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A-series engines don't have splines.

The 63A 5-speed for A-series engine uses the same input shaft diameter and splines as 63A 5-speed for L-series engine. So you need the same clutch disk as fitted to the L14 engine. It is 180 mm diameter and so takes a standard A14/A15 clutch pressure plate.

Or you can redrill the A12 flywheel to fit the more common L16 clutch disc and pressure plate (200 mm diameter)