10/18/2026
Put a 396 and a 427 side by side and you are looking at one engine twice.
They share the same block casting family, the same cylinder heads, the same valvetrain, the same bellhousing, and the same crankshaft with a stroke of 3.76 inches. The 402 that followed shares it as well. Three engines, three badges, one crankshaft.
What separates them is how far out the cylinders were bored. The 396 measures 4.094 inches across. The 427 measures 4.251. That is about a tenth of an inch of metal, and it is the entire difference.
Which raises an obvious question. If the bigger one costs almost nothing extra to make, why sell the smaller one at all.
The answer is a rule rather than an engineering decision.
General Motors had an internal limit of 400 cubic inches for its intermediate sized cars. Chevelles fell under it, and so did the Pontiac GTO and the Oldsmobile 442. Full size cars were exempt, which is why a 427 could go into an Impala, and the Corvette got through as well.
So a Chevelle could not have a 427. It could have 396, which sits four cubic inches under the line.
That is the widely accepted explanation for the number, and nobody has produced a memo proving it, but the arithmetic is difficult to argue with. Four cubic inches is not a margin an engineer arrives at by accident.
What happened next supports it. The limit was lifted for 1970, and Chevrolet immediately put a 454 into a Chevelle. The rule went away and so did the restraint, in the same model year.
The 396 also got slightly larger for 1970, bored out to 4.126 inches, which works out to 402 cubic inches. Chevrolet kept the 396 badge on the car anyway, because SS 396 meant something to buyers and SS 402 meant nothing at all.
The engine underneath was genuinely good. The cylinder heads are the part people remember, with the valves tilted in two different planes rather than standing in a row, which is why the rocker covers are such an awkward shape and why they earned the nickname porcupine. Tilting the valves lets Chevrolet fit larger ones into the same bore and gives the air a straighter path, and it is the reason the Mark IV breathed so much better than the engine it replaced.
None of which had anything to do with the number on the badge.
