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Mercury 255 specs: 255 ci (4.2L) 6-cylinder, 3.953 x 3.50 in (100.4 x 88.9 mm) bore and stroke, 9:1 compression. Rated 145 hp. Rod ratio and piston speed.
The Mercury Cologne (255) is oversquare — the 3.9528" bore exceeds the 3.5" stroke by 12.9% — a layout that favors airflow and RPM over leverage at the crank. The 3.5" stroke puts mean piston speed at 4,000 ft/min around 6,850 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 42.5 ci (696 cc).
9:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 10 engines in the Cologne family, sitting between the 170 ci and 256 ci members. Factory fitments include the Mercury Mountaineer.
| Displacement | 255 ci / 4.2 L |
|---|---|
| Bore | 3.9528" |
| Stroke | 3.5" |
| Cylinders | 6 |
| Compression ratio | 9:1 |
| Bore/stroke ratio | 1.129 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 145 hp |
| Bore/stroke character | 1.129 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,500 ft/min — conservative for a street engine |
| Displacement per cylinder | 42.5 ci |
| Static compression | 9:1 — low by modern standards, generally happy on regular pump fuel |
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