I'm having my SBC 400 block bored at a machine shop as we speak. When I dropped it off, I was in a hurry, and completely forgot to verify how much he was going to take the bore size over. I assumed .30 O was the most common, and didn't give it another thought. I am now in the process of buying pistons and rings for the new bore, and called to make sure everything was going smooth. He informed me that the bore was in fact NOT .30 O but .20 O. That seems like a bore size I have not seen, and I am having no luck finding a good priced set of pistons and rings for a .20 O cylinder. Any suggestions for cheap pistons? The guy sells them for $28 each. That seems high. Input? I have looked through Ebay, and even emailed a few vendors, but seems everyone has .30 O pistons and rings. Thanks a lot guys...

 

Okay, so maybe I'm the first to reply to my own topic, but I have found a set of each that will work. I still would like to hear your thoughts about the odd bore size, or if it really is odd at all. Could it have just been a machine shop trying to run up the bill by selling me pistons that cost a lot more at .020 over than they do at .030? Or is it just a guy that knows you only have to go so far to get a good wall, and not go all the way to a standard size bore like .030? Thanks guys...

 

I don't know why everyone is always so eager to waste the life of a block and go right for a .030 overbore. .020 is a standard overbore size, just not as common as .030.

 

Not that odd, At twenty over, you might get a rebore out of it later if something goes wrong..

 

Less is better on these engines on reboring. Have to agree though that everything seems to be standard at a .030 overbore, even with a stroker 383 engine. Nice thing about the good old style small block though, find about anything you want reasonable for it.

 

.30 is a lot more odd than .030

 

A lot of machine shops go to .030 over because it's easier that way, if they shoot for .010 or .020 over and don't set the machine up perfect then some bores might not clean up and they will have to start all over again. And sometimes the blocks are so badly machined at the factory that if your trying to blueprint the block you have to go .030 to .060 over to get all the bores in the EXACT position they should be and straight with the crank.

 

If it still has the numbers on the front pad it wouldn't hurt to deck the block about .020 while it's apart. Most are about .025 or so down so it would bring the pistons up for better quench and a bump in compression.

 

Prolly cuz its hard to find good pistons in .020.. ?

I know, when I had to put my block to .040, it was tough finding decent pistons.. Some folks will just slap any slug into the motor, but I like to put some thought into dome/dish area, quench, etc.

-- Joe

 

If there was a demand for .020 pistons you can bet there'd be a much better selection of them though.

 

when ever I look for pistions I never have a problem finding .020 over bore pistions. usually they like to give specs for .030 (like est. comprestion ratios) they always say in the fine print "standard and over bore sizes availiable, all comp. ratios based on a .030"- ect ect.


so what I would do is find the one you want and give them a call. it's that simple

 

It's really not that simple. When I wanted a specific piston that I liked in .040, they were like "no". Ok how about this one. "No". Seems as if .030 is the most available, and some in .060. Very few in .040, and less in .020.

And reading the compression ratio in the summit catalogue is not very reliable.

For compression ratio you need:

Actual bore diameter
Gasket thickness
Gasket diameter
Cyl head volume
Piston volume OR dome area
Stroke

The difference even in gasket bore, can be as much as .25-30 difference in compression ratio with some combos. (i.e, 4.080 vortec type gasket vs 4.160 bore sbc type)

Durrango boy: What size chamber heads are you using, and what would you like your compression ratio to be? Maybe we can help you find a .020 piston.


-- Joe

 

Thanks Anesthes for the offer, bu I have found a good pair of Speed Pro pistons, .020 on Ebay. They were not labeled as such, I just emailed, and he sait they carried the. I also found the correct rings for the pistons, and the combo looks to be what I need. Thanks so much for all the help and advise. Just so you know, they are pretty stock heads from a '87+ 350. I think maybe from a '89. They are getting cleaned up with new valve guides, and pushrod guides installed for my project. I wasn't told by the machine shop that I would need head work or different gaskets. I will have to drill the steam hole though. Thanks again...

 

Actually, you need ONE MORE VERY IMPORTANT NUMBER, often omitted like it is here, yet totally critical.

That number is the "deck clearance", which is how far down in the bore the piston is at TDC.

A stock piston with a 1.56" "compression height" is .025" down in the bore, more or less (usually more) at TDC.

Most replacement pistons hace slightly less compression height than stock. Most in fact will measure 1.535" - 1.54" .... in other words, .020" - .025" farther down in the bore at TDC, than stock. The reason is, most rebuilders feel alot more comfortable about building a motor with slightly less than OE CR, than they would about building one whose pistons crash into the heads. This is considered bad.

Go put those numbers (.025" for stock, .045" for a rebuilt motor) into your favorite CR calculator, on top of the .039" or whatever for the head gasket; and see what kind of a hit the calculated CR takes. The lower number is MUCH CLOSER to the real world.

As a result, the actual compression of a rebuilt motor, is VERY RARELY as high as it was stock; and EVEN MORE RARELY as high as the rebuilder claims it is. Be very careful about throwing around high compression numbers.

 

yes but, those are est. like they say. an estimate is just that. I can say I have never had a problem getting pistions in those bore sizes. they must be some thing very specific that you want.

the only pistions that I can think of which are hard to find in specific compresstion ratios and types are the 6" rod sbc 400 pistions and in many cases the 5.7 rod vertion as well

 

Sorry RB, forgot to type that one. But, i'm sure he would have realized hwen he went to use a compression ratio calculator.

-- Joe