Irvine 39 is an awesome engine, but a little finicky to tune until you are used to the needles...

Once set you will be rewarded with TT39 performance and OS32 reliability....

I would buy another Irvine 36/39 in a jiffy, already owned 3, and set one up for a friend at weekend... was a missile

OS32 is a plain piston, Irvine is ABC (ally piston Brass chromed liner) neither needs a ring, but ABC's are trickier to tune sometimes... more powerful tho...