oversize cylinder base O ring - why

[dirtsurfer]

I am about tot order the parts in preparation to replace the pushrod tube seals on my '79 r100 . I notice that my local parts supplier also features a + 0.2mm Oversize (cylinder base) O ring. Would that be to compensate for worn surfaces?

I notice that one of the previous owners has used liberal amounts of gasket cement around the cylinder base. In an ideal situation , no cement would be required , correct?

Thanks Max

[Kurt in S.A.]

Are you sure your '79 needs the large o-ring? My '78 R100/7 didn't...just completed a total top end overhaul. Even though the fiche shows a 2.2mm o-ring for my bike, it doesn't have one. I seem to recall some of Oak's ramblings about these o-rings and I believe they went from 2mm to 2.2mm...for some reason I can't remember. I doubt it would be to compensate for worn surfaces. If you have worn surfaces, you're replacing parts.

No "cement" but you should use some kind of non-hardening sealant use as Hylomar or Permatex Ultra Gray.

Kurt in S.A.

[chasbmw]

I use a smear of DreiBond, it keeps the joint oil mist free. Much better than Hylomar

[ME 109]

Does a 79 have an o ring groove?

I used the oversize on my 81 without trouble.
Three bond grey is available here Max and is good stuff.
Gotta have a thin smear of silicone as well as the o ring.

[Deleted User 62]

I'm seeing conflicting information about O-rings on various websites. A friend here on the island has a 1978 R100/7 that he said had an o-ring when he disassembled it, but I sure don't see a groove for one. There is a very slight machined space where I think a groove ought to be. Is that all there should be? Some sites say the o-ring groove started with Nikasil cylinders (1981?) others show it as used on 1974-on. What have you seen?

[Kurt in S.A.]

I've read both what Oak and Tom Cutter have written about this. Oak mentions two service bulletins. One in March 1982 says to use the 2.2mm for better sealing, then came back with another bulletin about a year later saying that if there was a leaking issue, consider using the 2.0 mm o-ring (-866 number). Oak indicates that the o-ring was cut for 5/1979-on bikes. It seems that the issue really is the cylinder must contain the groove. The 2.2mm supposedly will help with sealing but the 2.0mm could be used. Oak even thought that the 2.0mm o-ring was or should have been discontinued.

Tom seems to agree with the 5/1979 date through the 1980 models. This would be use of the -567 2.2mm o-ring. He lightly oils them to slip in easier. Oak differs from Tom, but Tom says that the -567 o-ring was also used on the early '80s Nikasil models. The problem comes with the '86-on bikes after the Airheads "came back" and those cylinders have an extra step in the cylinder. Care has to be taken when trying to use these cylinders on earlier engine cases. I suspect that these later Airheads don't take the large o-ring.

Kurt in S.A.

[ME 109]

I don't think the post 85 came with an o ring?
They had the groove but didn't come stock with an o ring.
Then again I could be wrong.

Both o rings will leak without sealant, IMO