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Thread: 1978 r100rs rebuild

  1. #1
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    1978 r100rs rebuild

    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?



    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.


  2. #2
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    1978 bmw r100rs

    Sorry.

    I appear to have posted twice.

  3. #3
    Professional Slacker
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    1978 r100rs rebuild

    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.








  4. #4
    Registered User
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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.....

  5. #5
    Rally Rat
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    Looks like fun. Keep us posted on the progress/issues. BTW, I love the engine supports you've rigged up. Brilliant!

  6. #6
    Registered User
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    Lightened flywheel...to make it rev easier

  7. #7
    Registered User gertiektn's Avatar
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    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.

    I like good stuff. No Chains here!
    1960 R50 -"Hanz"
    1977 R75/7 -"Gertie"
    2004 R1100s -"RFAR1"

  8. #8
    mymindsok
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    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!
    Last edited by mymindsok; 06-15-2010 at 07:25 AM.

  9. #9
    TGHSMITH
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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...

  10. #10
    On the Road
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    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.

  11. #11
    Liaison 20774's Avatar
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    Quote Originally Posted by tghsmith View Post
    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.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  12. #12
    535IS
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    Quote Originally Posted by tghsmith View Post
    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.

  13. #13
    TGHSMITH
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    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...

  14. #14
    John D'oh
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    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?
    Last edited by s09rwmb; 04-23-2010 at 03:55 PM. Reason: ...more questions.

  15. #15
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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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