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1978 r100rs rebuild

rowens816

Professional Slacker
I am probably sure that this has been covered before; however, I could not find any references to my exact problem.

I have acquired a 1978 R100RS "Project". This is my 2nd BMW, but the 1st one was the 1974 R90S in that burnt orange. Sure wish I still had that one.

There are many problems with this bike, but the most immediate is the rear main seal. This engine has been visited before.

Has anyone seen this flywheel before? This cannot be stock?

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Any ideas?

Secondly, the sn# on this bike is 6183284 indicating that the bike was built in January, 1978. However, the timing chain is double-row, and the engine has an electric tach. All parts fiches that I have seen indicate a cam sprocket that is double-row with a drive for the tach. What gives?

Any help is appreciated.

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Actually, I have alot of pictures, questions and commentary about this rebuild. I hope some of you guys are ready to comment on this project.

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Well, my 78 RS has the duplex chain and electric tach. 1977 was the last year for mechanical tach. Simplex chain arrived for 1979, along with new points set up.

I cant seem to get anything with the link.....
 
Looks like fun. Keep us posted on the progress/issues. BTW, I love the engine supports you've rigged up. Brilliant!
 
mod/lightened fly wheel

Looks like some extra mods... what about the hose clamps on the pushrod tubes??
Looks pretty oily..

Cosmetics on the tank and seat look good..

I have no knowledge on the cam chain question.. it can be researched.

Just take one thing at a time, keep it simple and do it right. Take your time.

:bikes
 
We know that BMW introduced upgrades a little at a time, so I wouldn't worry much about timing chains, tach drives and such.

Mostly we have to work with what we have, though you can install an updated clutch assembly pretty easily and that would get rid of that unsightly fly wheel!
 
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I'd check the numbers on the block... the breather set-up looks like a 80 on when the airbox was changed, but the early type cam is in place. the safety wired flywheel bolts suggest that it was set up for racing.... can't tell but those might be nikasil cyls, 78rs's also had differant pistons than anyother airhead...
 
Good project!

The electronic tach was introduced in 9/77, and the double row timing chain was standard until, I think, 1981. So these are likely the right parts for your '78 R100RS.

Check the vin and determine the build date from eg. Max or Real OEM, and consult not only the diagrams (which cover many models) but the parts lists, showing when parts were changed.
 
the breather set-up looks like a 80 on when the airbox was changed

That breather cap looks similar to my '78 R100/7. The vapors come up, go forward, and settle in a small well forward of the cap. The liquids seep through a hole in the case while the mist continues forward through an engine cavity. It exits the cavity in front of the starter and then is routed back to the R/H air snorkel via a long rubber hose.
 
I'd check the numbers on the block...

X2. In '78, they still matched the frame number. My '78 was built during the first week in January, near yours, but I have no idea what the cam chain looks like, never having had that part open.
 
I think you may be correct kurt, they had the long tube that went to the airbox (NLA part , I remember making one or two with copper flex for the tight bends) still should look under and see what breather valve is living there...
 
R90S tank looks real nice...

The stock BMW /6 sport tank and seat look nice enough to be worth the price of admission to that project bike. The flywheel is a custom lightened OEM /7 unit and is yet another idea of how to shave some mass and make the engine rev quicker. It looks like less than 20% of the flywheel material has been removed so its not a real radical cut. If the engine was run, it held together so the “engineering” if any seems sound enough. If the holes cut into the flywheel are sharp edged you’ll probably want to replace it because it was not finished properly. Did you find safety wire on the sump pickup bolts? Double Row chain is correct for that year. Electronic tach was a new feature in 78. Production for 78 RS US spec looks like 2500 machines. 6182501 - 6185000 (Snaab - 1978/79).

Your 78 has many things in common with the entire range of 1978, 1977 and even some of the 76 production. Should make good ride. Are you planning to make an RS with fairing? Could you show us a photo of what is bolted to the sliders on the front fork assembly?
 
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Well, the rebuild is continuing. The head and jugs are off, and they both look pretty good. I have to wait until this weekend to get the bore dimensions. I have the inside snap gauges, but the biggest micrometer I have is 3". I guess I will have to wait and use a friends gauge. TOO many years with 4-cylinders; never needed a 4" mike.

Remember the pushrod tubes? Someone even commented on the hose clamps on the tubes. Yea, that threw me when I first noticed it. Of course, all that oil should have been a dead give-away. As expected, it was a mess. One or more of the braised (sp) collars were coming loose ---- hense the hose clamps. My compliments to the previos owner --- however stop-gap the measure, it did not work long-term. One tube was long also. Look at the pictures.

I hope to get these cleaned up, blasted and rebuild the heads (with new valves) this weekend.

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FWIW, that flywheel is lightened in the pattern chosen by Udo Gietl-note how the drillings leave five untouched areas in line with each flywheel bolt. Udo believed it was important to avoid the flywheel flexing when the clutch was disengaged.

You have a bit of a collector's item.
 
Nikasil

.... can't tell but those might be nikasil cyls, 78rs's also had differant pistons than anyother airhead...

I agree those look like Nikasil.

Find out for sure BEFORE you rough the walls and ruin them with an abrasive hone.
 
In to the tubes.

yah, too bad it is not January or some such month. Good luck with the rebuild.

Jim, Ketchikan, AK
 
Iron Cylinders

To the best of my knowledge, these are iron cylingers. Actually, I have never seen the nikasil cylinders. Are there any specific marking on the barrels that would indicate nikasil?

As for the flywheel, I really want to keep it. Please look at these pics. The teeth on the flywheel are pretty solid EXCEPT for the front edge. They seem to be consistiently chipped, but not real bad. It looks to me that the started should have been shimmed back a little. I plan to clean up any sharp edges with some small files, and see how it turns out. I may also have to rebuild the starter. It's a Bosch starter --- stamped with 02 10 75. Can I assume that is the build date?

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