Unsticking a stuck motor

Bought this 76 kz400 and the motor is stuck. I'm not an expert on motors but I understand the basics for the most part. I just wanted some insight on some things to do to get it unstuck. It's just been sitting for about 10 years. The guy I bought it from said he got it turned over about 2 years ago with good compression But he never rode it just let it sit up until now. Goes through all of the gears and will roll in neutral no problem. Penetrating oil has been in the bores for about a week but its still not breaking loose. I've tried putting it in its highest gear and rocking it back and forth, And turning the crank shaft. Nothing. Do I just need to keep trying those things or should I pull apart the top end? Any insight? Thanks

Sounds like you have already done what you could. Time to pull the engine apart.

Probably the bore or rings are rusted so the damage has already been done. You might have just one pot really bad.

Got the head off. The bores were a little rusty but there was a ton of gunk on the pistons, I cleaned em up the best I could, I got one piston to move slighty but the other one is stuck. Ive tried taking the cylinder off but Its not budging

I've used a leakdown tester to free a stuck motor once. It's a bit of a redneck solution but 100psi in the cylinder got a stuck piston moving for me. There may be some safety concerns but if you can get the head on and mostly sealed up on that cylinder it could work.

Right, Ive heard of that method. I got one piston free the other one is still giving me a hard time. Ive used dot 3 brake fluid in the cylinders and it seems to be working...slowly but surely.

Acetone and ATF in half increments makes good penetrating oil. I'd puddle the stuff over the pistons and let them soak.

I used Marvel Mystery Oil to free up a tow behind compressor once. Had the engine sit for about a week full of the oil.

Really, marvels is the best with acetone or lacquer thinner.
But think past this. the whole engine? Most likely shot!

Take a block of wood and tap on the piston while putting upward pressure on it.
Steve

My old daewoo once blew a headgasket and so much coolant got into the oil that it seized the engine. I towed it up a hill, put it in fifth and got up to speed to roll start it - it took a couple of tries before the engine started turning over again. I then changed the oil and put some chemi-weld in the coolant, and I drove it daily for another year

If the engine is seized you can get it running again - but if you care about the engine, you really need to strip it down. If you can get it running you're just delaying the inevitable - you will need to strip it down later.

At this point the idea is to get the pistons out and assess the damage. If you can get the block to separate and wedge it up just slightly, you may be able to drive the piston out a little at a time.