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GMC L8T specs: 400.6 ci (6.6L) 8-cylinder, 4.065 x 3.858 in (103.3 x 98 mm) bore and stroke. Built 2021–2026. Used in the Chevrolet Silverado HD. OHV.
The GMC 401ci (L8T) is oversquare — the 4.065" bore exceeds the 3.8583" stroke by 5.4% — a layout that favors airflow and RPM over leverage at the crank. The 3.8583" stroke puts mean piston speed at 4,000 ft/min around 6,200 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 50.1 ci (821 cc).
6 factory variants share these dimensions across 2021–2026.
| Displacement | 400.6 ci / 6.6 L |
|---|---|
| Bore | 4.065" |
| Stroke | 3.8583" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.054 |
| Block material | cast iron |
|---|---|
| Head material | aluminum |
| Valvetrain | OHV |
| Fuel type | gas |
| Years | 2021–2026 |
| Bore/stroke character | 1.054 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,858 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.1 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 |
|---|---|---|
| 2021 | — | L8T |
| 2022 | — | L8T |
| 2023 | — | L8T |
| 2024 | — | L8T |
| 2025 | — | L8T |
| 2026 | — | L8T |
4 factory vehicles, 2020–2026. Each year opens that model's engine table.