nine16 wrote:Are MC21 and MC18 (r5k) axles the same? I have a couple PitBull trailer restraints myself.

I have no idea, but you could find out with some quick measurements. The left side of my axle has a 14mm hole. The right has a 27mm axle nut. The width of the axle (outermost part of the left side to the washer on the right side) is exactly 11 inches.