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 ↗Factory 7.5:1 versus CAD 9:1 reference — read before ordering
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. The factory 1975 PE 94J tuning page, PDF p. 17 specifies 7.5:1 compression, while JP1017 is cataloged at 9:1. The factory 1500 parts plate, PDF p. 58 identifies BHM1066 as the standard 7.5:1 piston set for U.S. and Canadian cars through 1976, and BHM1066 20 as its .020-inch oversize catalog entry. These are parts-book numbers, not markings confirmed inside this engine. JP1017 supplies an outside-dimension study, not evidence that its crown or compression matches the original 1975 engine or the parts now fitted. The Moss internal-engine catalog, PDF p. 33 lists separate 7.5:1 and 9:1 replacement piston sets with year and market restrictions, further confirming that displacement alone does not select a set. The CAD shows a plain provisional crown and cannot predict compression, valve clearance or piston-to-head clearance. Installed bore grade, oversize, crown form, pin retainers, ring gaps and groove positions remain unknown. The model’s deck clearance also depends on a provisional block height.
Compare the piston heights
A listed 7.5:1 replacement piston places its nominal crown 2.55 mm lower relative to the pin than the 9:1 piston used in the CAD. At a 73.7 mm bore, that height difference represents about 10.9 cc of cylinder space. It does not establish the compression ratio or identify the piston in this car.
CAD status: the public piston still uses the JP1017 envelope. Open the separate 19708 pin-height comparison in 3D ↗ to see how its catalog height changes crown position around the same wrist-pin center. This is an alternate assembly, not a drawing of BHM1066 or proof of what is fitted. Its crown surface and most piston details still use provisional JP1017-based geometry. At teardown, record markings, overall height, pin-to-crown height and the crown surface. The CAD piston inspector accepts the two direct dimensions for each cylinder and can generate a separate owner model from the exported measurement log. That changes the measured envelope only; the crown surface and compression remain unresolved.
Sources and fitment limits
Classic Engine Parts lists 19708 at 30.1498 mm pin-to-crown and 58.0898 mm overall, with an explicit 1973 U.S. Spitfire 1500 7.5:1 application and a broad MG Midget 1500 alternative listing. Deves' ring chart names AE 19708 and 20139V1 for 1975–80 Midget 1500 engines; it does not identify which belongs to this car. The JP1017 drawing gives about 32.7 mm pin-to-crown and 58.1 mm overall at a cataloged 9:1. The 10.9 cc comparison holds only if the pin center and bore are shared; head, gasket, deck and crown profiles also affect clearance volume. None of these listings cross-references the factory BHM1066 set to a dimensioned crown drawing.
Check a measured compression ratio
Use this after measuring one cylinder during a rebuild. The factory 73.7 mm bore and 87.5 mm stroke give about 373.3 cc swept volume per cylinder. At that bore and stroke, 7.5:1 requires about 57.4 cc total clearance volume; 9:1 requires about 46.7 cc. Those totals include the chamber, gasket, deck and piston crown together. They do not specify the dish in BHM1066.
Calculation: swept volume ÷ total clearance volume + 1. Total clearance = measured chamber + compressed gasket space + deck space + net crown volume. This simplified static calculation excludes crevice volumes and measurement error; it cannot establish the fitted engine’s ratio from catalog parts alone.
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. Identify the fitted crown and chamber, then confirm the intended compression ratio before considering a piston cataloged at 9:1. 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.