Measuring Pistons

Afordman31

Afordman31

Well-known member
I have various used pistons that don't have the size marked on the top. My question is were do you mike them, the ring lands seem to be smaller than the skirt, so it looks to me like the skirt is were you get the measurement. I have miked a couple pistons and the skirt is different from top to bottom.
If a standard piston is 3.8745 and you have a .020 oversize piston should it read exactly 3.8945? Any help would be greatly appreciated. Thanks
 
I always mike the full skirt pistons at right angle to the pin holes. (slipper skirt pistons are a horse-of-a-different-color) As well, the skirt diameter should be consistent from the bottom to just under the oil ring groove. Ring land area is always smaller in diameter. Check ring grooves with a new ring for excessive wear - especially the top groove. On used pistons expect .001 to .002 wear on the skirt. This can be stretched by a process called knurling out to a standard oversize. At today's shop labor rates, reconditioning a used piston for re-use is cost prohibitive.
 
I just checked 3 different pistons and all were consistent 1/4" from the bottom and at a right angle to the pin. I also pulled a new box off the shelf and the sticker on the box suggests measuring pistons in this manner.
 
As van Dyke mentioned, things are reversed these days: Used to be you had a ridge reamer to remove the ridge in the cylinder above the ring travel so the pistons would come out without breaking the ring lands. First, you honed that worn, tapered cylinder. Then you honed or reamed the rods and pistons for oversize wrist pins. Next you knurled the piston skirts. Finally you cut the top ring grooves wider and installed spacers. Every machine shop and many garages had the necessary equipment to do all this. So you just had to buy rings, wrist pins, and ring land spacers plus pay the labor to do this.
It was still significantly cheaper than buying rings, pistons and pin bushings!
 
The skirt actually has a slight barrel shape ) to it from top to bottom. The piston should be measured 90deg to the pin and usually 1/2" up from the bottom of the skirt (this measuring point can vary with different engine pistons and manufacturers). The ring land will be smaller then the skirt and usually they are tapered \ from bottom to top. This is so they will be straight | once they are at running temperature. The +.020" should measure 3.8945" as you said. The specifications show a max piston to wall clearance of 0.002" so with the +.020 bore size being 3.895" your piston could be anywhere between 3.893"-3.8945" and still be within spec.



http://macsautoparts.com/ford-model-a-specifications-page/camid/MDA/ca/215/#engine
 
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