Caterpillar pistons and sleeves can be had at 6 and 8 inch diameters, but they're much more expensive. And less readily available. I started looking at the Chevy pistons simply because of their availability, familiarity, and cost.
When I was investigating Stirling engines, the smallest piston I was considering was 8 inch. Recently, the cost of stock has gone through the roof and many sizes aren't readily available. This is what turned me toward the car pistons and sleeves. (Plus, the match-fit machining is already done, and you could easily have 30 spares laying around without emptying your wallet.)
But I totally agree with you on the pressure. I'd prefer a max of 100 PSI or so. To compensate for using the higher pressure (due to the reduced piston size), I was thinking of "just in time" steam -- meaning a very small capacity boiler. The steam must be produced at the rate of the engine anyway, so why not minimize storage volume. Also, the crater in the backyard would be smaller. (I figured an excess of relief valves along with regular maintenance would help keep it safe.)
Personally, I wouldn't run it completely unattended. Also, batteries would be charging anytime the engine is running (until they go bad). 20HP is the goal largely because of my TIG welder -- but anything above 5HP would be at least enough to run my lathes, mills, perform thin-metal welding, operate various power tools, LED lights, etc. And while 5HP is acceptable, I'd prefer 10HP because of the rating of my 3-phase converter.
I have been working in semi-dangerous environments for many years and make safety a huge priority. The dangers you cite were the reasons I stayed away from steam. I spent 6 months trying to concoct a Stirling engine (using data from everywhere). But to really get the thing to work you need to have 4 pistons (if using a Beta design like Ford and Philips built), or you need a massive double-acting piston like Robert Stirling's 1847 patent describes. Further, hydrogen or helium is the prefered gas with many atmospheres of pressure, etc. etc. Nothing comes easy.
Also, I was thinking the boiler and the engine are within inches of one another. I have read a few things which talk about the loss from the boiler to the engine. Since this is a matched-set, I don't see any need for a great physical separation between the two. I'd prefer to put it all on a single frame with appropriate isolation for vibrations.
Lastly, the oiling would be injected between the back-to-back pistons such that they see oil in the same manner they see if from a gas engine. However, oil is bound to get around the piston rings and contaminate the steam. Initially, the steam will be vented and lost. Hopefully it can perform one or two final jobs before being exhausted. That said, I hope to recover and condense it as a second project.
Sorry this is so disconnected -- random thoughts. I was also planning to have the engine heated from the same rocket stove. Perhaps even route a flue around it or through it to minimize differences between the steam temp and the engine temp. This likely means an overall reduction in steam temperature.
So the design is currently at the push-pull stage where all things are being considered, with safety, maintenance, reproducibility, cost, and effectiveness -- in that order as the goals in my mind.
BTW, having said all that with the vehicle pistons -- I can certainly make these items instead. But getting a sleeve and piston from a high-production vehicle means that the materials are matched not only physically, but also thermally. And datasheets are available for them.
Finally, I did consider more pistons, but this complicates maintenance, reproducibility and cost. That's why I looked at the higher pressure. As you know, everything's a trade-off.
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