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Mercury 250 specs: 250 ci (4.1L) 6-cylinder, 3.681 x 3.909 in (93.5 x 99.3 mm) bore and stroke, 9:1 compression. Rated 155 hp. Rod ratio and piston speed.
The Mercury Thriftpower (250) is undersquare — the 3.9094" stroke exceeds the 3.6811" bore by 5.8% — a layout that trades peak RPM for leverage and torque low in the range. The 3.9094" stroke puts mean piston speed at 4,000 ft/min around 6,150 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 41.7 ci (683 cc).
9:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 5 engines in the Thriftpower family, the largest at 250 ci. Factory fitments include the Mercury Comet, Mercury Montego, Mercury Monarch.
| Displacement | 250 ci / 4.1 L |
|---|---|
| Bore | 3.6811" |
| Stroke | 3.9094" |
| Cylinders | 6 |
| Compression ratio | 9:1 |
| Bore/stroke ratio | 0.942 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 155 hp |
| Bore/stroke character | 0.942 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,909 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 41.7 ci |
| Static compression | 9:1 — low by modern standards, generally happy on regular pump fuel |
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