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Mercury 255 specs: 255 ci (4.2L) 8-cylinder, 3.681 x 3.00 in (93.5 x 76.2 mm) bore and stroke, 8.2:1 compression. Rated 115-120 hp. OHV.
The Mercury Windsor (255) 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).
8.2:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
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 115 to 120 hp. Factory fitments include the Mercury Grand Marquis, Mercury Cougar.
| Displacement | 255 ci / 4.2 L |
|---|---|
| Bore | 3.6811" |
| Stroke | 3" |
| Cylinders | 8 |
| Compression ratio | 8.2:1 |
| Bore/stroke ratio | 1.227 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 115–120 hp |
| 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 |
| Static compression | 8.2:1 — 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 |
|---|---|---|
| — | 115 hp | 255 |
| — | 120 hp | 255 |