try getting a bolt, hardened one would be best allen-key type, and carefully using an angle grinder with a cutting disc fitted, cut 3 angled 'lands' (grooves), so that it resembles a tap, then using a lathe, turn a very, very slight taper on the end, tidy thread with a file where taper has been turned, ...hey-presto a cheap thread chaser!.... it usually works for me!:cool2: