BMW Rear Main Seal Nightmare

Discussion in 'Airheads' started by bmwhacker, Jan 12, 2012.

  1. bmwhacker

    bmwhacker Still on 3 wheels

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    Photo might be deceiving. The seal is flush with the case. If it was out any further it would be sticking outside of the chamfered edges of the bore. I installed it at the same depth the original one lived. I got my fingers crossed.:D
    #41
  2. wirewrkr

    wirewrkr the thread-killer

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    The seal looks fine to me as far as depth is concerned. I've installed quite a few. The only one that leaked on me was a N.O.S. silicone one that I got in a box of parts. (I was too cheap to buy a new one, never since though)
    Since then, I've only used the current factory supplied teflon seal. Pre-formed with my fingers, installed dry where the seal contacts the flywheel, no pits or gouges on the flywheel etc.
    Back in my VW tech days, when I was in VW factory training school, our instructor told us to check every case with a strong heat gun or propane torch for cracks. I have only seen a crack once or twice on BMW engines, but on VW cases it was a regular occurrence to have a cracked case. I think Pommie John from down under has posted a pic of his race bike cracked case.
    #42
  3. disston

    disston ShadeTreeExpert

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    Well I don't know where I get this from. I searched around in records but could not find any authority to quote for you. So I'll have to take responsibility for this myself even thought the idea is not original to me.

    The construction of the rear seal places the lip of the seal deeper than the top edge lined up with the case because of this most rear seals are found a little bit proud of the surface of the case and the recommendation is to install new seal to the same depth. That's fine if the seal being replaced is at the proper depth but if it has been replaced it may be at any depth somebody wants it to be.

    What I noticed when I replaced the rear seal on my 1975 R90/6 is I could see the contact worn area on the flywheel where the lip of the seal rode and did it's work. It was not really worn but it was shiny because this spot was rubbed constantly by the seal. The shinny spot was on the very edge of the flywheel that needed to seal the oil inside the engine. I noticed the seal was only proud of the case surface by about 1mm so when I put the new seal in I placed it so it was at least 2 mm proud of the case surface. I of course have not been able to check the shinny worn spot to see if it is nicely away from the edge like I hope because after reassembly it has not leaked and therefore there is no need to take apart again.

    I went and looked at the earlier pictures, noticed the original seal was installed flush. Don't have an excuse for myself. Hope this time works for you, I'm sure that doing stuff over and over is not fun, it's never fun for me. But if this has to come apart again in addition to looking for cracks with a propane torch also check the shinny area of the flywheel and see where that seal is ridding.

    Charlie
    #43
  4. H96669

    H96669 A proud pragmatist.

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    Me too, went through that two years ago on a K-bike teflon tranny seal. Could not reinstall at the same depth, it was leaking after benchtesting.

    Had to open up the tranny to see the 'Shiny groove"....not an area on that one, an actual groove.

    Careful measurements, remounted the seal so the lip wouldn't touch the "shiny area" , fine ever since. And looks like that same tranny seal had been replaced by many only to leak again shortly afterward.

    Could the newer seal lip ride at a different depth nonwithstanding the depth of the outer part....very possible.

    But interesting how they actually work/wear, some can extrapolate from that as to why they should be scrupulously clean and straight on installation:
    http://www.ewp.rpi.edu/hartford/~peetrm/FWM/Project/Reference%20Papers/wear%20and%20friction%20of%20ptfe%20seals.pdf
    #44
  5. jackd

    jackd Long timer

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    My crack was almost invisible on my '94 GS - and I get paid for a living to look for cracks on gas turbines. This crack was so fine that it only opened up when the engine was hot and oil spewed from the oil pump housing onto my flywheel to be slung everywhere.
    #45
  6. supershaft

    supershaft because I can

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    I think you have had a bad seal. You should be able to get your money back. it doesn't look right. Did you ever answer my question about the flywheel O-ring? Does it have one? Did you replace it?
    #46
  7. bmwhacker

    bmwhacker Still on 3 wheels

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    Yup, replaced the flywheel o-ring...twice.
    We just returned from a hard 60 mile ride and no oil loss noted. Must have been a bad seal. It came in an engine rebuild kit I bought a while back. Maybe it was "real" old, new old stock. :lol3
    #47
  8. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    This thread is good food for thought (or fodder for paranoia, whatever :eek1 ). One upcoming Spring/Summer project might be the rear main seal/oil pump o-ring when I do clutch and tranny work. This is the Methusaleh of oil seals-- it's a silicone seal that has been in since the mid-1980's and hasn't leaked a drop (touch wood). I'm tempted to "leave well enough alone", but since I'm going to be in that far, I'd be remiss if I didn't do the RMS.

