Hi all I am (slowly) moving towards building some unaflow engines and am in need of some advice. 1. In a (simple) unaflow, the exhaust gas that is not evacuated through the exhaust ports is compressed to a maximum at TDC. Presumably this pressure exerts a braking force on the piston returning, and is undesirable, though I've read that some say its useful to reduce shock ie on big end. (I know some designs used an 'auxiliary' exhaust valve.) Presumably that pressure can get up to steam supply pressure, in which case, steam would not flow until the piston had dropped till internal pressure was lower than steam supply pressure. Presumably also, a cavity must be left above the piston, because if not, exhaust gas would be compressed infinitely - and the engine wouldn't work. I'd be very grateful to hear any opinions or experiences on this matter. 2. Clearly, having a condenser on the exhaust ports would assist in drawing the maximum exhaust gas out at BDC. Any suggestions on condenser design? 3. A simply exhaust port would be a round hole about BDC. But that would take up stroke length. More, smaller holes makes better sense, maintaining or increasing cross section area of port and minimising stroke used up. Optimal exhaust ports would...presumably... occupy as little length in the cylinder at BDC as possible, allowing maximum length power stroke. By the same token, the should occupy maximum of the cross section. But of course there has to become surface for the piston/rings to move on. So what proportion of the circumference should be left solid? And what should be cut through to function as ports? I'm imagining long slots, occupying about 1/3 of the cross section of the cylinder we all. Does that sound about right? (I hope that makes sense).