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GMC L31 specs: 350 ci (5.7L) 8-cylinder, 4.00 x 3.50 in (101.6 x 88.9 mm) bore and stroke. Rated 255 hp, 1996–1998. Used in the GMC Sierra 1500. OHV.
The GMC Oldsmobile V8 (L31) 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 43.8 ci (717 cc).
It is one of 7 engines in the Oldsmobile V8 family, sitting between the 307 ci and 425 ci members. 3 factory variants share these dimensions, rated at 255 hp across 1996–1998.
| Displacement | 350 ci / 5.7 L |
|---|---|
| Bore | 4" |
| Stroke | 3.5" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.143 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 255 hp |
| Years | 1996–1998 |
| 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 | 43.8 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 |
|---|---|---|
| 1996 | — | L31 |
| 1997 | 255 hp | L31 |
| 1998 | 255 hp | L31 |