1978 R100/7 Leaking cylinder base

[Deleted User 62]

We got my friends 1978 R100/7 up and running last week, but a persistent leak at the cylinder base has us stumped. He says there was an o-ring on the cylinder base when he pulled it apart. I am not familiar enough with these later engines to know if there should be an o-ring there or not, but I sure don't see a place for it that has anywhere near as much room as for the top stud o-rings. He's assembled it 3 times, first with a white o-ring that came in a complete engine gasket set. That o-ring actually got squeezed out of the gap between the case and the cylinder in spots. He has since used up two grey o-rings, which seemed like a harder material, plus a thin film of sealant. Still the drip drip drip... Can anyone confirm that this thing should or shouldn't have the cylinder base o-ring? I think it's not needed and is causing the base not to seal. Also, should we be replacing the stud o-rings with every reseal, or are they good for multiple torquing?

[Kurt in S.A.]

My 9/77 build '78 R100/7 had no large o-ring...I don't think they came in until later. There was a recent thread on this. On my recent top end overhaul, the two small o-rings on the studs were quite mangled, but it had not been apart in nearly 100K miles. They should fit into recesses on the underside of the cylinder. If they look OK, probably could be reused...I think they get mushed a little just like the o-ring in the filter housing.

If it continues to leak/drip, I would suspect some perfection in either the engine case or the cylinder base. Or possibly the pushrod tube seals are not mating correctly. There is a process to pull the cylinder/head down which involves first tightening the bottom stud bolts with little force on the top stud bolts. If you pull all the bolts down, you'll end up cocking the seals in place. Once the cylinder base has drawn down evenly, then you can begin with some turns on the top studs/bolts.

There is also a proper positioning of the pushrod tubes relative to the base of the cylinder. If the tubes are into the cylinder too far, there won't be the right amount of pressure on the seals. Likewise, if the tubes aren't in far enough, they'll put too much pressure on the seals...they could split.

Kurt in S.A.

[Deleted User 62]

Success!!! He mounted everything minus the base o-ring and no more leaks!!! Thanks!

[Jean]

My "magic" book says the base O-rings weren't used until AFTER May 1979. (Factory issue: Auxiliary Catalog)
I imagine using the o-ring on an earlier engine would cause the pushrod tubes to leak in addition to leaking in other places!
They are bad to change little things in the middle of model years...

[Deleted User 62]

Yep, and it didn't help that the parts supplier lists it as "for 1976-on"...

[Jean]

Tim, all the other parts are the same.
I have the same problems here in the USA since my bike is a Euro-bike. At least my local shop KNOWS that, but that is why I bought the catalogue, long ago.