Unsiezing - Richard / Andrew

Jon Dalton /

No amount of whacking, soaking or torching would budge my siezed Motobecane engine, but here is what worked: I put the head back on, filled the cylinder up with water, put the spark plug back in and took the torch to it and left it there to heat up the water. Knowing from thermodynamics that water at 1500 degrees and 1 m3/kg has a pressure in the range of megapascals, something in there is going to want to move. I hoped that it would either move the piston or force the water / steam in through the piston / cylinder seal where the siezure is. I ended up losing it through the head gasket seal and the decompression valve, but when I took it back apart and hammered it again, the piston moved, and I wasn't hititng nearly as hard as I had been earlier.

I tried soaking it in solvent overnight, but it didn't make a difference. I can see how it would break down corrosion in there but if there was cold welding between the piston and cylinder, it wouldn't be affected. I don't know if this was the case or not, but based on what the piston looks like now, I'm pretty sure it was corrosion of the aluminum piston. I don't see how torching and then whacking would help because the aluminum piston would want to expand more than the steel cylinder sleeve would with the same amount of heat because of its coefficient of expansion, so you would somehow have to get the cylinder a lot hotter than the piston. I do see how the uneven expansion would create stresses that would break down the bonds, so what I did the second time was heat it up really good and then quench it with water. The piston still wouldn't move, even while bringing the hammer down with both hands from over my head, as hard as I could. It just split the block of wood in half and sent one half of it flying.

For curiosity, I tried to find the pressure that would result from water heated to 2500 degrees farenheit while maintaining the same specific volume. My thermodynamic tables from school didn't go up that high, and nothing on the internet did either, and to interpolate from 500 degrees up to 2500 won't get you anywhere close to the accurate result. Does anybody know where I could find this information? I know there had to be a great deal of pressure in that cylinder to get the piston loose.

Re: Unsiezing - Richard / Andrew

Kevin Harrell /

Good thing that the presure went out where it did. If it did not go out that way, or by pushing out the piston, there could have been an explosion.

Re: Unsiezing - Richard / Andrew

Ron Brown /

Jon.

I don't think I would recommend your method to anyone. Heating water in a confined space is very similar to throwing aerosol cans in the campfire. You may get away with it, but is it worth the risk?

As for the numbers you are throwing around here. you may want to look up the melting point of aluminum before trying to raise the water to 2500f.

I suspect that the water in your cylinder never reached a very high temperature. As soon as the head gasket began to leak, most of the energy would be used in converting the water to steam.

Anyway, I applaud your ingenuity and I'm amazed that you did not do yourself some severe injury.

Ron

Re: Unsiezing - Richard / Andrew

What about the same thing but in reverse...freezing? The big prblem I have is that the exhaust port is open about halfway so I can't keep in steam pressure. But I could colt a plate on it that "shoud" keep in the water enough to let it expand.

What do you think?

Re: Unsiezing - Richard / Andrew

Ron Brown /

Richard,

If you are not interested in saving the piston and the exhaust port is partially uncovered. Have you considered chipping away at the piston top and skirt with a cold chisel until it breaks free? Possibly helping it out by chain drilling through the piston top until you can knoch a chunk out.

Ron

Re: Unsiezing - Richard / Andrew

Yep. That's my method of last resort. I would ideally like to save it all, but the piston is the most sacrificial part. I do need to save the crank, con rod, and wrist pin and i'd like to keep the cylender as well. I'm going to give the freeze\thaw trick a tray and see how it goes. If it's no good, bood bye piston

Richard

Re: Unsiezing - Richard / Andrew

Jon Dalton /

Well, if Ron Brown says it's a bad idea, then it's probably a bad idea. I'm just glad it worked for me and my motobecane engine is not in 1,000,000 pieces, or a molten puddle of aluminum on the floor.

If you aren't worried about the cylinder, I would go ahead and try the ice method. The only thing I would be afraid of is cracking the cylinder because of the low temperature brittleness and the ice becomnig non-fluid. If that happens then it would still help get the piston out of there.

As for the risk of explosion, this is why they put safety valves in steam-powered espresso makers - heating water in a confined space isn't exactly safe. I was fairly confident that something would give way before it exploded.

Is there any way you could plug up your exhaust port? If it uses a bolt on flange, you could bolt on a piece of plate steel, or if it's the kind with the big nut that screws into the port, maybe you could find something round that would fit in under the nut. If your transfer ports are uncovered, that's a different story.

Did you try the heating and quenching method? I may be corrected but it really seems like that would work the best. If you just heat it, I'm sure the piston would expand even more into the cylinder walls. It takes a lot of heat to raise the temperature of that cylinder, considering the mass of it and the fact that it's designed to conduct heat away from the cylinder. If you torch it for a really long time, it could make a real difference. I found the best way was to set the torch up so that it's blowing diagonally down the side of the cylinder, and you can see a swirling of the gas all around the cylinder wall. Then you can switch sides to heat it more evenly. You could also heat the cylinder from the outside, and hopefully get more heat to the cylinder and less to the piston.

Good luck. After all this it should feel really good when the piston does come out.

Re: Unsiezing - Richard / Andrew

Jon Dalton /

It seemed like a good idea at the time. Being an engineering student, the space in my brain that would be allocated for common sense holds more technical jabble instead.

Re: Unsiezing - Richard / Andrew

Well..I took it into a motorcyclem machine shop. They said they could probably do it but they wouldn't guarantee that the parts would survive. They also required a minimum shop fee of $30 even though setup and the work would only be about 5 minutes. I sa the rig they would have used and I have a welder and a bunch of steel so I'm going to try and see what I can rig up. I think that I can get the same results as the shop, with the same guarantee for $30 less. I'll let you know how she works.

Richard

Re: Unsiezing - Richard / Andrew

Nope!!!! No good. I stood on the jack handle and hopped a little. I'm 260lbs and if THAT won't do it, it's not getting done at home.

Richard

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