I went through a lot recently to find a particular chuck key myself. I
think I found some tables for chuck keys in the MSC catalog. The shank
diameter of the key seems to be the "key". Measure the diameter of the
key's hole in the chuck. Also, in those tables, I think they go by
chuck numbers such as you have here.

If this 5/8" chuck has a 1 1/2"-8 threaded back, then you probably don't
want to fool with it and you should just get rid of it, cheap, to me.

Pete Stanaitis
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Alex wrote:

I just got a drill chuck marked RJ3-16L It's a 5/8 chuck.
What key will fit this chuck?

BTW
Is it any way I can check if it's a ball bearing chuck or not?