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I've got a pair of black 900cc cylinders in my parts pile with the number 731-1-1182 cast into them, and "B", 30/37 and 9 stamped on them. I can't find the part number anywhere on the fiche, and original R90S cylinders are apparently NLA. Can anyone tell me if they are original R90S cylinders? Also, are R90S pistons different from R90/6 pistons? Any way to identify them if so?

The R90s pistons are different from the R90/6 but I think you need to have them side by side to tell. You can't really tell in the Motobins pics. Maybe Mattcfish has will post some good pics for you.

The S pistons give a half point of higher compression or something like that. I have a pair of pistons that are good and I'm not going to use them. I had to go for second oversize. I think I also have a fresh set of second oversize smaller spigot cylinders (done by The Beemer Shop) and never installed.

Chuey

Chuey wrote:The S pistons give a half point of higher compression or something like that. I have a pair of pistons that are good and I'm not going to use them. I had to go for second oversize. I think I also have a fresh set of second oversize smaller spigot cylinders (done by The Beemer Shop) and never installed.

Chuey

Tim, don't listen to him, he's just teasing you!

Garnet wrote:The R90s pistons are different from the R90/6 but I think you need to have them side by side to tell. You can't really tell in the Motobins pics. Maybe Mattcfish has will post some good pics for you.

R90/6 look like this. Sorry don't have a picture of the S pistons, I sold them to a friend and didn't need a pic.
The domes of the S pistons are more pronounced than these. The valve pocket are also more defined.
[img=http://img834.imageshack.us/img834/3294/1000197z.th.jpg]
[img=http://img408.imageshack.us/img408/2205 ... 8km.th.jpg]

Ah, cool... compared to this image, my pistons do have a more pronounced dome and larger valve pockets. Thanks! Any ideas on the cylinders?

Tim, what I learned when The Beemer Shop was working on my parts was that the cylinders are the same as regular R90/6 parts. They are not available any longer. What that means is that if you have to do a clean up bore job and go over one or two sizes, you then need to buy the oversize pistons. So, if your cylinders are in good condition, you are, as they say, golden.

Chuey

Thanks Chuey, there is some discoloration and a very slight rough patch on the bottom of one of the bores. I'm going to ball hone it and see if that does the trick.

Tim Shepherd wrote:Thanks Chuey, there is some discoloration and a very slight rough patch on the bottom of one of the bores. I'm going to ball hone it and see if that does the trick.

I don't know this, so am asking it as a question. Is the rough area where the rings go or is it in the area where only the skirts go? If it's just the skirts, I'd think it could be cleaned up and them maybe have the skirts coated with that stuff they use for friction reduction.

Chuey

Chuey wrote:
I don't know this, so am asking it as a question. Is the rough area where the rings go or is it in the area where only the skirts go? If it's just the skirts, I'd think it could be cleaned up and them maybe have the skirts coated with that stuff they use for friction reduction.

Chuey

Oil?