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Rear Main Oil Seal Removal ~

Na Cl K9

John D'oh
I believe that folks are making more of a job out of this than is necessary. I had planned to disassemble an engine for some parts and finally got a chance to photograph the technique. Here in photographs is how I remove a rear main oil seal from the engine case of, in this case, a 1978 R100/7. Notice that the bottom lip of the seal is black from oil leaking past it and, the outside edge of the seal is even with the engine case. The seal is an original factory plastic seal correct for 1978 which has probably not been replaced before this.

First photo is the seal in place: Note oil on lip and seal position in case.
 
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So, 'blocking the crank' does NOT go without saying so...

I hope you block the crackshaft before doing this.
I did not block the crank shaft. The empty engine block and some of the parts are currently sitting in my SK machine. As illustrated there is no prying done to release the seal - its diameter is reduced and it's pulled from the pocket 'almost effortlessly' which was exactly the point of this post. Putting a new seal in or the fly wheel back on is quite another issue especially for anyone doing this job for the first and only time they are likely to need it done. Block the crank shaft as necessary....


Note that sometimes you must peal a bit of the plastic away to get to the metal core of the seal...
 
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Very interesting. I'm pulling the main rear seal on my bike this coming weekend, as it happens. This will help a lot. Thanks for the excellent pictures.
 
blocking the crank

I have changed the seal on my R90 back around 1987 and even with the BMW manual I have didnot know at that time I need to block the crack. Just got lucky that I didnot have any problems then.
 
Thanks for posting this. I'm performing the same repair on my '72 R50/5. I was wondering what the ring is that's mounted on the two pins behind the seal. I see one in your picture and I was wondering what this is and it's purpose.

Thanks!
 
Took my /5 apart to replace rear main seal and after inspection realized it was transmission seal that was leaking. So you might have one of those in hand just in case.

:usa
 
you can buy a tool to thread into the frnt of the crank, or use rags or something similar between the front cover and crank. i used pieces of bicycle inner tube taped in position.
 
how do you block crank

I took a spare 6 mm allen key, cut about an inch of it off. That piece of basically hex bar stock I then mounted in a wooden plug, half the length of a wine cork say.

With the engine front cover off (remove neg batt lead first), the 6 mm bar stock fits into the alternator bolt (which is crank mounted) and then I bolt the cover back on. You want to ensure the overall "tool" length is such that the cover is jammed against it, thusly holding the crank from coming forward. My cover has about a 1/4 inch gap when it's snugged up with the tool in place.

Have heard of the rag stuffing....I'd be afraid things still might move. I got to see these mystical thrust bearings and the pins they hang on inside a cutaway engine at Super Tech. You really don't want that puppy popping off. The crank has to be restrained from moving forward even 1/4 inch.
 
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Northwoods Airheads offers this, with the declaration that you can make your own.

Crankshaft Blocking Bolt

This is a small nylon bolt that I've cut to length and modified the
end. Jammed into your rotor mounting allen bolt head and with
the generator cover put back on and snugged up it will block
your crankshaft when you remove the flywheel and clutch etc.
The generator cover bolts will not be able to be tightened all the
way up and that's the idea the pressure is on the crank. You can
jury rig something like this yourself. Like many of my tools this
is a convenience to save you time and head scratching


$4.00 + $4.00 postage

IMG_1579-258x192.jpg

Disclaimer: I have no affiliation with NWA.
 
Special Tool???

OK - another question:

Where do you get the special tool?? BMW part #??


:buds


Just kidding!!!

:wave
 
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