Bore size vs piston diameter

I hope this is not a stupid question:

Which diameter is actual and which is over/understated in bore/piston combo? I assume the bore size is measured and the matching piston is smaller than the designation e.g. a .030 over piston is smaller than bore size + .030 - correct?

Yes the bore size is the actual measured bore size and then the piston is undersize by the clearance amount. The rings are typically oversize a small amount and need to be gapped (ends ground down) to get the correct spacing.

Thanks Andy! Here's my dilemma: my engine block (440) is at the machine shop and I don't have pistons yet. Normally the shop would size to bores to match the pistons but I would like to have them bore the block and then order the pistons (being in Europe doesn't make things faster/easier).
IIRC piston manufacturers are usually able to provide pistons in an exact size. My block should be fine with with a 0.010 overbore (I only have slight taper and still can see and feel the original cross hatch marks) - so a piston with a diameter of 0.008 (twice piston to wall clearance) less then the bore should work, right?
I was thinking about Autotec pistons.

Where are you going to find a .010" over piston? Or, any 440 piston that doesn't start at .030" over? Custom? It's likely to cost more than a shelf piston. And what about rings for a 4.330 bore? They're available but I only see 5/64, 5/64, 3/16 oil.

A replacement std piston is the same cost as a .030" over piston, and the machine work is likely to cost the same whether .010" over or .030" over. Plus a quality piston in std is more difficult to acquire than a .030" over piston. Most manufacturers don't offer std. pistons anymore.

Measure the taper in the bore. It may come round and straight with a good power hone with just a few thousandths taken off. Then run a forged piston a little loose with file-to fit oversized std rings.

So lets say is needs .005 to get straight and round 4.325". A forged piston is likely to have .003" clearance machined into it at 4.317". You now have .008" clearance and it's still a std. bore. No boring bar needed, just a hone.

The piston manufacturer should be able to tell you the recco skirt clearance for that specific piston and your engine. It's going to vary considerably with the aluminum alloy, forged vs. cast, cooling system, somewhat with the pin boss and skirt design.
Very important: the difference in width across the pin boss vs. across the thrust axis is the "cam grind" or profile. The most common cam profile is the standard Van Norman “C”. It compensates for the way the piston shape changes during hard use, and it's fixed during manufacture, and no correction for this can be made by changing the skirt clearance.
Your guess may be correct, or far enough wrong to cost you $$$.

I would like to bore not more than needed to have as much wall thickness as possible. Racetec/Autotec pistons are available in custome sizes so the .010 over shouldn't be a problem. The rings seem to be the real problem at the moment as I coulnd't find any 010 over rings in 1/16"

Minimum clearance required is .0035 to .0045

Pistons can be made to any diameter but rings are a different issue. You'll have to spend some time on the internet looking at catalogs but I don't know of any rings between 4.320 and 4.350. It just isn't a popular size. Sometimes you can get lucky and find the size you need from another engine but I don't know of anything in the 4.330 size.

You might talk to your machine shop and see if they are comfortable using a 4.320 file fit ring at 4.330 bore size. Maybe order a set of rings and see if it will work or not. There might be enough material in a 4.32 file to fit set to allow you to run 4.330 but I've never tried it. I've used file fit rings at 0.001 or 0.002 larger bore size but not 0.010 larger.

Also, if you are only going 0.010 over you'll most likely need to hone to the next size rather than bore. Make sure the shop is willing to do that. Depending on their hone they might not want to spend all that time.

In short I'd say that unless you are some sort of max effort Super Stock racer it is better to just do what everyone else does. Order 0.030 over pistons and bore and hone the block to match.

Suggest you at least identify the piston and ring first. If it is a custom you want from Ross, JE, Diamond or others, talk it over with them. Or if you want to use a shelf stock piston, pick it from a catalog before you bore. That way you will know what is available including rings.

I had a B block 400 bored for 20 over and that worked well... but the 400 has a bigger bore size to start with.

WRT file fit: remember the ring circumference is the bore X Pi, so +.010" ring set is -.031416" on the end gap. Even if you get it down there the radial tension is higher than you want.

It seems that I have the following options:

1.) Bore size .010 over and custom diameter pistons and shelf rings. Rings available only in 5/64 (pre gapped)

2.) Bore size .020 over and custom diameter pistons and shelf rings (pre gapped and file fit available) in 1/16

3.) Bore size .030 over and shelf diameter pistons and shelf rings

What's more important (option 1 vs 2) - wall thickness (.010 vs .020) or ring thickness (5/64 vs 1/16)?
I'll have to order custom pistons anyways (will be a dished piston) the price for all three options is pretty much the same.

Why are you concerned so much about wall thickness? At .030 , your wall thickness is not the primary area of concern for durability on an rb block. Many high hp .030 combos eventually beat the main caps out first. A .030 in various dish 440 combo piston is readily available. I also know first hand that getting a straight bore from a stock block at .010 or .020 in all 8 holes you would be lucky at best. Just my

Well I thought the same about a .030" overbore. Surely enough meat for that with our dragster. So I did not bother sonic checking the block. Well the end result was this. The cylinder wall was roughly 2X the thickness of a Pringle! This was a 69 HP 440 block. I too was more concerned with the mains than the wall thickness of the block. Apparently I should have checked the cylinder walls too. As for the question at hand. I would make sure you can get a good ring for the finished bore size. Pistons are the easy part, they will build the clearance into them when manufactured, provided the info you give them is accurate and the bores are round you should be good.

I like to keep the bores as thick as possible (minimum overbore). A custom piston helps a lot. That said, I think you have better chance of getting a nice straight clean bore if you bore and hone at least 20 over. I would go the 20 over route and use the more common ring package personally.

[quote=Al_Alguire]Well I thought the same about a .030" overbore. Surely enough meat for that with our dragster. So I did not bother sonic checking the block. Well the end result was this. The cylinder wall was roughly 2X the thickness of a Pringle! This was a 69 HP 440 block. I too was more concerned with the mains than the wall thickness of the block. Apparently I should have checked the cylinder walls too. As for the question at hand. I would make sure you can get a good ring for the finished bore size. Pistons are the easy part, they will build the clearance into them when manufactured, provided the info you give them is accurate and the bores are round you should be good.

That block was doomed at standard bore . boring it .030 was not the problem there. However that brings a great point A sonic check would go along way to total piece of mind. At .030 we are taking .015 of the radius of the bore If .015 puts wall thickness to critical tolerances than the block was no good from get go.

Correct using it at was the issue. But for out little 675hp pump gas deal was not concerned. I have sonic checked a bunch of stock blocks over the years and NEVER seen anything this thin. FWIW the rest of the engine looks great, well except the extra holes There is a reason I no longer use stock block deals in racecars. This was the engine from my 68D100 and was never meant to have 250 passes on it

quick look at the hastings piston ring site and I found 25 rings in the 4.330 size. but you better know your width and thickness before you have the pistons made.