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Ford J / A specs: 240 ci (3.9L) 6-cylinder, 4.00 x 3.18 in (101.6 x 80.8 mm) bore and stroke, 9.2:1 compression. Rated 150 hp, 1966–1967.
The Ford J / A is oversquare — the 4" bore exceeds the 3.18" stroke by 25.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.18" stroke does not reach 4,000 ft/min of mean piston speed until about 7,550 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 2.137 is long by production standards — less side-load on the cylinder wall and gentler piston acceleration around TDC, the geometry that tolerates high RPM. Each of the 6 cylinders displaces 40.0 ci (655 cc).
9.2:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 5 engines in the Ford 240ci family, all of them 240 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 150 hp across 1966–1967.
| Displacement | 240 ci / 3.9 L |
|---|---|
| Bore | 4" |
| Stroke | 3.18" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 9.2:1 |
| Rod length | 6.795" |
| Rod ratio | 2.137 |
| Bore/stroke ratio | 1.258 |
| Fuel type | gas |
|---|---|
| Firing order | 1-5-3-6-2-4 |
| Advertised HP | 150 hp |
| Years | 1966–1967 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1966 | gross | 150 | — | 9.2:1 |
| 1967 | gross | 150 * | — | 9.2:1 * |
Factory specification sheet. * Not yet cross-verified.
| Bore/stroke character | 1.258 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,180 ft/min — conservative for a street engine |
| Rod ratio | 2.137 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 40.0 ci |
| Static compression | 9.2:1 — low by modern standards, generally happy on regular pump fuel |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1966 | 150 hp | J / A |
| 1967 | 150 hp | J / A |
4 factory vehicles, 1966–1967. Each year opens that model's engine table.