I rebulit my 312 originaly in 1969 and bored it .040 over with pistons from my local Ford dealer, or at least thats what I remember. Rebuit it again in 1980 and had to replace one piston. Got it from Ford. Pulled the heads today to go through it again and discovered that the pistons are stamped .0425. Memory fails me on this one. All I can remember is it was .040 over. Depending on the condition of the pistons when I finish disasembling the engine I may want to go with new forged pistons. Am I going to be able to use .040 over forged pistons in the existing bore, or am I going to have to re-bore to .060 over? Checked my Ford parts book and they show the 312 pistons as .020 plus, .030 plus, and .040 plus. 292 pistons are shown as .020, .030, and .040. No plus. Anybody able to shed any light on this mess?

Surely somone on a Y-Block website knows something about this??

OK, leave the maybes out and deal with what your actual bore sizes mike up at? then you will know if you may only need to hone them for the forged pistons, they run more clearance than cast, but until you measure everything you,re guessing, also check what forged slugs are available, i,m sure something will work out. regards bill

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

As Bill suggests, mic the cylinders and the pistons and verify what you have.  Nothing says there were not some +002½”over 0.040 over pistons for special cases and you got a set.  If the pistons are indeed 0.0025" over a normal bore size, then in the back of my mind the 0.0425” pistons are made with either cleaning up an existing 0.040” bore in mind or being a fix for 0.040” bores that were overshot during the initial machine work.  Sort of like the red and blue pistons that were available from Ford in the standard sizes to compensate for variability in the standard bores.  With myself doing a large number of custom pistons, I do odd ball bores pretty regularly simply to minimize taking any more out of a cylinder than is absolutely necessary.  The key here is insuring that a ring set for whatever bore size I target for is actually available before ordering the pistons.

Lorena, Texas (South of Waco)

there were a lot of those .0425 pistons made fot the 312.it was the only size made like that i ever heard of.no idea why.

Did an approximate measure of the bore with an inside snap guage and an outside mike. Came up with 3.850 or 3.851. If the pistons are 3.8425 that would give me .0085 piston clearance. Does that sound about right? Will pull the pistons tomorrow and mike the pistons. Tying to locate an inside mike to get an accurate measurement on the bore. I am thinking  new .040 over pistons will measure 3.840 and end up .011 piston cleareance which I think is way too much. Need to do some research and find an inside mike.

First I'd get it measured exactly. Check for taper and eccentricity (out of round). But if it is that big, I'd say it's time to go to .050 or .060 over.


Lawrenceville, GA

Actualy the .0085 is about right for piston clearance. Think I was one decimal place off. Big difference.  My old Hot Rod Magazine engine manual recomends .003 to .007 for street and up to .012 for racing so the .0085 would be good.  Found an inside mike to borrow next week. Pulled the pistons today and miked them. They are definately 3.8425. Pistons checked out fine so far so my plan it to reuse them and re-ring it and install new rod and main bearingw while it is apart. ARP rod bolts, head bolts, and main bolts also. Problem now is rings. Checked bill from the 1979 and used .040 over rings then. I need to check the end gap on the old rings. If there is a problem, not sure if I will be able to get the .0425 rings. Could go with .060 rings and file them to fit but that would be a pain. Will measure and check availability. Good news is that the cam I thought was flat looks fine and I plan to re use it. Lack of performance was due to  low compression.

In going through an unfired Ford authorized remanufacturer's 312 in the early '80s before selling it, I found that it had Ford 0.0425 cast pistons in it, as I recall.



Weird!?!




NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Thats what I keep telling myself!