    I'll pay close attention to shiny spots and installation depth. I usually tap seals in with care, but I may invest in a seal installation plate with the depth stops.
    #48
  9. supershaft

    supershaft because I can

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    Everyone says how simple the job is. I think it is damn easy to all of sudden get the seal in super cocked and it F's up the seal's OD. I like a installation plate with all five bolt holes. It makes a big diff in getting the seal in straight the first time. Getting the seal out isn't so simple either if all the F'ed up seal bosses I run into are any indication. Drills and screws mess up a lot of cases. It's a shame because there is no need for them. All you need is a common hooked seal puller from the auto parts store.
    #49
  10. disston

    disston ShadeTreeExpert

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    Yeaaaah ! We were running out of obscure twilight zone repair ideas.

    charlie
    #50
  11. bgoodsoil

    bgoodsoil Dare to be Stupid

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    I've heard about the depth of the seal several times but never got a definitive explanation. I've done several rear mains and I've put them in flush. All of those were on 80's model airheads.

    So what's your 2 cents on the depth of the seal, SS?

    +1 on what SS said about using a 5-hole tool. I couldn't imagine bopping these things in with a hammer. I'm sure someone that does a lot of them could get good at it but that ain't me.
    #51
  12. bmwhacker

    bmwhacker Still on 3 wheels

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    I remembered I had one of the cheap little hooked seal pullers stashed in my "specialty" tool box. Made the seal removal a five second process. Only use it once in a blue moon and forgot I even had it.
    I had a thick aluminum disc, which is actually a pipe "diameter gauge" holder, the disc is just a few MM's smaller than the outside seal diameter. It worked perfectly to tap the seal in squarely.....with a dead blow plastic hammer.
    After becoming quite intimate with the seal & bore it appeared to me the the seal seemed to sort of "bottom out" when it was flush with the engine case chamfers.
    In the end I got pretty good practice tearing the bike down and reassembling. The last sequence took me from 1:00pm until 5:00pm from start to finish (not counting 2 "refreshment" breaks) and I didn't have any parts left over. :D.....and the SOB doesn't leak yet!:clap
    #52
  13. Cordless

    Cordless Two Wheel Addict

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    I have experience only with an '81 g/s but that was instructive as well as embarrassing. I put the seal in backwards, prompting a very tactful Anton to point out my (actually OUR--The Raven was involved) error. The second time took about 1/3 the time taken by the first replacement.

    I am glad yout seal seems to be holding.

    I find it interesting that by following your failed attempts I have learned much more about what to do and what to watch out for than I could ever have learned by you installing it correctly and flawlessly the first time.

    Such is the nature of learning how to wrench.

    Thank you
    #53
  14. supershaft

    supershaft because I can

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    I don't think the exact depth is that critical at all. Especially with the later style seals. The newer seals have damn near ten mm of the seal sealing against the flywheel/dohicky. Plus the seal boss chamfer varies considerably. I try to always get them in deep enough so that 100% of the seal OD is against the seal boss. That's damn near bottomed out in some cases and a bit proud in others. I don't think it matters as far as the seal's ID is concerned. But at the same time I always pay attention to how far the seal is in before I take it out and then look at the flywheel/dohicky to see where the seal has been riding on it. That will tell if you need to make any adjustments in the installation depth of the replacement. You just have to pay attention to what is going on but that is what a good wrench always tries to do. Keep your eyes open and think about how things work!
    #54
  15. bmwhacker

    bmwhacker Still on 3 wheels

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    Good advice..I sometimes get a little tunnel vision when up to my armpits with a final goal in mind....Have to remind myself to slow down a bit and be very observant. I have a friend (with 0 airhead experience) who always wants to "help" , but I don't want / can't have anyone else messing in the guts of my machine. I liken it to having someone else working on my airplane (if I had one). My passengers life and mine could depend on how that sucker is put together. Whenever my friend comes over and wants to "help" , I shut down for a break.....Am I out of line?


    BTW....looks like the leaking problem has gone away and the rear of the engine is......as they say in Rural Montana : "Tighter than a bulls ass in fly season"...Yeeeee Haw....
    #55
  16. GodOmelet

    GodOmelet Adventurer

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    Hey Charlie,

    I'm about to get started with this job (after getting that clutch lever arm weld squared away) and I have found a so much oil on the shelf that I'll be very interested in looking for cracks in the case also just to be safe, and now that I have a welder among my support team. How does one look for cracks on a case with a propane torch? Thanks, Mark
    #56