Dimension A = the lock length

Dimension B = the cam length

Dimension C = the offset

Measure the "A dimension" of your existing lock.  This is the lock length.  Measure the cam from the center of the mounting hole to the end of the cam.  This is the cam length.  If the cam needs to have an offset back or front to securely fasten the door, measure the offset. 

Click here to enlarge drawing.