The bike is almost 40 years old. There is a fair amount of grunge on the shelf under the transmission and inside the bell housing which suggests the rear main seal and/or the oil pump cover are leaking. So its time to replace them.

Parts

Here are the required parts.

Part #	Description	Qty
11 41 1 335 895	O-RING	1
11 11 1 338 342	SHAFT SEAL – 100X80X10	1
11 22 1 262 060	HEX BOLT – M11X1,5	5
11 22 1 337 093	GASKET RING – 59X3MM (from 09/78)	1

Tools

The tools are the same as those I used on the R75/6. You can see the list here:

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Tools

Resources

Refer to the R75/6 write-up for a list of useful resources.

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Resources

Remove the Clutch

First, I remove the clutch. You can see how I did that here:

• 21 BMW 1977 R100RS Remove, Refurbish, Install Clutch

Remove the Flywheel

WARNING:
Before I start work, I make sure the crankshaft is blocked from the front by putting a short M6 x 10 mm bolt in the alternator Allen head bolt installing the front engine cover so there is a small separation between the cover and the engine block when the cover bolts are just snug. DO NOT OVER TIGHTEN THEM. I also make sure the engine is at top dead center (OT mark) and paint an index mark on the flywheel across from the top right transmission stud. That way it’s easy to put the flywheel back on the crankshaft so the timing marks on the flywheel are correctly oriented.

The procedure for the R100RS is the same as for my R75/6. You can read about how I did that here:

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Remove Flywheel

I was able to remove the R100RS flywheel without using the Cycle Works flywheel removal tools. I can wiggle it off the cranshaft nose without much difficulty. I have to tilt the wheel back a bit at the top to get it to slide out of the bell housing as it’s a tight fit.

The flywheel on the R100RS is different from the one on the R75/6. It uses an o-ring on the inside of the hub to seal it to the crankshaft nose. And, the flywheel timing marks on the R100RS are retarded 3° compared to the flywheel on the R75/6. This was done to reduce pinging. Note the oil on the inside of the flywheel hub.

Crankshaft Bolt Hole Leaking Oil

Upon removal of the flywheel I found one of the holes in the crankshaft the flywheel bolts screw into was leaking oil.

This also happened on my 1973 R75/5 when I did the same work. Fortunately it’s an easy fix to stop the leak. You can read how to fix it here:

• 11 BMW 1973 R75/5 Remove & Install Flywheel, Replace Rear Main Seal, Oil Pump Cover & Cover O-Ring
  • Repair the Leaking Crankshaft Bolt Hole

Here is the repaired crankshaft bolt hole. This picture was taken after I removed the flywheel as described below.

There was oil on the inside of the hub (earlier picture) likely coming from the oil leaking from the crankshaft bolt hole. This helps explain why there is so much oily grunge on the inside of the bell housing.

It’s curious that the pattern of the grunge is between 9:00 and 1:00 on the engine block and not uniformly distributed.

The flywheel hub shows the typical groove caused by the original rear crankshaft seal cutting into it. This is why the seal leaks over time. The newer seal uses a different design to avoid cutting a groove.

Remove Rear Crankshaft Main Seal

The procedure for the R100RS is the same as for the R75/6. That said, this seal was hard to remove from the bore. You can read how I do this here:

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Remove the Crankshaft Rear Oil Seal

Clean Bell Housing

Before I remove the oil pump cover, I clean all the grunge out of the bell housing. I removed rear main seal first so I can plug the oil return holes at the bottom of the crankshaft bore. I can’t get to them with the seal installed. I put the engine is a large automotive oil drain pan to catch all the crud.

Here is the housing, with oil pump cover removed, after cleaning. Much nicer. 🙂

Remove Oil Pump Cover

The cover is the newer style with M6 hex head bolts instead of Phillips head screws, so it’s easy to remove the cover with a socket wrench. The o-ring is on the inside of the cover. Note the beveled edge at the top of the cover. The bolt holes are off center so when installing it, it only goes one way.

Measure Oil Pump Clearances

When I rode this bike a few hundred miles before starting the rebuild, I had one instance of the oil pressure light coming on. I tested the pressure with a gauge and didn’t see any indication of low pressure. Nonetheless, I want to measure the oil pump clearances in case there is a problem.

There are three measurements: the housing to outer rotor, the inner rotor lobe to the outer rotor lobe, and the housing height above the inner rotor. I measured these with feeler gauges. I inserted blades starting with the smaller ones in the published range of clearances and keep increasing the thickness until I find the blade that won’t fit. This is what I measured for the clearances.

• Housing to Outer Rotor Clearance:      0.004-0.005 inches
• Inner Rotor Lobe to Outer Rotor Lobe: 0.004-0.005 inches
• Housing Height Above Inner Rotor:      < 0.002 inches

These are all at the lower end of the allowable range according to the Haynes manual. The oil pump seems to be in good condition and not worn.

To measure the gap between the housing and inner rotor, I use an Allen wrench as a flat and insert the feeler gauge under it on top of the inner rotor. Due to the need for three hands to do this and take the picture, I only show how I place the Allen wrench. The measurement is taken under the wrench and top of the lobe of the inner rotor.

Install Oil Pump O-ring and Cover

The procedure for the R100RS is the same as for the R75/6.  Here is a link to the write-up.

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Installing New Oil Pump Cover and O-ring

Install Crankshaft Rear Main Seal

The installation procedure for the R100RS is the same as for the R75/6. Here is a link to the write-up.

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Install New Crankshaft Rear Seal

Install Flywheel

The bike has dual plug heads. Consequently the timing is different from the stock single plug head. The full advance normal is indicated by the center of the dot over the “F” mark. With dual plugs, full advance is retarded by 6°. I describe the changes required in these write-ups.

• 12 BMW 1977 R100RS Changes To Ignition For Dual Plug Heads
• 12 BMW 1977 R100RS Install Dyna III Electronic Ignition-Refurbish Automatic Timing Unit (ATU)

To make it easier to see the timing marks, I paint them.

I added a mark at 25° BTDC which is where the full advance should occur. The recommended method of setting the timing is to use the full advance mark at a high enough RPM to ensure full advance rather than at idle at the “S” mark.

The R100RS flywheel has a o-ring inside the hub. I cleaned the flywheel and then installed the new o-ring in the groove.

The flywheel installation procedure for the R100RS is the same as for the R75/6. Here is the procedure.

• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
  • Install Flywheel

Here is the engine with the flywheel installed. I verified that the “OT” mark is visible when the pistons are fully extended.