See other engines in this family
GMC 6-71 specs: 426 ci (6.98L) 6-cylinder, 4.25 x 5.00 in (108 x 127 mm) bore and stroke, 18.7:1 compression. Rated 238 hp. OHV. Rod ratio and piston speed.
The GMC 6-71 is undersquare — the 5" stroke exceeds the 4.25" bore by 15.0% — a layout that trades peak RPM for leverage and torque low in the range. With a 5" stroke, mean piston speed reaches 4,000 ft/min at only 4,800 RPM — the practical ceiling for a production bottom end — which is why a stroke this long makes its power low and runs out of safe revs early. Each of the 6 cylinders displaces 71.0 ci (1,163 cc).
At 18.7:1 this is a compression-ignition engine: the ratio is the heat source that lights the fuel, not a knock limit to be managed.
It is one of 2 engines in the Detroit Diesel I6 (Two-Stroke) family, the largest at 426 ci. Factory fitments include the GMC PD-4501 Scenicruiser, M4 Sherman (some variants), Various industrial, marine, and bus applications.
| Displacement | 426 ci / 6.98 L |
|---|---|
| Bore | 4.25" |
| Stroke | 5" |
| Cylinders | 6 |
| Compression ratio | 18.7:1 |
| Bore/stroke ratio | 0.85 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | diesel |
| Advertised HP | 238 hp |
| Bore/stroke character | 0.850 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 5,000 ft/min — above the 4,500 ft/min mark where most production bottom ends are considered at their limit |
| Displacement per cylinder | 71.0 ci |
| Static compression | 18.7:1 — high enough that pump premium or better is normally required |
Each link opens the calculator with this engine's specs already filled in.