Car Tech Questions - What Is Your Problem?

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Balance JobI understand that most Chevy 383 stroker kits incorporate external balancing by using a Chevy 400-style damper and flexplate. Even kits that are sold as balanced rotating assemblies frequently use the 400 damper and flexplate. Some kits, however claim to be "neutrally balanced." I have been lead to believe the GM Performance Parts HT383 crate truck motor is internally balanced like a standard 350. If this is true, can you clarify the differences and advise which is better, assuming a new crank is required?Robert l KeelingLouisville, KY

When balancing an engine, the mass of the components hung on the rod journals of the crankshaft must be brought into balance by offsetting weight on the counterweights of the crankshaft. The balance factor takes into account the motion of the components, factoring 100 percent of the reciprocating weight and typically 50 percent of the rotating mass. Unfortunately, the standard GM 400 crankshaft traditionally used in these engines does not have enough counterweight mass to bring the assembly in balance. While the crank's counterweights themselves lack the required mass, GM's clever powertrain engineers simply hung the required weight at each end of the crank with eccentrically balanced dampers and flywheels/flexplates, creating "externally" balanced engines.

The GMPP HT383, similar to the traditional 350/400 production-based strokers, is externally balanced, but the balance factor is different from either a 350 or a 400. GM lists a specific damper, PN 12498008, for the new HT383 application, while at the output end a PN 14088765 flexplate, common to '86-and-later one-piece rear-main-seal small-blocks, is specified. Follow the directions from GM and proper balance will be taken care of.

Production-based custom strokers typically used the eccentrically weighted production 400 dampers and flexplates to achieve balance, often employing way too much eccentric weight. Better too much than not enough, though, since the assembly can be brought into balance more easily by removing weight than by adding it. Expensive custom balancing can be one of the hidden costs of a pieced-together stroker. Some aftermarket stroker cranks have a "split balance," combining an externally balanced 400-style flywheel with a neutrally balanced harmonic damper to achieve overall balance.

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