So I'm disassembling a very rusty old 425. A mud dauber had made a nest in the #2 cylinder. As such, it is very rusty.

I've gotten rid of the rust with a combination of acid and penetrating oil. But the piston is still stuck. I mean S-T-U-C-K - like beating on it with a 4lb. dead blow as hard as I can stuck. (No I am not concerned about saving the pistons.)

I've read lots of methods, but I'm wondering what people here have used that really worked. Right now I'm leaning toward cutting the piston out, but I'm a little concerned about gouging the bore.

 

I'd take a chisel and start in the middle, parrellel with the pin.
Get the rod and pin out first, and then break-out the pieces.
Unless you've access to a fire saw - start in the middle!

 

Unfortunately I don't have acetylene. Is MAPP hot enough to burn it out?

 

I'd be cautious about using heat to shift the pistons.

I've done this job a few times, a 6lb lump hammer and a length of solid drift does a better job than a dead blow, this is no time for finesse!.
Once the piston has moved a fraction put more penetrating oil on and knock it the other way, repeat until it finally comes free.

Roger.

 

Yeah, I'm going to run by HF and get a proper sledge. I just happened to have an old dead blow that was already pretty beat, so I just finished it off. I've had the motor upside down, using a steel rod to beat on the underside of the pistons.

Did some Googling. Several guys in the vintage tractor crowd suggest filling the cylinder with a 50/50 mix of acetone and ATF, then burn it. Supposedly it heats the bore and expands it enough to allow some of the concoction to seep down to the rings. I'm a little skeptical since it would seem the piston expands too.