Hi Ian
Both of the 3 1/2s I have been associated with have Hillman Imp pistons fitted - with no issues. I know that I recommended these to Kevin, so I'm not sure what is going on. Maybe there are/were Hillman Imp pistons made with longer skirts? Hillman Imp engines were used a lot in sidecar racing outfits, so perhaps special lightweight pistons were made also, maybe this is what Kevin has ended up with? I have also always used 20/50 oil in the 3 1/2s, ie pretty thin oil.
Cheers,
Graeme