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I am building a 400 small block, boring out to .030. my question is, switching out to 5.7 inch stock 350 rods and speed pro flat top pistons made for the 400. will balancing be required? Being as its externally balanced already and that all 8 pistons and rods will be changed.

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tl dr: Yes, you should have the rotating assembly balanced.

There are two reasons (I can think of) which would require this:

  1. The 400 rotating assembly and 350 rotating assembly are balanced differently (as you most likely already know). Putting parts together between the two will not balance out.
  2. Whenever you put a new rotating assembly together with dissimilar parts (ie: parts which didn't originally come together from the factory, whether factory parts or not), if you want the rotating assembly to last, you need to have it balanced.

My theory behind engine building is, if you want to put it together and want it to last, you put it together right the first time. Balancing the assembly is the right way to do it. If you take shortcuts to the finish, you'll find the wear parts (ie: bearings) will take the brunt of it and you'll wear them out a lot quicker than if you do it right. If your purpose is to make power, not having a balanced assembly will hurt this area, too ... something more to think about.

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