It's a 13mm. I've replaced it once. In Ford terms, it might be a 1/2 since it seems that some bolts are metric and some are SAE. The 2 are interchangeable, but there is a slight difference in size. Go with whichever one has the best fit.
1990 F150 XLT Lariat
302/5.0L - E4OD | 3.55 Gears - Non-Limited Slip | Stock GT40 Heads | .030 Over (Speed Pro H120CP30 Pistons) | T-Moss Ported Lower Truck Intake | Comp Cams XE258HR-14 (35-512-8) | Hedman Shorty Headers | Will install my Trick Flow 58cc heads when I quit being a lazy bum.