In an ideal world I'd recommend the spacer was hardened as they do wear, particularly so if the engine shock absorber isn't working correctly...Mark Cook makes the 'std.' thickness spacer to that specification...
If hardening facilities aren't available a material such as EN19 would make for a reasonably durable component..In other words avoid basic mild steels such as EN1A....Ian

email (option): ian@wright52.plus.com

Thanks again for all helpful replies. I definitely will check chain case clearance when I get the cush spring installed. Not sure if the shim was hardened. He used some shim stock that he had. He’s out of town this weekend but I’ll check with him when he returns. Thanks again.
Barry

As it seems there are three different thicknesses of these hubs, it might be more cost effective to make only one size and two spacers?

Mark

email (option): pes.sales@btconnect.com

Definately more cost effective considering the cost of cutting the splines and grinding the inner race region
However unless you want to ship them asa set , which wastes materials it will need to be well documented on the PES web page so purchasers can order the correct one .
As a person educated in the engineering sciences and a true conservationalist I hate wasting materials .