Easy way to make a piston

[tmk]

Just thought i'd share with the group here.

i am (slowly) building a stirling, and i was in need of a piston. The copper + epoxy idea seemed ideal, but copper pipe is expensive, and i don't know any plumbers i can bum scraps off of. Besides, i don't have a good way to push them out once the epoxy is cured.

So i went a similar route; I used PVC pipe and wax paper:

* cut a ~1" length of PVC pipe, 1"od, 3/4" id (you can use whatever size you want)
* cut a bit of wax paper into a strip a few inches long and 1" tall. The longer the paper, the more times it will go around the pipe, the smaller diameter the finished piston will be, and the looser it will fit in the cylinder when it's done.
* roll up the wax paper (carefully! don't want wrinkles) and put it inside the PVC pipe. It should be flush at the bottom, and stick out a tiny bit at the top
* put that assembly on flat piece of wax paper.
* bend a bit of coat-hanger wire into an "L" shape and skewer a bit of packing peanut on the wire. the peanut is just to take up space and keep the weight down. It's totally optional
* mix the epoxy on yet another piece of wax paper, and make a little piping bag out of it. (like for frosting or cake icing)
* squeeze a bit of epoxy into the PVC with the wax paper(just to coat the bottom), put the wire/foam in, then put a bunch more epoxy in.
* twirl the wire (counterclockwise for me, but it depends which direction you rolled the wax paper) so the wax paper unrolls as far as it can go.
* move the wire up and down a bit.. this removes air bubbles, and should also push the epoxy up the sides, which will create 'wings' all the way around the cylinder without anything in the middle. This effectively makes your piston longer without much weight.. longer piston should = better seal but slightly more friction i would think.
* clip the wire to something to keep it centered, and come back after your epoxy has dried
* the epoxy piston should push right out (Wax paper is slippery!). you'll need to remove the paper and clean up the edges a bit, and afterwards you should have a nice piston. Use a different (longer) piece of PVC for the actual power shaft..

Granted, this leaves you with PVC pipe as a piston-shaft, but you could do the same thing with copper or any other metal i suppose.

I got 3 pistons out of a small hardware-store tube of epoxy.

Also i haven't quite figured out how to make a good joint to the rod that connects it to the flywheel, but that shouldn't be too hard.

I was thinking a bobby pin might work better than coat-hanger wire for the attachment point, but anything will do really.

-tmk

[egriff]

I'm going to try this for my first piston

[EL34]

Hi tmk,
Excellent, well written instructions!

Just asking about the expoxy.

JB weld is the only epoxy I know of that can take very high heat, and I have seen it burn at very high temps.

Most epoxies will not take any sort of heat.

What type of epoxy did you use?

[CanadaPhysics]

The best type of piston is a metal piston from another device. I have not had the epoxy burn but at high temperatures it has gone soft (permanently) and deformed.

What I got the best results from was a piston from an air conditioning compressor. I went to a couple of different mechanics shops and asked if they had used pistons laying around and they both gave me a used air conditioning compressor. Each compressor had 7 pistons (that makes 14 free pistons) in it so there was even room to screw a few of them up during building. These pistons also have a very low friction rod connected to them. It is not long enough to be used alone but makes for a great pivot point.

The piston did not fit perfectly in the cylinder so I put o-rings in the grooves where the original seals were. To get a perfect fit o-rings that were just to small were obtained then one or two wraps of Teflon tape were used to bring it up to the correct diameter.

If I have learned one thing from building a Stirling engine its don't cheap out on the piston. The rest of the parts can fit loosely but if the piston doesn't make a good seal the engine will not run.

[EL34]

I have seen quite a few post where people have had great success with JB Weld power pistons that were formed in the same way as described above.

JB weld is hard as a rock and very high temp.

Fairly easy way to make a close tolerance piston with JB Weld.

Regular two part epoxy will just soften up or burn.

[Aviator168]

You can buy very short (1.5 inches, 2/3 inch ID) copper joins for less than 50cents at home depot. I grease the inside with vaseline before stuff it with eproxy.

[Bidwell]

Pictures!

[jimlarsen]

JB Weld slow setting epoxy claims to hold up to 500 degrees F. JB Kwik says it will hold until about 400. I suspect that the PVC may become soft before the epooxy does. I have used other epoxy that was not rated for high temperature and I did not have good results. JB Weld is great stuff. There are a few competitors on the market that make similar claims, so it pays to read the package if you can't find JB Weld on the local shelf.

[Ian S C]

I'v never used epoxy/ JB Weld pistons, but if the power cylinder is thermally insulated from the displacer cylinder, it should be OK even in a fairly high temp motor. Other wise a small meths lamp, or a candle should do.
Canada Physics has the idea, rather than use an O-ring with teflon tape to pack it out, I would just use the teflon tape, or you can get teflon packing. The teflon has much lower friction than the rubber of the O-ring. Better than that is if you can make a cast iron piston that will fit a steel or cast iron cylinder with out any packing. Ian S C