clearance on cast pistons

I have been screwed by some machinists that would not bore without having the pistons on hand,which I could understand. But this know it all had cast pistons in hand and bored it out for alum. ones.This was before I had my own mics to double check these guys,now I have an rebuilt engine that still will develop blow by when worked hard. Those cast pistons just won't swell up enough to tighten it up. DOUBLE CHECK EVERYTHING you have done. I had a head for a 60 resurfaced once and it came back farther out of flat then it was before they did it.

cylinder diameter: 4.251 - 2
piston dia at skirt: 4.245 - 1
piston dia at 1 & 2 rings: 4.235 - 2
piston dia at 3 & 4 rings: 4.235 - 2

where the "minus 2 means minus 0.002" tolerance".

The nominal bore is 4.250 (4.249 - 4.251)
nominal piston dia at skirt (4.244 - 4.245)
which implies a clearance of 0.004 - 0.007".

I would expect these tolerances to be the same for the 0.045" over bore.

If you have any kind of shop service manual for that tractor, see in the front part a section with the heading "Condensed Service Data." Then, under Sizes-Capacities-Clearances, you will find Piston Skirt Clearance. And if you do not have a service manual, and doing the kind of work you are doing, get one!

What troubles me more than your quest for the number is the question of how large the bore should be! If you take the pistons and the "number" to your machinist, he should have no further questions! If he does, find another machinist. My 2 cents. (PatB)

By "2-4" Piston Skirt Clearance, you mean all the way around and in thousandths, right? I think .002 to .004 clearance total on a 4.295" bore might be great.... for about 10 minutes, then she'll likely start to drag!

I agree that consulting a book in this case would be the best method, but it might not be possible to obtain solid service data for an unstyled "A".

I have in my notes from a number of sources over the years that .001"-.0015" per inch of bore for cast iron piston clearance measured 90* to the wrist pin (most pistons have extra relief on the pin bore face sides), and .003" to 004"per inch of bore for piston ring end gap.

So, for a 4.295 bore, the clearance for the vertical measurement of clearance would be .00425" to a high of .00644, so call it .004-.007 and he would be safe. End gap should be .013" to .017. If he intends to work it hard, I'd lean toward the high end, but if he intends to break it in slowly and without a lot of immediate heat, he could lay towards the low of .013.

The over bore is exactly that; an over bore. If 4.250" was standard, then that plus .045" would be the final, honed, finsh size, which would be 4.295. Machinists who are used to building high revving, high compression engines tend to want to treat this job like a major blueprintting effort, but you just have to tell them that it is just simple gas engine turning less than 1250 rpm and under 100 pounds of compression.

Frank