Piston Question
 

Hi All,

Sorry if this is a dumb question this is my first time dealing with this hopefully someone can help me out. Me and my dad got an motor block that we was told was machined a while back to .080 over and never assembled or anything. Anyways we measured the bore and 2 cylinders are 3.955 according to what i have seen that is .080 over the other 2 cylinders we keep getting 3.952 a little smaller. our question is will .080 Over rings and pistons still work even if the bore is a hair smaller?

Re: Piston Question
 

Most (should be all) machinists have to have the pistons before boring a block. Do you have the pistons to go with the block in question?

Re: Piston Question
 

No we got it at a swap meet it was just block and crank

Re: Piston Question
 

Measure the pistons and compare to the bores.
The real issue is having proper clearance.
On race motors, we bore the minimum to clean up the hole and have custom pistons made to fit. In some cases we can have 4 different bores.
The rule of thumb is .001 clearance for each inch of bore diameter.
Some specs, especially for older pistons with split skirts will call for total .0015 - .002 clearance.
I have never witnessed a problem when the clearance was loose as compared to tight.

John

Re: Piston Question
 

See link below.

Pistons should be cam ground which means that they are not round but oval. The clearance in the fore and aft direction is more than side to side. That is so the thermal expansion has some place to go and because the side to side part of the piston is where the forces are.

You can measure the clearance with a feeler gauge. I recommend 0.004 or more in the side to side direction. You can hone out the cylinders a little to enlarge them or better yet take it to a reliable machine shop to have the pistons fitted.

You also need to set the gap for the piston rings.

The red book (Model A Mechanics Handbook, Volume 1) is very valuable when putting together an engine.

https://www.fordbarn.com/forum/showthread.php?t=85463

Re: Piston Question
 

As stated above, the important thing is piston-to-wall clearance. Original Ford specs were a max of .002". Modern pistons are made differently, and our driving habits are too, and need more clearance. For stock engines, I bore to .003" and hone another .0005". Racing pistons and conditions require more, often much more. Bigger is better in this case.

Re: Piston Question
 

To answer your question. No .080 rings and pistons will not fit in those bores. As has been said get your pistons and rings. Use the the piston manufacturers recommended clearance to determine what final size the bore should be and then make the cylinders right. Pistons are cam ground and there are many different profiles. Tapered from top to bottom and with slit, partial slit, or no slit. And then there are coatings!

Re: Piston Question
 

You have to give the pistons to the machinist and have him bore to fit, to do it right. Since you need new pistons anyway, I would forget the .080 bore that may or may not be right, get .100 over and start there and get it right.

Re: Piston Question
 

Thank so much for the information. I reached out to Snyders and talking with them their .080 over pistons and rings should work just fine i just need to do a final hone and on the 2 larger cylinders when hone the small ones out.