Hi Ant, your showing symptoms of hot air disease (good on you). Sorry I'v no drawings of my free piston motor, it's down for a rebuild. There is an artical in Model Engineer 1st of Dec 1995 with a diagram that could be scaled up to maybe 1" to 1 1/2" bore.
My first attempt was a BETA type, but I found it a bit of a hassle fiddling with the springs, so I put it asside and built a GAMMA one, and as it has a linear alternator I found that the surrounding structure must be non magnetic. Thinking back, this was the main problem that prevented the first motor running properly.
If you can get onto the stirlingengineforum, there is a resent thread about thermo accoustic motors, with vidio, It shows one running without a crankshaft, as a free piston motor. Ian S C