Done a little bit of work for my S90 in between all the other projects.
The plan is to build another engine, bored and stroked to 122cc by using ATC110 parts, cylinder, crankshaft and head. Only the ATC110 head has separate valve seats which are pretty thin, which means there is not enough material in them to install a bigger inlet valve.
A while ago I recieved a head from someone, probably C90, that had a full cast in combustion chamber, much easier to put a larger valve in there, but this head didn't fit the ATC110 cilinder.
I had to machine the space for the top of the cilinder, and also the studbolt hole where the oil comes up, as the ATC110 has a large rubber collar there and the C90 head didn't have a space for it.
ATC head on the left, C90 on the right.
the valve I plan on using compared to the original S90 inlet valve
when made to fit the head, this motor will breathe a little easier.