I assume you are asking because you need to replace a lost one (otherwise why care?)
Well I thought I'd answer even tho' I am not a metallurgy expert. Since its main function is to separate the aluminum cranckcase from the aluminum prop driver when using an electric starter, it just needs to be something different from aluminum, thin and flat and smooth---different to keep similar metals from galling when rubbed against each other (also don't want the crankpin hitting the backplate), thin to fit into the alloted space and still leave a tiny gap between the crankcase and prop driver, and flat and smooth to minimize any binding between those two aluminum parts. The one I have for an OS40 is magnetic. My guess is to keep it thin, you end up using some type of steel.
If you don't use an electric starter, you could just do without, at least for a while. Basically as long as the prop is pulling, the gap spacing should prevent the parts from touching.
Topic: OS Thrust washer- what is it made of? (Read 843 times)