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421 Pistons
Pontiac V8 421 Pistons
Big Journal, Small Bore — 421 Pistons for Pontiac V8s
421 Pistons are their own problem. The 421 uses the big 3.25" main journal block but a smaller standard bore than a 428 or 455, so a 4.150" piston is already .057" over on a 421. That single number changes which sets fit, how much cylinder wall you have left and what compression ratio you can chase.
Butler covers the 421 almost entirely with Ross Racing Pistons built on the Pontiac forging Ross developed for Butler, including Quick Ship flat tops and CNC milled dish tops, plus fully custom 421 pistons for 4.000", 4.210" and 4.250" strokes.
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More About 421 Pistons
1. Know how far over you really are
The 421 catches people out because it shares a block family with the 428 and 455 but not their bore.
| 4.150" bore | Butler lists this as .057" over on a 421. On a 400 the same number is a .030" overbore. Sonic test the block before you commit to it. |
| 4.200" bore | The upper end Butler shows on 421 piston data, and only worth chasing on a block that measures out with enough wall thickness to support it. |
| Custom bores | Ross will build a 421 piston to whatever bore the block finishes at, which is the usual answer when a 421 has already been cut once. |
2. Stroke and compression height on a 421
Butler lists three custom Ross 421 combinations, and each one changes the compression height you need.
| 4.000" stroke | 1.595" compression height using a 6.625" rod, on a 1/16, 1/16, 3/16 ring pack. |
| 4.210" stroke | 1.495" compression height. |
| 4.250" stroke | 1.295" compression height, which is also the height Butler's Quick Ship 421 pistons are built at. |
3. Quick Ship flat tops and Butler milled dishes
| -8cc flat top | Butler Ross Quick Ship in a 4.250" stroke, 4.150" bore, 1.295" compression height, 1/16, 1/16, 3/16 rings. Pins and rings are not included on this one. |
| CNC milled dish | The Quick Ship dish pistons arrive at Butler as a thick flat top forging and get finished on a five-axis CNC mill to the dish volume your combination needs. Pins are included, rings are not. |
| Real numbers | With an 87cc chamber, Butler shows the milled dish landing around 9.27:1 at -18cc, 9.13:1 at -20cc and 8.99:1 at -22cc. That is how you dial a 421 into pump gas territory instead of guessing. |
| Compression ceiling | Butler recommends no more than 9.5:1 on cast iron heads and no more than 10.5:1 on aluminum heads for pump gas. |
A 421 is worth building carefully. Measure the block, decide the stroke, tell Butler the chamber volume and the compression ratio you want, and the piston can be finished to hit it rather than settled for.
Not sure what your Pontiac needs?
Email our tech team at tech@butlerperformance.com. Tell us your engine, heads, car, and power goal, and we will put the parts together as one combination that works.