The engine is from a 89 camaro its a 350 and I want to change the pistons and maybe the rods do I have to balance the engine.

 

Yes, it they have different weights.

 

So how do you balance it by the flexplate or on the crank
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there's two ways to balance and engine. one is by weighing the rods and pistons as an assembly (one rod and piston assembled) then find out which one is the lightest. then lightly grind the counterweight (it looks like a metal block) on the bottom of the rod cap until all the rod/piston assemblies are very close to each other. thats called internally balanced. externally balanced engines use a dampner and flywheel/flexplate ofa certain weight to minimize vibration, however the internally balanced engines are far superior.

 

so if I interially balance a post 87 engine do I have to use a new flexplate.
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You can also balance the crank and flex plate. If you look at the counter weights on the crank you will most likely see holes that look like it has been drilled. well it has been to remove metal to lighten it up. But your 89 (86 and up) is externaly balanced. So be sure your flexplate/ flywheel and dampner is for that.

 

I looked at the flexplate and it has a good size weight welded to it. If I change the internals do I have to change that weight?
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