I’ve removed the airbox, transmission, clutch and flywheel to gain access to the rear main seal so I can replace it. You can see how I did that work here.

• 13 BMW 1983 R80ST Remove Air Box & Pulse Air System
• 23 BMW 1983 R80ST Remove Transmission
• 21 BMW 1983 R80ST Remove Clutch
• 11 BMW 1983 R80ST Remove Flywheel

I’ve published documents on replacing the rear main seal on other projects. Here are links to those documents.

• 11 BMW 1983 R100RS Remove Flywheel & Rear Main Seal
• 11 BMW 1983 R100RS Install Rear Main Seal, Oil Pump Cover & O-ring
• 11 BMW 1977 R100RS Replace Rear Main Seal & Oil Pump Cover O-Ring
• 11 BMW 1975 R75/6 Replace Crankshaft Rear Oil Seal, Oil Pump Cover & O-ring
• 11 BMW 1973 R75/5 Remove & Install Flywheel, Replace Rear Main Seal, Oil Pump Cover & Cover O-Ring

Since the 1983 R80ST engine is similar in many ways to the 1983 R100RS, I will link to the 1983 R100RS documents as appropriate in this document.

Tools

I typically use the Cycle Works tool for removing and installing the rear main seal.

• 1983 R100RS: Rear Main Seal Removal Tool From Cycle Works

That said, on this bike, I had to resort to getting a “bigger hammer” as the seal was very tight in the bore. So I used a hooked seal puller to carefully remove the seal without scratching the aluminum bore it fits in.

Parts

The new rear main seal is part of the Euro MotoElectrics seal kit, EME Part # GSK-EngineKit421, and is BMW Part# 11 11 1 338 342.

It has a plastic cylinder inside the seal to keep the lip of the seal at the correct diameter to fit around the guide ring that mounts on the nose of the crankshaft. I remove the white plastic ring before installing the seal.

Video

Here is a short video of how I do this work on the 1983 R80ST.

VIDEO: 1983 BMW R80ST Replace Rear Main Seal

Preparation

Before I start work, I clean the bell housing so I don’t get any dirt, grunge or grit inside the engine when I remove the rear main seal. Since the top engine compartment was also dirty and oily, I removed the starter motor so I can clean it at the same time. I also removed the rebreather hose, which I plan to replace, so it is easier to clean the top engine compartment. I plugged the hole in the rebreather spigot with some blue shop towel to keep water out of the engine.

I use kerosene and a stiff brush to remove the hardened gear oil. Then I use Gunk Foamy Engine Cleaner to remove the kerosene and a lot more of the dirt and baked on gear oil. Finally, I use a toothbrush and some brake cleaner to get into the corners of the casting and get out stubborn dirt and gear oil. When I’m done, the inside of the bell housing shines again.

Remove Rear Main Seal

This link shows how I do that work.

• 1983 R100RS: Remove Rear Main Seal

Here are some pictures from the 1983 R80ST. On this bike, the steel sleeve of the seal was so hard I couldn’t screw in the puller screws of the Cycle Works tool. Instead, I tightened the puller screws as tight as I could by hand to mark the edge of the seal. I put a piece of blue shop towel behind the seal to keep the steel shavings from falling inside the engine. Then I drilled 7/16″ holes in the face of the seal and screwed in the puller screws.

I was not able to remove the rear main seal using the Cycle Works tool. It was stuck tightly in the bore and the screws pulled out of the edge of the seal. I repositioned the puller and and drilled new holes in the face of the seal. Then I used my MAP gas torch and heated the engine block around the perimeter of the seal. Again, the screws pulled out of the seal.

So, I used my seal puller. I used the short hooked end and carefully pried the seal out of the bore by walking around the seal to get it loose. I was very careful to put the point of the hook of the seal puller inside the channel at the edge of the seal so I don’t damage the aluminum bore in the engine block the seal fits inside of. That technique worked.

Install Rear Main Seal

This link shows how I do that work

• 1983 R100RS: Install Rear Main Seal

Here are some pictures from the 1983 R80ST.