BC - Bolt Circle: Diameter from the center of one stud to the center of the stud directly opposite of it
PH - Pilot Hole: Large Hole in the center of the rim. The diameter is measured from the center.
SS - Stud Size: Diameter of the stud.
ID - Inside Diameter:
Measure inside the flat portion of the rim.
OD - Outside Diameter:
Measure inside the narrowest part of the rim. (Where the slight flair starts, drop center.