Small Block & Big Block Chevy Specific - Need Help. 383 strocker issue

My dad just droped a 383 into his 71 Monte. Got it running and it just vibrates like no tomorrow. We are thinking maybe fly wheel. Not sure if it might be something to do with valves or something either. Anyone got an idea.

is the balancer vibrating? is it externally balanced? if so the externally bal. flexplate and harmonic balancer must be balanced with the crank. what part of the engine is vibrating.

It's externally balanced. They weren't balanced with the crank eithers. It vibrates mainly back at the tranny and back by the tail shaft is where you can see it. But when you get up to about 1.5k to 2k rmps you can really feal it. We threw some regular rockers on there and it helped a bit. Still vibrates though.

Sounds like you should have ballanced it with the flywheel and ballancer.

Sounds like you should have ballanced it with the flywheel and ballancer.

yep what he said. you will probaly wear the bearings out really quick.

what is the fylywheel from/for. if its internally balanced, then the flywheel needs to be xeroed out which requires knocking the wheights off it and sometimes drilling holes to zero it. extenally balanced you need to talk to your machine shop and get some recomendations

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I always balance an external engine with flex plate or fly wheel. Too easy to be a little off.
Especially if you just throw some used piece on.

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Check the flexplate for a counterweight, report back and DONT run it again as you might rattle the bearings out of it.....