Here’s a cool little video from HowToLou.com (I’m guessing this guy is named Lou) with a third prototype of a homemade engine made strictly from hardware store parts. It’s not quite working yet, but it’s rather interesting and I think he’s on the right track.
I’m more electronically inclined than mechanical and have some ideas on how to make the timing of the spark and the exhaust valve better, using electronics. But I probably couldn’t do it with just hardware store parts, unless perhaps the hardware store sells radios that I can scrounge parts from.
One of these homebrew engines coupled with a homebrew generator connected to a QRP rig would make a nifty little project to show at your local ham club or field day.