Offset valve spring retainers or keepers ?

I did this on an odd ball head in the US (Opel CIH type head) and it used old 9 mm valve stems and so I could not find an arbor to fit. So I made the guide bit for a standard hole saw arbor from brass rod stock (so it would wear and not the guides) and turned it down by chucking and spinning it in a hand held drill, working it against a grinding wheel, and checking up and down the length constantly with a dial caliper.

Once that was done, the holesaws are standard bi-metal types. The cast iron was cut down dry at moderate drill speed. You'll need 2 or 3 successive sizes and then smooth out the ridges between with the dremel.

Of course, it will go a lot faster if you just buy the ready made bits and arbor from and outfit like Comp. You can buy those in the sizes you need and avoid so much rigamarole like I needed to do.