All small block Chevy's are internally balanced with the exception of the 400 which is externally balanced on the flex plate and also require a correct “harmonic balancer”. The 400 is the only small block that actually uses a harmonic balancer which, in conjunction with the flex plate or flywheel “balances” the engine. A more accurate description for this component on all other Chevy small blocks is a “harmonic damper” or “torsional vibration damper”.

454/502 Big Block Chevy engines also use an externally balanced crankshaft.

Regards,
Glenn