Dan,
As far as interchangeability of (A/B/C) C113 overbore kits and the C123 (x30/x40 series) standard kits; I have heard that the factory ones will not inter-change, but the after market ones will. Never took the time to verify this myself, but the piston differences that Justin is referring to would make that believable as the firecrators have a much more pronounced dome/step height. I put the 3-1/4" stepped head pistons in the last 3 motors I built, and it"s been awhile since I"ve had my hands on a 3-1/8" aftermarket to compare. This is total guesswork, but I would bet that the problem would be trying to use a firecrator designed for a Farmall C in a x40 motor as they (later motors) probably have a smaller combustion chamber. I know they have larger valves and a diff manifold.The last one I built was a 240U block I rebuilt and put on a 140. I didn"t go with the 3-1/8" pistons because I wanted to punch it out to a C133. So we opened up the O-ring seal area and ordered the 3-1/4" kit (Super C, stepped head kit), then I custom sized thinner O-rings. When I compared the Tisco pistons to the old firecrators (by connecting them together on the same pin), the OEM"s were taller to the top of the piston and taller on the dome. The comparison isn"t apples to apples though since the OEM has a dish beside the dome and an odd shapped profile. More like the side view of a telephone handset, as opposed to a rectange as on the aftermarkets. And remember there are different firecrators; the ones I pulled out of a 230 motor (374339R1) did not have nearly as tall of a dome as the 240 firecrators (374367R1). If you can afford the high altitude firecrators that would probably be the best way to go, if not, try the stepped head aftermarkets. Worst case is you"ll be down a little on compression and you might need to bevel the step a little with your dremel tool; but I doubt it based on my experience. Might also note there are 3 compression rings on the aftermarkets instead of two on the firecrators. Bottom line is that I got 170psi out of it, and it runs great with alot more power than the 140 can hold down. Hope that helps,
Stan(VA).
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