Too little cylinder base sealant?

[tunnelrider]

Hi all,

I re-fitted my top end only 3000km ago, using a new gasket kit and a very thin amount of Hylomar blue around the engine case mating surface where the cylinder sits.  About 500km ago I noticed there was a bit of a combustion stain going on from the right cylinder, showing on the engine casing, at the front of the cylinder.  I'm just about to fit recond. heads so I've got a good chance to fix any wrong doings I did with the fitting initially.  Can anyone shed some light on to why there may be some combustion gas staining at the base of the cylinders?  I oiled the large o-ring just before fitting so I don't think that would be the cause, at least I think so...!  Could it be not enough sealant? I only applied it to the engine case by the way. The other side is OK.
Cheers.

[Tony Smith]

I have exactly none at all, To my mind the secret is the careful placement of the O-rings to make sure they are not displaced or "crimped".

Not that I have any leakage as yet, but if I did I would expect that the occasional CT-8 wash would deal with it.

[tunnelrider]

Oh well, there's the sealant theory out the window, thanks Tony!  For sure, it comes of easy with a squirt of degreaser.  Would you know if it has any effect on compression?  I'm thinking probably not to me since it's down at the base of the cylinder...

[georgesgiralt]

Hello Tunnelrider,
IMHO the stain is an oil leak. I've had the very same.
Two BMW repair man told me to apply a very thin coat of Loctite or Permabond sealing product.
I used the red Permabond http://www.permabond.com/products/anaerobic-adhesives-and-sealants/ because it was readily available at the industrial parts dealer near my workplace.
Clean the cylinder base and the motor casing thoroughly then oil the O-ring gaskets, place them on the cylinder base then make a thin line of sealant around the cylinder. The important thing is that the line must be continuous. Play a particular attention at the oil supply holes.
Moderation is a key here.
It was a permanent fix for my motorcycle.

[tunnelrider]

Cool, thanks George for that. Wonder why Tony doesn't get the same with using no sealant - maybe my barrel or engine case is slightly out of round so the o-ring doesn't seal properly?... I'll know more when I see the o-ring on the disassemble, next weekend.  Is there any disadvantage after using Permabond in future top end work that you know of?

Cheers.

[Barry]

If BMW had got the O ring dimensions and manufacturing tolerances of the groove correct sealant wouldn't be needed.

Back in Jan 82 they tried to resolve the problem by increasing the O ring size from 2.0mm to  2.2mm but in some cases that caused distortion of the cylinder barrel so for production they went back to 2mm.  Parts fiches still show 2.2mm O rings so there seems to be some confusion over the correct size.

Anyone know what size O ring they used ?

[tunnelrider]

In all fairness Barry, I've always suspected that a PO dropped her on the right side, by the right head having some big impressions of the rocker cover retaining bolts, actually cracking the forward stud housing.  Also the head mating surface rocker side is raised somewhat in the center bolt section, on the tail side, probably an impact damage.  Who knows for sure what happened but it can be well assumed the bike's been used and abused while ridden!  So it should!

[mrclubike]

Mine is doing the same thing It doesn't bother me at all  
I would use one of the Ultra Grey or Black Permatex sealants.
Like George already said just be careful not to use to much that you block oil passages or  get big strings of it that oozed out and get into the oil pan and clog things up.
The old Blue silicone sealants were prone to doing this
 The newer Ultra sealants are tougher and less likely to  separate from the seam if they ooze out  

[georgesgiralt]

Well, actually, these sealants are perfect for the job because they need the absence of air to cure. So if you put too much of them, the oil will carry them out and they won't cure in the passageway because there is some air into them.  *
The problem with putting too much of them is when dismantling. As the glued surface is somewhat large, you will need much force to break the joint. Limiting the amount as much as you  can gives a very weak joint and, so, easy to break.  The two guys recommending them have no problem dismantling engines they sealed that way.
I do not know for the Loctite products but the Permabond have an expiration date on the box, so watch out before buying a tube ... And do not store them before use. You may find they've gone bad when needing them.

* : Of course, you do not know what they would cause when mixing with oil. So it is better not  to have them mixed with oil ;-)

[Tony Smith]

Oh well, there's the sealant theory out the window, thanks Tony!  For sure, it comes of easy with a squirt of degreaser.  Would you know if it has any effect on compression?  I'm thinking probably not to me since it's down at the base of the cylinder...

I expect that eventually the cylinder base joint line will leak, it is in the nature of the type of engine, is harmless and easily dealt with. When I was assembling my engine I decided that the Permatex "form a Gasket" was probably not appropriate and was too lazy to go and buy an appropriate anaerobic sealant. Additionally I expected to be pulling the engine apart again fairly quickly (which never happened) so I wasn't fussed.

The sealant will have a minuscule effect on compression - easily ignored.

I have spent a lot of time building VW flat four engines and one of the things you keep on hand is different thickness base gaskets and even a few shims to ensure the compression height in the cylinders is correct. I constantly marvel at the way BMW was able to produce (or have produced for them) thousands and thousands of identical and interchangeable barrels - no two VW barrels were every identical.

[Kookaburra]

Like Tony, I reassembled with correctly prepared new O rings but without using base sealant and have had no oil blowing at all on a 1985 R65.  Clearly the precise fit varies between particular airheads.

[tunnelrider]

Ok thanks for all your replies, and interesting to know it's an oil leak.  Am thinking I'll do the top end rebuild with permabond as the base sealant, as I don't plan on doing it again for a while, hopefully...  I think with the whack this cylinder has probably had, it'll always leak unless I seal it, which is not a biggie anyway.  Cheers for all the info.

[montmil]

Unless there is some unusual damage effecting a complete and full surface seating, the only thing you may wish to use is a very thin smear of the easily obtained YamaBond.

On my R100S, some doofuss had gobbed on way too much "blue" sealer. I found bits of it inside the engine case. Don't be that guy.

[tunnelrider]

Ah, that guy!  Must be the same guy who dripped glue on the lower frame and didn't notice it until it was dry!  Will not be him!  I will just do the same as last time, a really thin streak spread by fingertip, continuous and well away from the top studs.  

[nhmaf]

Slight case imperfections, tiny nicks from a connecting rod drop, or small glob of hardened goo that wasn't totally removed from the cylinder base/engine case surface could cause this sort of oil weep, as well as a slightly cock-eyed o-ring.  

I just did the job on my 78 R100 (does not have any base o-rings)  and I used the modern standard sealant, Yamabond.   A paper-thin schmear on the engine case where the cylinder base mates with the engine case (I avoid getting too near the upper studs as the PO had goobed in some sealant there - fortunately not enough to block up the oil supply to the rockers, but I was picking sealant out from the o-ring wells there).   What seems to work for me is to go up to near the bottom of the upper 2 studs, and along the case in between them, but not to actually put any sealant around the outer edges of the studs.   I will also put a really thin paper-thin schmear around the base of the cylinder.  With a model that uses a cylinder base o-ring, I'd skip that part and just be sure to lube the o-ring with a bit of engine oil before putting it in its groove.  For the upper stud o-rings, I use a small dab of silicone grease to keep them in their wells during reassembly.  So far, perfectly clean, oil-free cases!