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Mercury Windsor 4.2 specs: 255 ci (4.2L) 8-cylinder, 3.681 x 3.00 in (93.5 x 76.2 mm) bore and stroke. Rated 120-122 hp, 1981–1982.
The Mercury Windsor 4.2 is oversquare — the 3.6811" bore exceeds the 3" stroke by 22.7% — a layout that favors airflow and RPM over leverage at the crank. The 3" stroke does not reach 4,000 ft/min of mean piston speed until about 8,000 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 31.9 ci (522 cc).
It is one of 24 engines in the Windsor family, sitting between the 221 ci and 351 ci members. 2 factory variants share these dimensions, rated from 120 to 122 hp across 1981–1982.
| Displacement | 255 ci / 4.2 L |
|---|---|
| Bore | 3.6811" |
| Stroke | 3" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.227 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 120–122 hp |
| Years | 1981–1982 |
| Bore/stroke character | 1.227 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,000 ft/min — conservative for a street engine |
| Displacement per cylinder | 31.9 ci |
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 |
|---|---|---|
| 1981 | 120 hp | Windsor 4.2 |
| 1982 | 122 hp | Windsor 4.2 |