Purchas this engine from http://www.ministeam.com

MiniSteam

The Monahan Heron, is a double acting, slide valve, twin cylinder 3/8 in bore, 3/8 in stroke steam engine. We have chosen to use a Stephenson linkage for reversing due to the smooth ease of motion, which is great for servos. Not to mention they just look great. All steam intake and exhaust outlet thread sizes are 1/4 in-40 and use 5/32 in copper tubing for plumbing.

This engine is for a live steam model boat launch such as the Krick Borkum