In starting off, …a little background: The engine I am building is a large bore , low p.s.i., (40 p.s.i.) low r.p.m. grasshopper steam engine, .using dry steam but no superheat.

Regarding porting design, the inlet steam will be seperate from the exaust steam. I am using the pre existing D slide valve port surface as the base to build a type of spring tensioned swing valve that seats against the outgoing exaust pressure rather than seating inline with the exaust pressure. In this way it will do double duty as a water-slug relief valve..

In order to attain the most effective sealing against vacume loss and because steam temperatures will be well within the opperating limits, I am thinking of using metal backed viton rubber for the valve material. Anybody caring to take pot shots at that idea, opinions invited

Am familiar with balanced D valves, having built one for my double acting 5 x 8 – simple and self seating. Question I am wondering about is weather they leak or how much they leak ( as I've been told they all do to an exent. ,. Probably neglible in most high pressure applications. Am feeling though that the success of the low pressure, large bore grasshopper will partially have to do with maxamising vacume and so I'm casting about for the best seal for the least machining. Also feel that there is less friction and better seal in the stuffing box when valve rod is designed to rotate rather then reciprocate. Of cource the over riding part of the formula is that I just want to do something different.

Indeed a very accurate observation about the inexorable allure of steam and the creative force.