I had both engines rebuilt and he balanced the rotating assemblies. I just replaced both harmonic balancers (externally balanced) and just last night a friend of mine said that I should have brought all 4 back to my builder so he could balance match the new ones to match the old ones. I cant do it now regardless because I threw the old ones out. Just wondering to what level I may have F'd up. Thanks for your input.

First of all what engines are we working with ? Unless your builder modified the dampers when he balanced the original assemblies (he should not have) and your replacement dampers are a quality brand, you really should not have a problem. What brand dampers did you buy ? Call your builder and ask him if he ever messes with the dampers when balancing assemblies. When I balance an external balance assembly I never mess with the damper or flywheel. That makes replacement a lot easier.

Gen V 454's. I can call him on Monday. He balanced the rotating assembly included the coupler and flywheel. I bought these:

Scat Street Performance Dampers SCA8005

Those should be ok. Scats stuff is typically pretty decent as far as the balance goes. I rarely see an issue with external balance Chevy stuff. Fords are another story Seems like nobody can get them correct.

Too late now but the one word of advice I give to all of my customers is to NEVER throw old parts away until you are absolutely sure that the new parts are working properly.

Find a new friend. Or don't talk engines with said friend. Lol.

The balancers you replaced them with are fine.

Thanks Tom. He is pretty anal. Even the dust in his shop is organized 🤣