I'm swapping over to a GM crate 350 from my original 305. I went to install the harmonic balancer using the bolt from the old engine and it doesn't fit. As best I can tell, the original bolt is a 7/8 x 2 1/4 x 20 and the new one appears ro be 7/8 x 2 1/4 x 18. This is a best guess since I don't have a tap that size. Can anyone confirm this or tell me what size I need?

Thanks!

 

You should be able to call the supplier of the crate motor, or Chevy dealer and they can tell you the exact thread size.