Engine balance refers to how the rotating assembly (crankshaft, pistons, connecting rods, flywheel/flexplate) is balanced to minimize vibration. Getting this wrong can destroy an engine quickly.
INTERNAL BALANCE: An internally balanced engine has all of its rotating assembly mass balanced within the engine itself. The crankshaft counterweights account for all of the rotating and reciprocating mass. The flexplate or flywheel is a neutral, zero-offset component — it does not need any additional counterweighting.
EXTERNAL BALANCE: An externally balanced engine requires additional counterweight on the flexplate or flywheel (and sometimes the harmonic balancer) to complete the rotating assembly balance. The crankshaft alone cannot achieve balance — it relies on the external components to compensate.
WHY IT MATTERS: If you install an internally balanced engine with an externally balanced flexplate — or vice versa — the rotating assembly will be out of balance. This causes severe vibration that will damage main bearings, damage the transmission, and can shake the engine apart. It will feel like a catastrophic failure almost immediately.
COMMON EXAMPLES: • LS engines: Internally balanced — use a neutral flexplate • Early small-block Chevy (283, 327, 350 pre-1986): Many are externally balanced • Ford 302/5.0: Externally balanced (28-oz offset) in most applications • Ford 351W: Externally balanced (28-oz offset) • Ford 351C/M and 400M: Externally balanced (50-oz offset) — DIFFERENT from 351W
Always confirm the balance type of your specific engine before ordering a flexplate or flywheel. If you're unsure, call us at 1-844-XPARTS-1 or email support@expediaparts.com and we'll help confirm it.
