why dont i ever see motors rebuilt 10,20 over ? its always 30 from stock.

Mostly due to more common piston/ring sizes but, my 400 was origionally bored .020 and now is at .030 9 years later.

yep standardization. Ulimately lowers the cost to all of us, but the down side is that a lot of blocks are cut more than need be.

most common over bore makes the most common parts available. usually takes .030 to clean up so most shelf pistons are .030 or .060

I have always been a pain in the butt for the machine shops. My 360 accually needed .030 over, but many of my builds only get what they need for a clean up, and true cylinder. It costs more for pistons, but even with my harleys I want a thick cylinder wall.

keeps things truer, and cooler in my opinion.

Kasey

Not unusual to have .008-.009 of bore taper in at least one worn out bore and .030 is really .015 per side so it's really not that much ot punch one. I try to go .020 if there is a readily available piston, I'd always opt to bore a 4.04 340 to a standard/common ring size 4.06"....the hard part is finding a 40+ yr old motor that will clean up with that little of a cut. And the ridge above the top ring That doesn't see ring wear can be pretty severe to try to clean up with less than that.

Also remember that the last couple thou is your clean-up hone.....020 doesn't give you much wiggle room.