Right way to measure Piston to Wall clearance?

[jking29]

I have never heard of a machine shop boring a block without the pistons in hand or at the very least the measurements off of the pistons going to be used. It is the machinists fault and it should be him eating the cost another set of pistons or another block.

 

[bluegs03]

Well i spoke with the machinist today and he is driving out to meet me with a bore gauge and a micrometer, now im just praying that me and the shop i took this block too made a mistake when getting the numbers. Its so sad but i would be sooo happy to hear that i made a mistake or read the gauge wrong. Ill let you know if it was indeed the machine shops fault or if it was the user of the measuring instruments.

 

[treebonker]

ok now you really have me confused. Why would measuring the pistons before you bore the block help things? The pistons will be the same size no matter when you measured them they dont change? Im also not understanding what you mean when you say they have a clearance built in? I have never heard of such a thing and am curious. Is what your saying that the pistons can vary as much as .004 in there size from the manufacturer?

No, "clearance built in" means when you buy 20 over pistons for 3.366(85.5mm) bore, the pistons are made .004 smaller and will measure 3.362. Pistons are not made perfect, so if you find some are 5 tenth smaller, then you need to factor that amount when boring the block.

 

[BoredDSM]

ooooh yea i see what your saying now i think tree just put it in a confusing way. My machinest has done alot of work before so if he made a elementary mistake like that i would be appalled but its sounding like thats what happened... also you have to account for the fact that the cylinder walls could of had material scrapped off in its 120000 miles of useage and know what the block size was to begin with, then you can tell what it is now and subtract the 2 to get the measurement you need to bore it .020 over. I imagine what happened is the cylinder walls had .005ish of material scrapped off of them from usuage, my machinest just stuck the bore in there and took an additional .020 over out and then honed it out another .003

I thought you were saying that you had to bore the cylinder out depending on the size of the pistons and how much they varied from the manufacturer, which is wrong of course.

Even so its not my resonsibility to tell the machinest to do that, i mean if every person who had machine work done knew every single detail of every step that he needed to take then they would just rent out machines and do it themselves thats why we take it too a expert machinest because they have these steps memorized and we wont mess up. Im still convinced it is the machinists fault

There is a lot more to it than just boring a block out. The machine itself is pretty big and would be pretty hard to rent. You also need to know how to make proper cross hatching and learn a lot of metal properties. There is a lot of controversy in machine work and it's up to you to make your own decisions as a machinist to see what is better and better for your customers. Not as easy as you think.

 

[Defiant]

Okay. This is getting just too far away from the topic, and apparently from reality. bluegs03, any engine shop manual will explain how bore is measured, and how piston clearance is determined.