Stock valve stems are 3/8th and I've already reduced them to 11/32 (bronze guide inserts). During that decision process I looked at 5/16, 8mm and 7mm (maybe even 5mm) options. I consulted with two different head porters and both stated that the reduction from 11/32 to 7mm (or was it 5mm?) might yield 5cfm and it was more about reducing valvetrain weight than increasing air flow. I feel confident I'll be able to maintain valvetrain stability with 11/32 stems to what ever rpm the ports choke at so didn't see a need to go to a smaller diameter stem. It requires custom valves no matter what (longer stem above the retainer notch) and it appeared the cost of custom smaller stem valves and retainers was higher than custom 3/8 or 11/32 stem valves and standard retainers.
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Rebuilding an OHC Pontiac 250 with EFI and a Turbo