They go under valve springs in modified engines and up till now I've been making them on the Student but it's a bit of a pain without a decent parting off system. On a cnc they'd be a minute or so each to do after setting up. Another way of doing it would be to have round blanks pressed out of slightly thicker plate and then finish to thickness and do the boring. If anyone fancies quoting me for the job here are the dimensions.
o/d 34.0mm i/d 13.75mm thickness 4.0mm
With a counterbore on one side 2mm deep x 24.8mm diameter. Needs to have a fairly sharp inner edge, i.e. bored with a 0.4mm or 0.2mm radius tool. Quantity 120
o/d 34.0mm i/d 24.4mm thickness 4.0mm No counterbore. Quantity 64
Tolerances not very critical. A couple of thou on any dimension. All corners to be lightly chamfered. Material, mild steel or any other steel that comes to hand.
-- Dave Baker - Puma Race Engines