Re: K-27 Running Gear/inside and outside admission

Inside and Outside admission refers to the relative position of live and exhaust steam to the valve seats. A flat slide valve is an outside admission valve in that the live steam is on top of the valve exerting pressure downward on the seat and out to the steam chest surfaces and against the valve rod packing.
Piston valves can be built either way - with the live steam on the outer edges of the spool and working on the valve chest covers and valve stem packing (outside admission) or the live steam can be introduced into the center of the spool and the exhaust steam is on the outside and against the stem packing (inside admission).
Most piston valve applications were inside admission to avoid the problems of keeping the valve stem packing tight against superheated live steam. In addition I belive there is a balance problem inherent with outside admission piston valves.
Some locomotives that recieved after-market piston valve conversion kits (such the IRCA 2-8-0's at the Loop and Nevada Northern #40) utilized an outside admission piston valve bolted to the original slide valve seats.
In converting a slide valve K-27 to an inside admission piston valve, the valve gear would have had to been changed to operate 180 degrees out from what it had to do with outside admission. This could have been easily done by changing the eccentrics on the axle.