340 crank with 318 harmonic balancer?

swingingdart said
It is a 340 crank shaft in a 318 with all 318 rods and pistons.
However they all spent time in an engine builder's shop together and he did his magic to make sure they would all work together.

I thought the harmonic balancer off the 318 was fine (thanks for verifying what I was hoping) but at some point with checking the timing I realized it spun on the rubber. And driving it i feel a vibration, its hard to describe, but it starts small then vibrates up harder, then drops small and vibrates up harder, over and over and over and the instances increase with RPMs.

Caused by bad harmonic balancer?

Here is my thought on your vibration. The vibration is being caused by the 340 crank not being balanced to the 318 rods and pistons. And I say this because your harmonic balancer is neutral balanced even if the ring slips it's still neutral balanced. Your vibration is going to knock out your rod bearings and you're going to have a piece of junk for a motor sitting in that car if you continue to run it with that vibration. I think you need to pull the motor apart and have it balanced