Tight piston skirt

YES!  That's a Davis piston holding fixture that causes the piston to "mushroom out". After expanding / ruining a piston or 3 using that fixture
I decided to try re-setting pistons using that tool without the fixture. IT WORKS! I threw that fixture away & have had no problems since...
I now use a flat surface on my steel vice as a platform for placing the piston on top of.
You raise a most interesting point about exactly WHERE the expansion is on the piston. I (perhaps wrongly) assumed the expansion was at
the piston crown, but never thought to measure it.
If you have no other pistons to swap with, it is possible to save the piston with some light lapping, assuming of course you're able to do that.

I almost wish I hadn't thrown out my holding fixture because I have the odd good cylinder that NEEDS large pistons of which I don't have, so perhaps
this might allow expanded pistons to work...

Good luck.

Cheers,

Paul  (aka Mr. Open Mouse)

p.s. When I refer to "large" pistons, I'm only meaning 50 to 100 millionths of an inch larger.
pps. All the above is ASSUMING the cylinder is OK.

pkrankow wrote:You inadvertently put a burr in the exhaust port with the wrench.

The reset sounds like the right way to do it more or less.  As long as you did not hot the skirt directly the skirt did not change dimension.

Phil

I'm trying the piston in a spare cylinder with a good fit to it's piston, Phil, so no burrs.
The spare piston's crown and skirt both measure .4058", so it's definitely the skirt which expanded somehow.
And the expanded piston fit it's cylinder properly before resetting.

From now on, Paul, I'll use my vise instead of the Davis base - thanks for the tip!
I also assumed the top until I tried to fit it upside down for lapping.
Surprise, surprise!  
I can't imagine how it happened.
And if anyone runs into a tight piston after a rod reset, check the dimensions to see if the crown or the skirt expands and please let us know.

p.s. - I'm using dial calipers and interpolating the .0000ths.

Listen to Paul, he truly is an authority on these engines.

I never heard of the tool expanding the SKIRT before. I have had a burr in my exhaust before and I am glad you do not.

Phil

See this thread on wrench fit/cylinder burrs:
https://www.coxengineforum.com/t10250-wrench-exhaust-web-question#129657

This is the one I have:


It has a raised boss in the center for the crown to rest on, but I still have ruined many of the older ones I've tried to reset. It takes the perfect firm Lick to get it right, and older work-hardened ones are most difficult to do without distorting. I have the most success with new ones that have loosened during break-in, and that's the last reset I ever give them.
Rusty

Do your rods not loosen in use or do you just not reset them again, Rusty?