Cylinder Bore - Machinist Question

I'm trying to decide what bore my 351W block is, stock or .030, .040 bored over etc. With a digital caliper at the top of the cylinder I get roughly 4.045 on most cylinders some a few thousands more some less.

I know there has to be some over/undersize tolerance for the piston to fit and stroke easily that the rings make up for.

So knowing a stock 351W has a 4" bore is my block already bored .040 over or is that normal to accomodate for a 4" piston or is a 4" piston really .0x thousands undersize or is a stock cylinder bore really .0x thousands over piston size?

 

Sounds like your block is already a worn .040 over, and hopefully it will clean up at .060. You might be better off finding a standard bore core block to start off with, especially if it is a late model block. A good core 351W block should go for $100.00 or less. As for the pistons, they are normally slightly undersize.

 

^^^^^ What he said^^^^^

 

I ***ume if you're using calipers then you are actually measuring the "virgin" section of material at the TOP of the block - aka the "ridge". If your measurement is accurate - no offense: a) are your calipers any good and b) are you proficient at using them) - if so then it would sound like you have a block that was machined to a set of pistons. Normally a ".030 over" block is exactly that .030" over - the clearance built into the piston itself - being different for whatever type of piston being used. However it is not unheard of to have things the other way around - meaning the clearance being machined into the block - not the way I'd want it done, but it's hard to tell what may have happened in the past. In any event - IF you are in fact measuring at the TOP of the block then what I jst said applies. If on the other hand you are actually measuring down into the worn area then what has been posted previously makes perfect sense.

 

To answer the original question, the bore is the bore size, the pistons are undersize by the amount of clearance. So the original bore would be 4.000", and the original cast pistons would be about 3.998 or so. Forged pistons would be a few thousandths smaller than that.

If someone used a ridge reamer on the block, then it could be any size at the very top.

Normally the bore will be worn only a few thousandths of an inch below about the first inch of the bore. The rings wear the bore near the top the most, because that's where cylinder pressure is highest. (pressure pushes the rings out against the cylinder wall)

You don't have the old pistons? if you do, just look at the oversize stamped on them.

 

Ebay sells cheap cylinder bore gauges that go to 6 inches or so that way you can go deeper into the bore and check measurements and also you can check out of round.

 

keep in mind that if you use a dial bore gage, you need a micrometer or accurate caliper to set it!

 

Thanks for the answers guys. I bought the block for $100 to use as a core. Its definately been apart before as in sheet metal has been off not sure on internals. Its an 84 block, I measured the top of the bore but did not remove any pistons. There are no markings on the top of the pistons to indicate they are over sized.

The reason I was asking is I was debating to ask the machine shop to rebuild my block if it had not been over bored already versus taking what could be a .060 block from them.

 

If it is a worn block, check to see if it is even round. The bores don't wear evenly. The dia. w/be greater the farther down the bore you go. They w/be more round from front to rear than from side to side. If you bought the block as a core, let your machine shop measure and advise you, as they do it everyday and have the right equipment to measure it with. Good luck..............