Just got done measuring everything 40 times. Both barrels and pistons measurements are almost the same barrel to piston. The variance on the barrels is from 3.624 to 3.618 with a max of .001 out of round and top to bottom. Both pistons are the same clearance to the barrels too, .0047, just a tad big. Everything I find say .0045 max.

Looks like it's new piston time.

 

Is that a 96” with 3 5/8 bore?

 

Yup. 3 5/8 x 4 5/8

 

Just trying to figure your measurements, just wondering why there would be .0047" clearance with under a 3.625 bore, the pistons skirts look good?
I don't understand what the 3.624 - 3,618 variance is.

 

I'm guessing measurements were "thumb sight and eyeball" that is, without torque plates just to get an idea of what you have.

Put them in plates and they can twist, bell, hourglass, barrel or stay pretty close to what they were. And even though they've been run on the same engine it doesn't mean they will each behave similarly. You never know what they will do till you torque them in and let them sit for at least 1 hour before doing the critical measurement.

But even out of plates, having a piston to cylinder clearance of over .004, regardless of the type of piston, is all you need to know.

 

My curiosity has got to me. What kind of measuring equipment are you using to take those dimensions?

carl

 

That's "the out of round" measurements on the bore. The clearance piston to bore is the .0047 figure, which is out of spec. The skirts have scaring on them.



Yup, no tq plates, I was more interested in basic figures. It was done using calipers and a bore gauge, I was more interested in the piston to bore figure. Didn't think tq plates would matter much for that figure, I was just trying to get a close figure for reference. If it was closer to .004 I would have proceeded on and used tq plates. But at .0047 I need to go to oversized pistons and have the barrels machined. I could probably go by "loose" fit figures, but I don't think it would last very long.....lol.


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Vernier calipers, I.D and O.D calipers and a dial bore gauge.

 

What do your pistons measure 1 " from the bottom oil ring on the skirts and are they cast or forged?

 

How close is your repeatability with calipers? Do you get closer than 1/2 a thousandth measurement to measurement?

carl

 

I am a machinist. ID measurement with calipers is not applicable for the clearances you are trying to realize. Even a premium quality Mitutoyo caliper has limits. Telescoping bore gauges are problematic as well, even so they should be measured by a micrometer after a gauge block qualification. I do not work with cylinders but I would guess without a torque plate you would get irrelevant readings anyway.

A shop that specializes in doing this work should have the proper bore measuring tool and/or CMM to get you set up right.