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jcwright 04-27-2017 10:23 PM

Notes on Overhauling an Atomic 4
Hello Afourians.

I am writing to thank Don Moyer, MMI, and all of you for your contributions to the forum, which made it possible for me to overhaul my engine. Without the advice you have assembled here, I would never have attempted this job.

Below is a link to my notes on the overhaul, which I hope expresses my appreciation for your help.

Best regards,

Jack Wright

Note on overhauling an Atomic 4:

lat 64 04-28-2017 01:51 PM

Hey, great document.
It should be a good example to others.

I ran through it fairly quickly and noticed you were not sure about piston measurements.
Pistons are not round at all, but often have complicated shapes to allow for different rates thermal expansion in hotter areas during running.

The only real critical measurement for determining piston wear is the widest dimension of the piston skirt. That is at the bottom of the skirt, 90-degrees from the wrist pin axis. This is where typical(normal) wear occurs most and clearance* is smallest. This area is the bearing surface when the piston is pushed down the cylinder. This skirt-size measurement, plus clearance, is what a machinist will use to determine the size of the cylinder when fitting new pistons in a newly-bored cylinder or determining acceptable wear in an "overhaul".
Sometimes measuring at the top of the same area will get you different measurements even on a new piston.

*piston-to-cylinder wall clearance, Measured by subtracting piston diameter from cylinder diameter.

Cheers and salutations,


BunnyPlanet169 04-28-2017 04:06 PM


Originally Posted by jcwright (Post 105492)
From the Document: "The overhaul was a case of mission creep."

Jack: Welcome to the forum, and what an epic first post!!!

Absolutely beautiful job on the engine. Nice work.

Stick around - won't you?! :D

jcwright 04-30-2017 12:08 PM

Russ @Lat 64:

Thanks for the details on piston measurements. There are so many things I wished I'd known during the overhaul. In hindsight bringing my questions to the forum as they came up would have been smarter. And more fun.

Your note helps me interpret Hess' comments on this:

"Piston skirt diameter (bottom of skirt near expansion slot at right angles to pin). Note: pistons are not round when at room temperature because they are cam ground oval so they expand to round when at engine running temperature. Std: 2.560 in."

I wasn't sure how to interpret them at the time, but my (average) measurements for skirt diameter at that location were: 2.553, 2.554, 2.559, and 2.560. Hopefully I didn't make a major mistake in not getting new pistons.

Jeff @BunnyPlanet169:

Thanks for your comment.

Yes, there was a lot of mission creep here. And I had some explaining to do when the family's CFO started asking questions about rising mission cost. :)

Best regards,


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