I'm almost sure they are 3/8 - 16 bolts. You might need to run a tap into it to clean out the accumulated gunk/dirt - they are usually pretty nasty in there.
Edit - ok, I admit it, I was bored - I went out and ran a 3/8-16 tap into mine - fits perfectly.
2003 F-250 XLT Crew Cab 7.3L, Chrome BigTex Grille Guard, Quad pillar - 3 ISSPRO gauges (trans, pyro, boost) and DP-Tuner F6; Roush fuel pressure / temperature / oil pressure gauges, Ford Severe Duty AIS, 31 row 6.0 transmission cooler, ScanGauge II, Marinco mod, Walker BTM
Last edited by RT; 03-19-2012 at 05:23 PM.