The piston is more
than a cylinder.
The engine CAD now separates four hollow piston bodies, their wrist pins, eight compression rings and four oil-control rings. Select a part to see its role and the dimensions behind its shape.
Explore piston 1 ↗JP Pistons’ JP1017 drawing, PDF p. 162 lists Midget Mk III 1493 cc applications and provides the piston height, pin location and ring widths. The factory specification, PDF p. 11 gives Grade F/G bore and skirt sizes plus top and bottom clearance ranges. The CAD uses Grade G midpoint values as one example. It does not prove JP1017 pistons or a Grade G bore are in this car. Crown dish, actual compression ratio, installed size, pin retainers, ring gaps and groove positions remain unknown. The model’s deck clearance also depends on a provisional block height.
Explore the assembly
Piston shell
Open underside, pin bosses and three grooved seats replace the former solid cylinder. The manufacturer’s height and pin location set the overall scale; internal wall and boss shapes are provisional.
Select piston ↗Compression rings
Two separate bands per piston use the 1/16-inch width in the replacement drawing. Split positions are visual cues, not installed gap measurements.
Select compression rings ↗Oil-control rings
One 4 mm band is shown per piston. The CAD does not resolve the real ring’s expander or construction.
Select oil rings ↗Wrist pins and bushes
The factory pin diameter scales the separate pins. The rod small-end bushes remain separate too, but their fits and the pin retention method need direct inspection.
Select pins ↗If the engine is opened for a real job
- Identify before ordering. Record the engine number, piston markings, bore oversize marks, crown form and any part number. Photograph each cylinder and keep parts indexed by cylinder.
- Measure the interfaces. Use the Midget factory method to measure bores, piston-to-wall clearance, ring grooves, ring end gaps, pin and bush fit, and piston position at top dead center. The CAD’s provisional coordinates cannot substitute for these measurements.
- Document disassembly. Before removing any cap or pin, record orientation marks and how the fitted pin is retained. Preserve ring order and which face points up.
- Rebuild from identified parts. Match replacement piston, rings and bearings to the actual measured engine and its workshop procedure. The virtual explosion shows relationships; it is not an installation sequence or fitment gauge.