This piston assembly set of 4 follows the Austin Healey 100M high-compression specification. Supplied in +040 oversize, they maintain an approximate 8.6:1 compression ratio and suit engines rebored by 0.040 inch.
Correct for engines rebored by 0.040 inch:
Exclusions: Not suitable for early BN1 3-bearing engines prior to chassis 221536, or for standard low-compression 100/4 engines unless converted to 100M specification.
Professional fitting is recommended. Confirm final bore size, set ring gaps, and verify piston-to-bore clearance. Use a piston ring compressor tool (TOL111) to protect rings and speed assembly.
Specifically designed for 100M engines, these +040 pistons provide the correct compression and crown geometry after a 0.040 inch rebore, supporting reliable sealing and performance.
Yes. These are for blocks rebored by 0.040 inch. If your machining is 0.020 or 0.030 inch, use the corresponding oversize set.
The piston diameter is 0.040 inch larger than stock to suit cylinders bored by the same amount.
Yes, each piston includes its ring set, plus gudgeon pin and circlips.
8.6:1 HIGH COMPRESSION
