Mailbag: Understanding Internal Balance vs. External Balance Chevy Flexplates

Q: The 350 engine in my 1990 Chevy truck needs a flexplate. Some guys tell me it’s internally balanced, while others say it’s externally balanced. I don’t know which flexplate to order. Can you help?

A: Prior to 1986, all Chevy small blocks were internally balanced (except the 400). These two-piece rear main seal engines had a balance counterweight cast onto the back of the crankshaft. In 1986, the 305 and 350 moved to an improved one-piece rear main seal design. Even though the rear crankshaft counterweight had to be relocated externally to the flexplate or flywheel, the one-piece seal engines are still referred to as internal (or sometimes internal/external) balance. The corresponding flexplate or flywheel on the other hand, will be listed as external balance because they come with attached counterweights. Summit Racing’s heavy-duty, SFI-approved flexplate is a direct replacement for your application.

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This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.