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Ok, so I've decided on 2" piston with a stroke of 2.5" (for some unknow reason). And a Displacer 1.75" diameter and 5.25" long (I think I used someone's ratio).

If there is something wrong with those dimentions right off, please speak up.

Now what to build a chepo prototype out of to see if those sizes are compatable with one another. If the thing works at all, even for a minute or so, I'll build a better quality one out of better materials and closer tolerences next for further testing.

In the meantime, I was thinking of muffler pipe for the cylinder and piston, with no rings. I could cut the end off of a reducer and weld the ends closed for the piston, then hone the crap out of the inside of a 2" muffler pipe for the piston to work in. Maybe Graphite for a lubricant on the piston so it will at least work for that minute or two. Does that sound like a reasonable cource of action? I don't know of any other pipe that fits together so nicely without costing me an arm and a leg. I want to stay cheap for the prototype because I'm sure changes will have to be made.

The displacer is another matter though. I could probably scrounge up some thin aluminum of the size stated but I've never welded aluminum before and don't have the means necessary anyway. But I do have a 115v wire-welder and can get aluminum wire for it. Would that welder work for making a Displacer? Or maybe I could just plug the ends of the Displacer with anything I can find, JB weld or the like?

Any suggestions?

I'll start with the displacer construction, don't waist your time with aluminium, first off; it conducts heat too fast, the idea is to keep the hot at the hot end, and the cold at the cold end. Second; if your motor is to work at its best, it will get too hot, the displacer will, if not melt, will collapse, I know, I tried three of them on one motor before I got the idea and changed to stainless steel. Mild steel will do quite OK.
Your power cylinder needs to be accurately made round and parallel in the bore, and the piston then made so it's an easy fit, it should slide down the bore under it's own weight, and when you block the end it should almost stop.
The swept volume of the displacer should be 1.5, and the swept volume of the power piston is 1, ie., a ratio of 1.5 : 1, you might be best with your measurements to reduce the power piston stroke, or else the displacer stroke is going to be very long. An example, both cylinders same bore: 2" bore, power piston 2" stroke, displacer 3" stroke, and if the displacer is reduced in diameter, the stroke must be increased to maintain the volume ratio. For the displacer cylinder exhaust tube will be OK, stainless the best, but ordinary steel will do. You can MIG weld a disc on the hot end of the displacer, and it's cylinder.
Ian S C

Let me see if I have this right.
A 2" diameter Piston with a 2" stroke will displace 6.28 cu. in. of air. (pi r squared h)
If the ratio is 1.5:1, then the Displacer needs to displace 9.42" cu. in. of air right? (6.28 x 1.5 = 9.42)
So then my 1.75" x 4" Displacer with a 4" stroke displaces 9.62 cu. in. of air. Close enough?
Do I have it right?

Close enough for me. Ian S C