Is this a R90S piston or not?

[StephenB]

Need the assembled knowledge base to figure this one out: I have a lead on supposedly R90S pistons. The same seller sells R90S cylinders but ... they aren't black. Upon my questioning he replies that everything cam be returned if I am not happy. The heads he's selling do have the longer spigots for the Mikunis, but he bought it all from another guy years ago ... genuine R90S, he claims of course. Never mind whether he is trying to fool me or himself, he gave me the numbers embossed on the inside of one piston:

090602 which I believe is a cast number
"+2" underneath the cast (?) number
275 which I believe is month and year of manufacturer,

I can't make heads or tales whether it is a R90S piston or not. And I really wouldn't care if he wasn't demanding "rare" prices!

Can anybody solve this riddle?

Maybe you can tell from the picture, cause I ain't never seen either a 90/6 or a 90S piston.

[mattcfish]

I can't tell from the picture. The dome looks /6 to me from that angle.
I'm curious about the base of the cylinders. Do they have a foot that extends into the block? From that angle it looks like they are missing, like on a 1070 kit from Siebenrock and others. If there is a "foot" make sure it is the diameter you are looking for (97mm vs. 99mm).

[Jean]

I have an old catalog...an R90s piston is ID by numbers that are of the series 11 25 1 259 8xx, the /6 is 11 25 1 259 2xx.
Likewise the cylinders are ...ummmmm.....not listed.
Those numbers SHOULD appear someplace on the pistons, tho. The last 2 digits after the "8" (or the "2") tell you the oversize.

[Chuey]

Most of what I know, you probably already know and all I know is stuff I learned when Ted Porter led me through his rebuild of the parts for my R90S. The cylinders are worth quite a bit of money even if they need to be bored. The pistons are very expensive. I have some original R90S pistons that he said were fine but my cylinders had already been bored to first over so they wouldn't work for me. I had to buy pistons and they cost me about $700.00 or so. I've seen a cheaper source since then but that was still $500.00 or so.

The 97 vs 99 spigot size issue made me have to change gears in a big way. I ended up going with the newer, bigger size stuff so I had to buy some 99mm spigot size cylinders.

There is no difference between the cylinders as far as standard or "S" goes other than the black color which you would probably want to repaint anyway.

Chuey