ALL SBC with one piece rear main seals are EXTERNALLY balanced. All two piece seals are INTERNALLY balanced.
If you build a 383 stroker - that would be EXTERNALLY balanced.
Use the correct flexplate / balancer.
You asked about what SIZE of balancer - and that depends on clearance if any issues depending on what you're putting the motor in. And you should talk to your engine builder to determine exactly what balancer to use. Some balancers have different weight IMBALANCE etc so this is a trickier question than just "what size" -- getting it wrong will have bad consequences so don't guess.
|