my 400 is bored 20 over . using the tools i have on hand i measure 4 inches and 3 cm. is this within specs to just replace the rings or is a re-bore required.

 

your measurements are still a little rough to determine if bore is within spec. If you have the block out, take it down to a machine shop and get them to mic the bore with a dial bore gauge. They should mic the bore in at least 6 places, top midle and bottom, then at right angles. A good shop will be able to tell you if the bore is round,free of tapper and the correct size for thepistons you are using.Most shops will do this for free.

Greg