Eddie Shermer rebuilt my engine last year (or was it the year before?) with pistons obtained from the SOC Shop. I imagine that they had to be machined, but if you are an engineer, that should pose no problems.
I suggest that you have a chat with Eddie (his telephone number is inside the front cover of Yowl).
Failing that, I may have a set of 600cc long stroke TT Rep pistons,
Chris.
