the old motor was stock, the new one has almost double the HP,

Make sure you do not have a couple plug wires mixed up, out of the 1-8-4-3-6-5-7-2 firing order. Was the cam a 4-7 swap?

Just for my future ref… If you have two same gen Chevy BB motors. The balance on the flywheel and the balancer are exactly the same? And interchangeable (always) I know it wasn’t that way on a couple older Mercruiser 302’s but I do mean older (73) 888's...

Yes that's what i'm saying . Gen for gen ,and displacement..If you were to buy a reman without a balancer and flywheel, you use your old one,right .In a hi hp situation it is obviously better to have the entire rotating assemble balanced. But if that was the case in this thread the engine builder should have asked for the op's flywheel and balancer...Wonder if the builder in this situation just put on some different pistons without worrying about balance ...Or builder bought an internal balanced assembly and op put on his external balanced flywheel and balancer. curious

If the engine came from the engine builder with no balancer or flywheel then its more then likely internally balanced. Especially if your saying its a high hp engine. If it is external then the engine should have been balanced with the flywheel and balancer. I have seen also some shops work on the flywheels to get a crankshaft balanced and that's the wrong thing to do because if you ever have to replace the flywheel the crank will be out of balance.

ok, i went down and pulled the balancer off the motor and its an externaly balanced, i talked to the engine builder last week and he said the engine is externaly balanced and any BB externaly balanced flywheel and balancer will work, my best bet would be to just go to his shop and look at the one's he used to balance my motor. dont know what else to do.

here is the engine that is in the boat www.ebay.com/itm/230951113710

i believe your builder is mistaken,the counterweight on a mark 4 is different than a gen 5 or6.

fixx
 

+1 and from what that pic looks like it has a 396/427 ballancer on it if that is the exact same engine..it should only have that ballancer on it if it has a forged crankshaft and internally ballanced.

this has always confused me as most balancer manufacturers show the same balancer for MK4 and Gen5 and some also fitting Gen6 (although I think that is if you dont have the trigger wheel on the crank) I have a balancer that is off a Gen5 and it is the same balance and my mk4 with the only difference that I could find was the counterbore for the pulley was not on the Gen5 one. I know the flywheel IS different due to the loss of the cast on counter weight on the rear crank flange but it seems the balancer is shown as the same and that would make sense. this is for my 454 so the 502 may be balanced different.