you should measure the hole diameter in the bridge and the slot width on the tailpiece too. Also, you need to know whether its metric or sae thread in your guitar.
If its a nice guitar, you might want to consider spending a little extra for Tonepros locking parts...they really do work:
I'm not sure if they are metric
or standard. I don't have proper
holes in my guitar. I had them
held on with some random screws
and washers before. I'm refinishing
my guitar now and don't have the
studs. I might just buy a whole new
set. Who knows.....