Steel or brass?

[tevans9129]

In a cast brass cylinder, should one use brass or steel for the valve? Is one better than the other? I have a PMR #3 that I have started on and since this model engine building is new to me I like to make a prototype with on-hand materials. I just made a valve from some old 3911 steel but not sure if there is any advantage in using steel vs brass.

 

[Brian Rupnow]

The valves fit into the cylinder head, not the cylinder. I generally use a valve made from mild steel running in a brass valve housing that incorporates the valve guide and the valve seat in one piece. Since the valve slides in the guide area of the valve housing, the valve and the piece it slides in should be made from different materials to avoid "galling". Brass is softer than steel, so that even if the valves leak a bit, once the engine starts the force of combustion will drive the valve face down against the seat hard enough to deform the brass seat a bit and create a perfect seal.

 

[tevans9129]

The valves fit into the cylinder head, not the cylinder. I generally use a valve made from mild steel running in a brass valve housing that incorporates the valve guide and the valve seat in one piece. Since the valve slides in the guide area of the valve housing, the valve and the piece it slides in should be made from different materials to avoid "galling". Brass is softer than steel, so that even if the valves leak a bit, once the engine starts the force of combustion will drive the valve face down against the seat hard enough to deform the brass seat a bit and create a perfect seal.

Thanks for the info Brian and for responding. I had the idea of the different materials in the back of my mind but thought there must be a reason that PMR would use brass. BTW, I enjoy your postings, they are quite interesting.

 

[larryg]

Brass moving on brass will gall. That said I doubt that the casting is the same brass that the valve material is. So in a model it will be fine. Steel will also be fine until it rusts. I built the PMR #4 and used 304 SS for the valve. The #4 is a cast iron cylinder. Don't over think this, have some fun and if it fails then make it over. Remember it's the journey not always the destination.

lg
no neat sig line

 

[Peter Twissell]

Am I right in understanding the PMR #3 is a steam engine with a piston valve, not IC?
I can't see brass valves lasting long in an IC engine.
For the piston valve, my preference would be steel running in brass / bronze. Steel has lower thermal expansion coefficient, so is less likely to become tight as everything warms up.