of the cover are at the bottom while the bevel edge on the outside is at the top.

I clean the engine matting surface around the oil pump. There is no gasket since the o-ring does the sealing. I also clean out the screw holes with brake cleaner and blow them dry with compressed air as blue locktite is used on these bolts to prevent them from loosening.

This is the orientation of the cover; just rotate it upward and add the bolts.

I found that the edge of the cover near the lower right bolt was bent and pushed it out with a screw driver so my socket would fit around the bolt head. The metal is very thin at the edge.

The bolts are torqued at a low value: 72 INCH/Lbs (not FOOT/Lbs). I use an INCH/Lbs torque wrench and tighten them in a criss-cross pattern in two stages.

Install New Crankshaft Rear Seal

I use the Cycle Works tool to install the crankshaft rear seal.  It uses the five crankshaft bolts to pull a black plastic disk toward the engine case to force the seal into the hole.

Seal Preparation

The old style seal had a sharp edge. The new style seal has a wide flap and is made from PTFE. It needs to be preformed around the flywheel hub before it is installed. I use some 600 grit wet/dry paper and dress the outer edge of the seal hole. I want to remove any burrs around the beveled edge of the hole so the seal will slide in smoothly.

I form the seal around the flywheel hub.

I use a propane torch to heat the housing around the hole. Since this is a large seal, it makes it easier to install the seal flush.

When I get the aluminum to about 100-120 F, which takes about 5 – 10 minutes of heating, I put a bit of oil around the inside of the hole.

Insert Rear Seal

I get the seal started by hand so it is flush in the hole.

Then I use the five flywheel bolts and mount the black ring on top of the seal. I tighten the bolts in a criss-cross pattern 1/2 to one turn at a time to push the seal into the bore.

I keep tightening the bolts until each of them stays snug as I go around to ensure the seal is installed all the way inside the bore.

When I’m done, the edge of the seal is recessed just a bit from the top edge of the bore.

Install Flywheel

Previously, I cleaned the hub on the back of the flywheel and then used metal polish to remove the discoloration. I put a light smear of oil on both sides of the hub and on the outside of the crankshaft nose so it will slide over the crankshaft nose easily.

I put three clutch bolts into the flywheel to make it easy to rotate it on the crankshaft hub when I align the holes for the flywheel bolts.

I put the flywheel in the oven and heated it to about 200 F. I want it slide on easily and to let me rotate it a bit on the crankshaft nose when I get the bolts installed.

I use some welding gloves so I can handle it when it is heated. Before I install it, I make sure the alignment mark on the flywheel and the crankshaft nose line up. I just need to get the correct holes aligned on the flywheel and the crankshaft nose.

The heated flywheel slides on the crankshaft nose smooth and easy. I verify that the “OT” mark is visible in the timing window since I have the crankshaft at top dead center.

I finger tighten the bolts quickly. I check that the flywheel moves easily. If it doesn’t, then likely the rear thrust washer has come off it’s pins. It rotates easily. I loosen the bolts a bit so I can wiggle the flywheel and adjust it so the bolts are centered in the hole. You can use an edge of a hex head on a bolt to see how much the flywheel rotates to the left and right of the edge of the hex head, then center the hex head edge. I tighten the flywheel bolts 1/4 turn in a cross-wise pattern and again make sure the flywheel can still move easily.

As I wait for the flywheel to cool to room temperature, I install the Cycle Works flywheel tool to immobilize it. I use a ratchet strap across the rear shelf behind the clutch housing to hold it down on the work bench when I torque the flywheel bolts.

When the flywheel has cooled, I torque the bolts in 3 stages in a cross-wise pattern. I tighten it to 20 FT/Lbs, then 50 FT/Lbs and then to the final 75 FT/Lbs.

Here is the flywheel installed. You can see the registration marks on the flywheel and the crankshaft nose.

Next step is to install the clutch. I’ll document that work in a separate write-up.

Revisions

2019-04-30 Removed reference to soaking new seal in hot oil. Not required