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A friend of mine has begun the process of restoring his 19975 R90S (built 10/75). He was advised that both cylinders required a .20 over re-bore, and he acquired the appropriate pistons along with other required parts in a single purchase several years ago prior to any machine work. One cylinder cleaned up nicely, the other did not. He now has a .20 over piston looking for a cylinder. He has two potential donor sets of cylinders available, one from an R60/6 and one from an R90/6.

The question is: can a cylinder from either set be re-bored to .20 over the R90S spec and be used on the R90S?

He would prefer to use an R60 cylinder, since it comes from the remains of a parts bike. The R90 set is from a bike that is a fairly minimal project that he would prefer to keep intact.

Any advice or observations would be greatly appreciated.

Hank
’72 R60/5
’95 K75S

IIRC, I don't think the 600cc cylinder has enough meat to go that large. You can take a 750cc to 900 or so.

Kurt in S.A.

Like Kurt said the 60 won't work.
I had a similar problem not long ago and looked till I found a pair of standard cylinders that would clean up 1st over.
From what I understand, R75 cylinders will bore to 90 std, but 2nd over won't leave much beef.
The thing that will make them harder to find is those particular cylinders were only used two years, 74 & 75. In 76 the block started its transformation to 980cc and used bigger cylinder spigots.

Before he buys any cylinders have him measure the spigot size of his engine. It may already be 99mm which would make finding R90 cylinders quite a chalange.

Garnet wrote:Before he buys any cylinders have him measure the spigot size of his engine. It may already be 99mm which would make finding R90 cylinders quite a chalange.

Good point, 10/75 is a late date.
Ken Whitehawk had some 76 cylinders last week... don't know if he sold them.

I have two barrels from my R90S project that are second over. They were done by Ted Porter. I ended up switching to the 99mm spigots. The ones I have are for the smaller spigots. If those would work, I'd sell mine.

Chuey