Bore, stroke, displacement and compression for all 9 GMC 403ci engines, compared side by side. Plus rod ratio and head specs where published.
| Displacement | 401–403 ci |
|---|---|
| Cylinders | 8 |
| Bore | 4.055–4.065" |
| Stroke | 3.858–3.898" |
| Compression | 10.8–16.8:1 |
| Advertised power | 250–470 hp |
| Years | 2001–2026 |
| Engine | Displacement | Bore × Stroke | CR | HP |
|---|---|---|---|---|
| LB7 | 403 ci | 4.0551" × 3.8976" | — | 300 |
| LLY | 403 ci | 4.0551" × 3.8976" | — | 310 |
| LBZ | 403 ci | 4.0551" × 3.8976" | 16.8:1 | 360 |
| LMM | 403 ci | 4.0551" × 3.8976" | — | 250–365 |
| LML | 403 ci | 4.0551" × 3.8976" | — | 397 |
| L5P | 403 ci | 4.0551" × 3.8976" | 16:1 | 445–470 |
| LLY / LMM | 403 ci | 4.0551" × 3.8976" | — | — |
| LGH | 403 ci | 4.0551" × 3.8976" | — | — |
| L8T | 401 ci | 4.065" × 3.858" | 10.8:1 | 401 |
Both bore and stroke vary across the family, so these are separate castings rather than variations on one block.
Compression spans 10.8:1 to 16.8:1, so fuel requirements differ meaningfully between members — worth checking before assuming parts interchange.
1 of 9 are oversquare — bore larger than stroke, which favours revs over low-end torque.
Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.