Gordon Smith Safety Valve Design

I am making some new safety valves for my 7 1/4 NG loco to the Gordon Smith design. I was wondering whether it is intended that the ball should be fixed in the guide/shroud or allowed to rotate. Any thoughts? Also any thoughts on whether to use the light hammer tap to seat the ball or whether to burnish the seat using Brasso or lapping compound, what do you think?

The existing safety valves were made to the EIM drawing scaled up, but this time I am making them a two piece body to the 7 1/4 NG design from Polly. I have never been happy with the old valves because they don’t reseat properly until the pressure has fallen by 15 to 20 p.s.i and even then they leak a bit!

On several Gordon Smith valves I have made, I left the ball free to rotate, and did not burnish or lap the seats. I did make the seats with the best finished cuts I could manage, and bought stainless balls with best surface finish and roundness that I could afford. Never a minute's trouble with the valves I have made. Good luck with yours,

Thanks Jeff, that’s useful, Alain

I agree with Jeff in that a perfectly made seat should not need the ball to be tapped in. The problem is that you need a perfectly round hole that is dead square to the flat face portion. That means all done in one stage on the lathe and a reamed hole.

Yes, use stainless balls but you must use a 'throw away' steel one to tap the seat. There is no harm in striking a small tap. Use the balls and springs supplied by Polly.

You can't really burnish the seat because it is deep down inside the upper body.

The Gordon Smith designs should work superbly because, in the few designs of his that I have tried on 5" models, he has got the chamber in which the ball and guide sits just right so that coupled with the spring pressure they are a quiet and gentle semi-pop. Take care to get the ball seated in its guide EXACTLY to the depth specified.

Thanks Norm, the old ones were single piece body, but the new ones will have a two piece body so that it will certainly be possible to get a good finish and concentricity on the seat.