Hello, I am rebuilding an old caddy 390 mill. Motor does not have too much wear. If honed to 0.010 over stock do the pistons need to be upsized or just use larger rings? Will there be too much play in the cylinder?Sorry I'm new to this, hope its not a stupid question. G
In my opinion .010 is a lot to hone and hold the bore straight, that said I bet it's been done before, I would pay a reputable machine shop to go .030 over and do it right.
Look up the stock piston clearance. Add .010 to that and see if you are still in spec. If the pistons are all good, and the ridge is not there, a glaze busting hone will do what you are trying to do.
Yep, you'd have way too much clearance at .010 oversize. Once the engine is disassembled, you could check the bores with a dial bore gage to see how much wear and taper is present, and if you can get by with just running a glaze breaker type hone thru it and replacing the rings (with stock size rings). Any automotive machine shop should be able to check the bore dimensions for you, as well as doing the honing if that turns out to be an option in their judgement. You wouldn't spend much more for them to clean, check, and hone it than you'd spend just for a good quality glaze breaker hone. One thing to be aware of is the presence of any ridge at the top of the bores. If there's a ridge, it needs to be removed with a ridge reamer prior to pushing the pistons out. The ridge is typically smaller than the bore size itself, and is made up of hard and abrasive carbon products of combustion. Forcing the pistons by a ridge can render them useless when they may have been fine otherwise.
.010" is too much clearance for a stock-type piston. The engine will run, but it will tend to be noisy, the rings won't seal well, it will use oil, and the piston life will not be very good. The highest bore wear is near the top of the ring travel. The wear is typically about double on the thrust side vs the opposite side. So, even if it measures..... say .008" oversize where I mentioned, a minimum of about .0105" must be removed to make the bore straight and round again. Nothing against you; the average person will look at a worn bore and think it looks good. Before you go too far make sure what you have and do some careful measuring. Finally, a positive pressure hone in the hands of a skilled individual is needed to straighten/resize a bore. A "regular" spring hone can't do that.
Get the bores measured up accurately first. It may only need a good honing and you can use a stock ring set. Bob