Airhead Tech Pages


Replacing The Rear Main Seal

by Dale A Duvall and Don Eilenberger

I can't believe all the misinformation floating around about rear main seals. I hope I can clear up some half truths.
  1. You do not have to remove the engine or cylinders to do the job.
  2. You DO need a special tool to install the seal.

    It is not an absolute - you can do it if you are careful and have a vernier caliper and know how to use it. The manuals aren't REAL clear on this.

  3. The seal must be installed dry, as it is a teflon impregnated paper sealing surface and has to be pre-formed-hence the special tool.

    I don't know which seal you're talking about - BUT - the newest seal (one I got about 2 months ago) is BLACK, and must be installed WET. The instructions are VERY SPECIFIC about soaking it in engine oil for an extended period of time in order to soften it up before installing it. This was supposed to be the hottest "trick" seal from BMW. Mine was for an R65 - but I believe the exact same seal is used on all the later boxer's (/5 on).

    The special tool that they currently use is to set the depth of the new seal (which is different from the old white rubber seals), not the shape. The new seal be about 1mm out from where the old seal was in order for the lip to seal properly on the flywheel boss. If you are a REAL clever guy - you could use an O-ring of the correct diameter and thickness UNDER the new seal to set this spacing easily, or you could do it by measuring with the vernier calipers (like I did) before removing the old seal, and while installing the new seal. Once the new seal is in place (and soft from soaking in oil) installing the flywheel will form it's shape.

  4. The flywheel bolts do need to be replaced on a /5, but if you torque them to 7 ft/lbs your flywheel will really fly off. (The correct torque is around 55 ft/lbs. Check with your dealer.)

    There are more than JUST the flywheel bolts here - and it appears there is some confusion. There are also bolts holding the pressure-plate to the flywheel. These are 6mm bolts. If you tighten them to 55 ft/lbs, you'll be holding the heads in your hands. The manuals call for something like 16 ft/lbs. This is WRONG. It will ALSO lead to holding the heads in your hands. Correct torque for this size bolt is ~7 ft/lbs. On some bikes these are not bolts, but large flat head screws - again the 7 ft/lbs is a reasonable number to torque these. The large bolts (19mm head I seem to remember) that hold the flywheel to the crankshaft may very well be around 60 ft/lbs - I would have to check my manuals on this one.

    The bottom line is that the books call for ALL the above bolts/screws to be replaced - along with the 4 holding the driveshaft to the rear output of the tranny. They're cheap - do it.

  5. You do not need to remove the swing arm and wheel. Just remove the pins (with a 27mm socket ground down-not 21mm) unhook the brake rod and hold the wheel/swingarm assy back with a tiedown strap.

    Uhh... true, with a caveat. The swingarm is a BEAR to get back in place with the wheel/tire attached. I did it. I wouldn't do it this way again. I would remove the wheel and swing arm. Once you've got the wheel off there is NO extra work involved (over what you describe above) in removing the swing-arm.

    While you're at it - remove the swing-arm bearings and clean and repack.

    If this sounds complicated, it is. For a novice this may take 8 to 10 hours. An expert can do it in about three. Did your dealer quote you flat rate? If not try another dealer.

  6. Always replace your driveshaft bolts. They are cheap and need to be retorqued also. 25 ft/lbs.
Down with myths and conundrums!

Since I've been married for over 20 years I haven't needed a conundrum! Some myths would be nice though!

Dale A Duvall duvall@efn.efn.org
Don Eilenberger dje@mail.bellcore.com


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Last Update: 11 September 2010