Drilling holes in valve stems is a tricky but satisfying job. I buy a new drill bit each time (1/8 for smaller stems, 5/32 for larger), then use a piece cut off the plain (chuck) end as the pin. No problems for veteran-strength valve springs. I mark the length I want, put the drill bit in the lathe on a very low speed, then use a thin cut-off wheel in the Dremel to cut it off.
Leon