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Mercury H-code specs: 351 ci (5.8L) 8-cylinder, 4.00 x 3.50 in (101.6 x 88.9 mm) bore and stroke, 9.5:1 compression. Rated 250 hp, 1969–1971.
The Mercury H-code is oversquare — the 4" bore exceeds the 3.5" stroke by 14.3% — 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 8 cylinders displaces 43.9 ci (719 cc).
9.5:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 7 engines in the 335 Series family, all of them 351 ci — the differences between them are in compression, heads and rating, not displacement. 3 factory variants share these dimensions, rated at 250 hp across 1969–1971.
| Displacement | 351 ci / 5.8 L |
|---|---|
| Bore | 4" |
| Stroke | 3.5" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9.5:1 |
| Bore/stroke ratio | 1.143 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 250 hp |
| Years | 1969–1971 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1969 | gross | 250 @ 4600 * | 355 lb-ft @ 2600 * | 9.5:1 * |
| 1970 | gross | 250 @ 4600 | 355 lb-ft @ 2600 | 9.5:1 |
Factory specification sheet. * Not yet cross-verified.
| Bore/stroke character | 1.143 — 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 | 43.9 ci |
| Static compression | 9.5:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| 1969 | 250 hp | H-code |
| 1970 | 250 hp | H-code |
| 1971 | — | H-code |
1 factory vehicle, 1969–1971. Each year opens that model's engine table.