Here are Puff1's specifics:
Gears: Tamiya Gearset (modified). The gear set was in my junk box .. if you have to buy it, that will cost $14.95. You could probably make your own from some model car gears (the hobby shops have tons of them). You will have to make an adapter for the steam engine in any case. This one has a brass plug where the motor housing snapped in and it uses piano wire for a drive shaft to the gear set. The pulley, drive shaft and brass plug were fabricated.
Wheels: Left overs from the Skinny Project (see Skinny)
Cost: Left over parts and a few pieces of hobby brass = $10.00 (if you don't count the $7,000 worth of shop equipment to make the steam engine).
I, very briefly, thought of putting a boiler on Puff to get him off the air tether that he currently runs on. But something about a live steam boiler with pressurized fuel burner toddling around the house or garage seemed to tell me the Darwin Awards people would be looking for me if I did it.
Puff, running on about 5 PSI, seems very reliable. I plan on putting a neural network on him and hope to someday make him self replicating - perhaps even sentient...... ;-)
Close up of the engine.
Real Movie of Puff1 Puffing