A short story...

About 20 years ago, I was sitting around one day staring at an engine, and it occurred to me that there was one part of the engine that I had not checked in every way: the piston!

For a number of years, I had been checking (and re-machining and straightening) all the components of an engine -- (lineboring the main bearing bores, squaring the case deck to the crank centerline, squaring the base of the cylinder to the bore, checking connecting rods for alignment, etc.) -- but I had neglected to check the piston itself for square!

Like many people, I had measured pistons for size, shape, clearance over the small end of the rod, wristpin fit, etc. -- but I had also simply "assumed" that a piston was a "good" component. I was wrong! (what I'm referring to here is how "square" each side of the piston is to the wristpin bore -- or more precisely: how "similar" the two sides are when comparing them to the wristpin bore)

The tool...

Here is the tool I made: (more pictures at the end of this article)

The purpose of this tool is to check how each side of the piston relates to the wristpin bore.

I made the main "beam" out of a piece of 1" wide aluminum... around .450 thick (1" x 3/8" flatstock would probably work fine). This one is about 5" long (more as a convenience to hold rather than out of necessity). I milled a ¼" slot down the center of the aluminum "bar". I also drilled and reamed a .375" hole for the indicator shank, and then drilled and tapped a ¼-20 cross-hole for a setscrew to clamp the indicator shank. (Don't forget to grind the end of the setscrew smooth and flat so it doesn't butcher the indicator shank!)

I got fairly carried away making the "pin" that goes in the wristpin bore; mine is made out of a piece of heat-treated tool steel. I ground this to the correct size in a cylindrical grinder (just .0001 or .0002 clearance on a standard wristpin bore), and then kissed the "shoulder" that would be touching the piston. If you look at the photo showing the pin, you can see that there should be a "relief" down in that corner so that there is no chance of this pin "hanging up" on the outside corner of the piston. The shoulder needs to contact the outside face of the piston and nothing else. The larger end of the pin is tapped (¼-20) so that it can be attached to the aluminum part.

On the pin I made, the wristpin diameter is around 2.125" long, and the larger part is ¾" diameter and about ½" long.

The indicator I used was a Mitutoyo model #1506 with a 1½" face. The shank is 3/8" diameter, and this indicator reads to .0005 with .125" travel (not much travel required, obviously). It's easy to see .0001 or .0002 on this indicator as the .0005 lines are quite a ways apart.

Simply position the "wristpin" in this tool so that the dial indicator contacts the piston just under the ring groove.

How to use it...

Pretty simple, really; just slide the pin into the wristpin bore of a piston, (holding the shoulder of the pin tight against the piston) and rotate it back and forth to find the "high spot" on the indicator. You can zero the indicator there if you want... then just pull it out and put in into the other side of the piston, and rotate it back and forth to see the high spot on that opposite side. Note any difference in indicator readings from one side to the other. That's how far the particular piston is out of square!

Some pistons are perfect, and some are a ways off. Since the distance being measured is quite short (basically from the center of the wristpin to just under the ring groove), a difference in readings of .0005 (five tenths) from one side to the other actually represents a fairly significant amount.

Here's the bad news: what do you do if you find that a piston you have is significantly out of square? Well... you probably set it aside and find one that checks a bit better. Hopefully this article doesn't start a "run" on piston sorting that leaves many pistons on the shelf!

Here is a QuickTime video (with audio) of the tool in use. (12 meg)

Or if you'd prefer a smaller file but lower quality, here it is in Windows media format. (2 meg)

Additional photos...

Click on any of the photos in this article to see a larger version of that photo.

Note: the reason my piston checking tool looks a bit "beat up" is simply because it's about 20 years old.