Big Valves

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Stock valves for A12 engine are 35 mm intake, 29 mm exhaust. You can fit the A14-size 37mm inlet/30mm exhaust. And valves up to 40mm have been fitted for ultra high-RPM use.

Overview

The stock A12 valves (35/29) are good for small-bore (1200cc) use below for up to 6000 RPM. For larger bore engines (or higher RPM) the 37/30 valves from an A14 are better.

For racing at high RPMs, more power can be made with larger valves, but care must be taken for proper valve flow.

The 40s may require relief/flow notch in the side of the cylinder wall?

Feral

Feral used 40mm steel Inlet valves, tuliped and swirl polished, with 35mm Exhausts steel polished.

42mm Valves

42 mm intakes are perhaps the limit for A-series cylinder heads.

Dual Valve Springs

Large valves work best at high RPM. When revving over 6000 RPM, dual valve springs are called for. The stock A12GX springs are good for 6600 RPM.

Racing/aftermarket dual springs are usually good for 9000 RPM and shouldn't be used in a street engine as they cause accelerated wear on the valve guides.

Triple Valve Springs

Photo Index

Big Valve

Valve Spring