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:
http://www.tonepros.com/
What I have:
Tonepros nickel SAE locking studs for stop tailpiece, used for $10
Gotoh aluminum metric studs for stop tailpiece...new for $5
...Oh, the freedom of the day that yielded to no rule or time...