Cyclocross lever clamps come in three diameters:

26.0mm for mounting on the bulge section of road bars
24.0mm for mounting on the non-bulge section of road bars
31.8mm for mounting on the bulge of oversized road bars

People that have to run their lever 'flipped' are using 24.0mm levers, which of course won't mount on the bulge of bullhorns. It should be noted that if you have bars with a 'sleeved' clamp area (like all Nitto products) then a 24.0mm clamp would mount right next to the stem.

Here are some clamp diameters of common 'cross levers:

Tektro RX 2.0: 24.0mm
Specialized: 24.0mm
Empella Frogglegs: 24.0mm (I think)

Paul: 26.0mm
Radius: 26.0mm (I use one of these)
IRD Interceptor: 26.0mm (with shim for 24.0mm)
Pyramid: 26.0mm

Tektro RX 2.1: 31.8mm
Salsa: 31.8mm, but includes 2-piece shim to make it 26.0mm


A lot of these levers can be found here: http://www.store.yahoo.com/cyclocros...untlevers.html