There was some pretty neat stuff come out of our model engines over the years. O&R used a deep drawn stamped steel piston in the last of their engines. Some thought it was sort of a takeoff on the Crosley car engine that used stamped parts oven brazed together to make up the block. The GHQ however also used a piston of the same technology years prior to the O&R's. It should also be mentioned the O&R developed diecasting to a high degree and produced some of the lighted engines ever made.
Or how about the M&M and Atom engines that had no bypass (transfer) ports? These used an inner piston that formed a valve at the crown. The outer piston would hit a stop near BDC and the inner piston would go a little farther opening the valve allowing the fresh charge into the cylinder.
The Aero .35 with parallel cylinder and crankshaft.
The Rocket (May) used a yoke to attach the wrist pin (gudgeon pin) to the piston, no wrist pin bosses. The threaded portion passed through the piston head and a stamped piston baffle was held on with a nut threaded onto the yoke. I've often wondered how many guys took them apart and put them back together wiht the baffle out of place!
Allot of independent speed and race car guys developed porting over the years. There were some really wild looking engines with their weird looking bypasses. The Doolings were of that ilk with their spherical bypass.
The Bantam .19, Arden .099 & .10, Atom .09, and others used magnesium cases, some were prewar.
Some forgetable technology: The OK .29 with the cylinder attached with 2 little screws through the goofiest looking "wings". The plain bearing Drone .29 diesel, the slag engnies; Thor, Genie, Rogers, Buss, Syncro, Brown, aluminum of the same mystery alloy used for piston and cylinder. They were cheap and very compact for .29s. Some have been run and used on models and SAM even has an event for them. And what forgetable list would be complete without the Gotham Hobby Deezil! A low in manufacturing technology.
I love this stuff!
Steve