Remove Stuck Piston From Cylinder Bore

Article by Mark Trotta

There are several ways to remove a piston frozen in a cylinder. Here's what worked and didn't work for me.

remove stuck piston in small engine

What Didn't Work

First, I tried filling the top of the cylinder bore with penetrating fluid (PB Blaster) and letting it soak for a few days, but that didn't seem to do anything. I also tried WD-40, but that didn't seem to work either.

After those two penetrants didn't work, I tried Marvel Mystery Oil, but it was too thick to penetrate past the stuck piston rings.

remove stuck piston from bore

Finally, I tried plain household vinegar, which surprisingly worked better than the other fluids. Not only did the vinegar penetrate past the rings, it also dissolved some of the built-up engine sludge.

Patience Is The Key

After letting the vinegar soak in the bore for a few days, I checked on progress, filled it up to the top of the bore, then waited a few more days.

After two weeks of letting the vinegar soak down and then refilling it, I decided it was time to try the hydraulic press.

remove frozen piston from small engine

Using only minimal pressure with the press, the piston moved slightly in it's bore, perhaps 1/8 of an inch. I had to be careful, because the aluminum block could easily crack.

So I stopped and played it safe.

I let the piston and rings soak in vinegar for another week.

And then my patience paid off.

remove seized piston from engine

Although the piston assembly came out with rings intact, the 50 year-old rings did not want to come off the piston, so I had to break them off in pieces.

small engine piston removal

The top of the piston was cleaned gently with a soft wire brush ( it should be as smooth as possible). The ring grooves should also be cleaned, and checked for excessive wear with a feeler gauge.

Hone Cylinder

If your cylinder wall has a shiny finish, you'll need to break that glossy glaze and leave a nice cross-hatch pattern. This allows the new piston rings to seat properly.

A 3-stone hone like the one pictured below is commonly referred to as a glaze breaker.

small engine cylinder honing

The tiny peaks left behind by the honing process provides just enough roughness to allow the rings and cylinder wall to wear together.

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Clean and Oil Cylinder Walls After Honing

When you are done honing, clean the cylinders thoroughly and dry with compressed air. After they're completely dry, apply a light coating of oil (10W30, ATF, or whatever) to the bore to prevent rusting. If you're not installing the piston for a few days or a few weeks, put the oiled cylinder in a plastic bag and out of the way until needed.

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You Can't Beat Fun!


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