Metal base gasket on R100GS

[RandyB]

Pulled the jugs, etc. on my 89 R100GS to do some work. Found metal base gaskets and white orings. IIRC, there were no metal gaskets on this bike from the factory.

A bit of history: The PPO allegedly built this bike to ride to Tierra del Fuego. So far, every mod I have found is well executed.

Could he have installed those gaskets to lower compression for poor gas? They aren't that thick. Should they be on there? There was no sealant on the gaskets or cylinders and they didn't leak.

:lurk

[lostboy]

He probably did install the base gaskets in anticipation of poor octane. You are correct that they are not original. I would remove them-unless you discover that he was running high compression pistons

[RandyB]

Here's what I found:

Metal gasket .74mm thick. No ridge in the cylinder.
DOMED (I assume hi comp) pistons. Everything looks great so far.

Now....

I have a set of balanced conrods and pistons for my 84 R100RT in a box. What would be the pluses and minuses of swapping them into the GS and putting the hi comp into the RT later? The reason I ask is there is a 50-50 chance I will be taking the GS overseas to live (Wish me luck. More to follow.).

Pluses:
Lower comp in GS can use any gas.
Higher comp in RT might be fun.

Minuses:
Won't work.
Break in issues (may be mitigated by switching conrod bearings as well).
Change riding characteristics of both for the worse.

They're both getting new rings. GS parts go out to be balanced this week.


:lurk

[GeorgeRyals]

A dome on the piston does not necessarily mean it is a high compression piston. If you could compare it to a known OE piston, then you would know for sure. It is so easy to R&R cylinder and heads, put it back together without the shims and see what happens.

BTW What work were you doing that made you pull the heads. Hot-rodding??

[RandyB]

A dome on the piston does not necessarily mean it is a high compression piston. If you could compare it to a known OE piston, then you would know for sure. It is so easy to R&R cylinder and heads, put it back together without the shims and see what happens.

BTW What work were you doing that made you pull the heads. Hot-rodding??

Higher comp may have been a better term.

Held next to my RT piston there is a pronounced dome. The Rt is not quite flat, being more truncated from the piston body up to a flat top. I don't have a good handle on squish band but these shapes may have something to do with it.

I'm removing the shim.

It needed new pushord tubes and one cylinder tested low comp. I measured the rings that were in and they were worn. New rings on the bench.

[mmmalmberg]

If you don't know what the domed pistons are, turn the engine by hand a few times before firing it up without the base shims, to make sure there's no interference anywhere...

[RandyB]

Once I get them cleaned up I'll know if they're OEM, I hope. Thaks for reminding me about hand spinning. I always check first.

[seniorasi]

Here's what I found:

Metal gasket .74mm thick. No ridge in the cylinder.
DOMED (I assume hi comp) pistons. Everything looks great so far.

Now....

I have a set of balanced conrods and pistons for my 84 R100RT in a box. What would be the pluses and minuses of swapping them into the GS and putting the hi comp into the RT later? The reason I ask is there is a 50-50 chance I will be taking the GS overseas to live (Wish me luck. More to follow.).

Pluses:
Lower comp in GS can use any gas.
Higher comp in RT might be fun.

Minuses:
Won't work.
Break in issues (may be mitigated by switching conrod bearings as well).
Change riding characteristics of both for the worse.

They're both getting new rings. GS parts go out to be balanced this week.


:lurk

If you are moving the GS to Europe I'd be tempted to leave the HC pistons where they are. Your GS will love the 95 octane rating of European gas.

[seniorasi]

Once I get them cleaned up I'll know if they're OEM, I hope. Thaks for reminding me about hand spinning. I always check first.

I'm hoping you'll reconsider before removing the shims and running the bike. First check the part numbers on the pistons and rods to be certain it is not a stroker motor. If he went to the trouble of putting domed pistons in the bike there is more than likely a good reason for the shims. Not trying to tell you what to do, just a humble suggestion.

[RandyB]

Thanks for the advice. I checked and all parts are BMW. One more torquing and valve adjustment and the break-in controversy can start.

Edit: Nothing blew up, flew out or fell off on a 70 some mile break in ride. Ran it up to 4K for 40 seconds or so then got on it and rode it like I stole it. Time will tell.