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GMC 261 specs: 261 ci (4.28L) 6-cylinder, 3.875 x 3.688 in (98.4 x 93.7 mm) bore and stroke, 8:1 compression. Rated 140 hp. OHV. Rod ratio and piston speed.
The GMC Chevy Straight-6 (235/261) (261) is oversquare — the 3.8752" bore exceeds the 3.6882" stroke by 5.1% — a layout that favors airflow and RPM over leverage at the crank. The 3.6882" stroke puts mean piston speed at 4,000 ft/min around 6,500 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 43.5 ci (713 cc).
8:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 3 engines in the Chevy Straight-6 (235/261) family, the largest at 261 ci. Factory fitments include the Chevrolet Task Force trucks (heavy duty), GMC trucks (heavy duty), Chevrolet Bel Air (export/heavy duty).
| Displacement | 261 ci / 4.28 L |
|---|---|
| Bore | 3.8752" |
| Stroke | 3.6882" |
| Cylinders | 6 |
| Compression ratio | 8:1 |
| Bore/stroke ratio | 1.051 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 140 hp |
| Bore/stroke character | 1.051 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,688 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 43.5 ci |
| Static compression | 8:1 — low by modern standards, generally happy on regular pump fuel |
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