Updated Sketch Up model and piston assemblies

I have shared the model in Google and will upload it to the web site. The pistons now have 4 nylon screws that protrude through the cores and shell to keep its base centered. This should prevent the piston crown from scraping the plastic seal, which is just a polyethylene bag, because the pressure on it should center the front.

The seal bag is not included in the model, I am still trying to figure out how to represent it.

The current heat exchangers are also shown attached, but without the hoses for the hot and cold water.

Added the piston core caps and the push rod guide's bearings. Push rods have cutouts to allow snapping them on and off the wrist pin, to make changing the bags easier.

It now pretty much represents the current state of the real prototype. I still have to rebuild 2 more of the pistons to install the new shells with 50 mil clearance and the Nylon spacer screws.

I hoping once back together this time, it should be able to hold pressure long enough it to get it run. It has a low displacement cam, so should run with a pretty low temperature difference ~40 degrees F. So far I only have one pair of heat exchangers, which I pump ice water through. Still need a heating means for the hot cylinders. I was just heating the heads with torch, but since most of other parts are plastic or rubber, that is not a good idea. I may just use some high wattage resistors, it would be less messy than using hot water.

New insight into bag seal failures

I looked at my collection of leaking bags. The hole were close to crown of piston or located where the crown of piston hit the bag.

Plan: Make the piston outer shell smaller to allow more clearance between the piston and cylinder.
Previous version had 20 mils. Turning new set with 50 mil gap.

The pressure on the bag centers it in the cylinder but the piston can tip causing the crown to scrap the bag. To prevent that I am adding some diameter to the tail end of piston. For now I am going to use nylon screws that would also attach the piston outer shell to core. Later may use Teflon ring fitted to a groove at the end of the piston.

Status: Turned shells, still need to drill the holes for the attachment screws. Going to make another piston assembly for my piston test fixture. That way I can see if the piston twisting problem has been fixed.