Base Gasket Surface Woes

[Guest]

I recently got these cylinders for my '78 GS750. They are bored to accommodate oversize pistons, so I am hoping to rescue these and not just 'look for another'. Got to cleaning them up last night; Base gasket came off really easy, head surface looks good....but i uncovered a PO's horrendous deeds underneath. Many of the scratches are deep enough to feel with a fingernail, but nothing sticks up above the surface. The whole mating surface is boogered up like this.
I want this to get this right the first time and not have to tear it apart again. Will a machine shop be able to skim this, with the cylinders in the way?? (I know I have to think about adding additional base gasket thickness if they take much off).

[Nessism]

To skim furface that the liners would have to be pressed out of the cylinder first. I wouldn't do it. I don't think you have much risk of leaks because the oil pressure goes up through the outer stud holes and are sealed by O-rings. I'd coat the base gasket with sealer and roll the dice.

[Guest]

To skim furface that the liners would have to be pressed out of the cylinder first. I wouldn't do it. I don't think you have much risk of leaks because the oil pressure goes up through the outer stud holes and are sealed by O-rings. I'd coat the base gasket with sealer and roll the dice.

I'd agree with what Ed said, they're not that deep. Where did you get the it from?

[posplayr]

copper coat the base gasket.

https://www.amazon.com/CRC-401612-Copper-Gasket-Compound/dp/B000M8NZ8E

[Guest]

copper coat the base gasket.

https://www.amazon.com/CRC-401612-Co.../dp/B000M8NZ8E

I've never heard of such a thing, interesting product.

[Guest]

Thanks guys. It was an ebay purchase, so always a bit of a gamble! The cylinders themselves look great, measure round, and just need a light hone. The pistons that came with it are unknown make, ~69.5mm, high compression, and in good shape. They are in this thread: http://www.thegsresources.com/_forum...ylinder-advice.
If I can dig up some rings for those pistons I will probably go with this set up. Will probably end up having the pistons machined to lower compression ratio a little. There are a lot of unknowns, and it will take a little work and figuring.
I also have the 850 cylinders and pistons I had been planning on swapping in, those would be a straight over swap. So right now I'm trying to decide which one to go with...

[posplayr]

I've never heard of such a thing, interesting product.

You can pay twice as much from a copy-cat product from Permatex.

https://www.amazon.com/dp/B00W98KR9A?psc=1

[Guest]

yep. not a whole lotta pressure down there anyway.

[Brendan W]

Mine was like that two years back. Sprayed the base gasket with hi temp aluminium paint. No problems so far.

[Guest]

Thanks for the input everyone, I feel more confident about using these jugs now!

[Dogma]

I dug a gouge into my 1000G, deeper than those, right where the O-ring meets the cylinders (of course, the ONE slip). I filled it with 1207B silicone, leveled it with a gloved finger and let it cure before reassembling. I have well over 10,000 miles on it since then, with no sign of a leak. Those scratches can be sealed, and they only have to hold whatever pressure your crankcase vent sees, plus some transient pulses from the pistons shoving air around. If the vent is routed to an airbox like stock, it'll be vacuum in the crankcase.

Even with scratches where the O-ring seals, these roller-crank GS engines only have to hold something like 8 psi of oil pressure. The way those O-rings fail is if they're made of the wrong material, absorb some oil, swell, and pop out of the O-ring seat. We know that OEM O-rings do not do this. We do not know of a non-OEM O-ring that does not. I know first hand, the Vesrah O-rings fail this way (on my 850). It's worth a few bucks to get the OEM O-rings, if your gasket kit isn't OEM.

[bobgroger]

I've never heard of such a thing, interesting product.

Really sticky and gooey. Used to use it with steel head gaskets in the 70's. Came in a container with a round brush as well. Think I still have a can....

[gs11ezrydr]

There is s company that is called Copper gaskets unlimited ,send them your old base gasket. tell them how thick you want it and they will cut it out. Thicker if you want to drop the compression like if you have the head surfaced or thinner if you want to raise the compression a little. Copper coat both sides and you should not have a problem. This cylinder block had some pretty deep scratches also. I used them one time when someone had cut the head so much that the pistons were sticking out of the bores, and it rattled when running where the pistons were just touching the head. I had them cut the base gasket to give me -.010 below the head surface and ran perfectly. Used them on race bikes a lot. Just a thought.