Swapped 305 for 350 in 1988 TPI. used original T5 tranny and flywheel. Are these flywheels interchangeable or should I have purchased new flywheel for the 350? Since GM didn't make a 350 and T5 combo is there a source for flywheel? Also, parts catalogues talk about internal or external balancing. What does that refer to?

 

If the flywheel you had bolted up, then you're good to go. It is not possible for a 88 flywheel to just bolt up to any motor that it isn't correctly balanced for.

The correct source for a 350 swap is a 305 flywheel. i.e., what you have.

The 400 small block and 454 big block cannot be internally balanced, in their stock configuration; there isn't enough room inside the block and its moving parts to fit a large enough counterweight. Therefore some of the balancing for them must be done by the flywheel or flex plate, and the harmonic balancer.

The 87-up 305 & 350 are internally balanced; except that their crank flange is absolutely round (for the rear main seal). The earlier generation of 350 and 305 had a small balance weight made into the crank flange, and so was not perfectly round. The flywheel you have has a very small "unbalance" that is equivalent to that older one. It is the same for a 350 or a 305, which is why a 305 flywheel balances a 350 perfectly.

 

305 flywheel bolted right up to the 350 so we're good to go. Thanks for the help!