If your valve is like this:



then here's your item list:
Item Part Number Description
1 51013811 Valve stem complete w/ handle
1 51014311 Valve stem complete w/ handle, brass
2 51013011 Nut
3 51013500 O-ring, nut
4 51013111 Reducer bushing, 2 in. male x 1-1/2 in. slip
5 51003100 O-ring, cap
6 51012911 Cap
7 51014011 Body, with bulkhead fittings 8.0"
7a 270052 Body, with bulkhead fittings 7.5"
8 51005000 O-ring, valve stem, 2 in., 2 req.
9 39102900 O-ring, bulkhead (2) * 8.0" Model only
10 51013500 O-ring, bulkhead (2) * 8.0" Model only
11 274494 O-ring, bulkhead (2) 7.5" Model

Keep in mind that it's still true that if the surface the O-ring seals against has been damage, simply replacing the O-ring may not help.

If you have another valve, you may have to wade through the PentairPool valve manuals: http://www.pentairpool.com/pool-owne...ction.php?s=51

Pentair bought both American and StaRite, and has their slide valves. I don't remember whether PacFab (what Pentair used to be) had a slide or not, but I didn't see one listed.