Determine if your U bolts are round, square or semi-round
Measure A - Diameter of Rod - Many times it's easiest to use a socket or wrench to determine the diameter (See reference below)
Measure B - Width of U bolt from inside to inside
Measure C - Length of U bolt from bottom to top
11/16 Socket = 7/16 Rod
3/4 Socket = 1/2 Rod
13/16, 7/8 or 21mm Socket = 9/16 Rod
15/16 Socket = 5/8 Rod
1-1/16 or 1-1/8 = 3/4 Rod
1-5/16 = 7/8 Rod
Includes 2 Nuts and 2 Washers per U Bolt