Hmmm. As compressor head valves, these shim washers are going to see severe duty conditions. The cycle time, rate, and duration is high. You will want an alloy steel for this. The reason I suggested damper shim stack washers is that is the duty they are designed for, in addition to being available in sizes and thicknesses to accommodate. You might have to Google around for damper shims, but they are cheap, they come in assortments, and they won't crack after hours of flexing in the manner that those valves do.