Finally got around to posting this up. Here's the Joy valve linkage up and working. I used a screw jack system to model the reversing engine. I used the Joy valve linkage because it doesn't require an eccentric, but instead derives its motion from a point on the connecting rod. Here's a good link with animations explaining steam valve linkages: http://www.lner.info/article/tech/valvegear/valvegear.shtml