I acquired a merc 377 engine that is missing a flywheel .Anyone know what flywheel I should look for.(internal or external balance).I would like to not pull engine apart for balancing. Thanks

That flywheel, has a counter-balance on it. It's cast into the back. I would spend the extra $ and get the factory flywheel, the last time I balanced a 377 making it a 383. I forgot the 377 flywheel and tried a 350 flywheel that was counter weighted and it was not the same between the two when double checking on balancing machine.

you can get 377 2 ways. stock 350 bore with 3.75 stroke. or 030 over 4-1/8th bore with 3.48 350 stroke. you need to know which first. one piece or 2 piece seal? if 2 piece, is there a dowell pin sticking out of end of crank between bolt holes? makes it more likely long stroke ext balanced flywheel. if one piece, it is either stock 350 crank or aftermarket 3.75 crank. you need to measure your bore and stroke to know what you need. a head gasket isn't a huge expense. pull a head.

Should have said stock 377 merc mpi

Anyone know for certain the long block is not the same as the HT383 ?

http://chevy-crate-engines.com/SBC-C...evy-HT-383.php

The Merc specs list them as having 3.75 stroke for the 377 cube vs the 3.8 stroke in the crate you have shown.

I know the listed specs.....but could they really be the same longblock ?

Here's what I'm getting at:

There is a massive chance that any particular black motor starts as a long block from GM. We know this.

If the Merc 6.2L/377cid is from GM, then they might have a flywheel available, And if so, GM is usually alot cheaper than Merc.

Here's my knowledge on the 6.2 MPI. 3.75 stroke, powder rods, cast pistons. The flywheel has a small weight pad, I tried a flex plate that was weighted for a 350, because I forgot the flywheel. There was a difference in the counter weight, between the two. I balanced it with the original 377 flywheel.

Not sure if this will help or confuse.....I had a 350 Mag EFI/MP Bravo 1 motor I turned into a 377 (standard bore stroker, NOT destroked 400). I used the original flywheel and had the balance weight milled off. Worked and still works great. So you could get a 350 flywheel and mill off the weight or follow SBs advice regarding the GM part.

check to see if the harmonic balancer had a counter weitght on the inside of it or if theirs a bunch of holes drilled on one side..if it has a weight then get the flywheel merc perf said to get..