5/8" steel axles are supplied in the kit and are machined for a woodruff key which anchors the caliper. It will be necessary to drill a hole in the inboard end of the axle to anchor it to the gear.
The XL plans call for the axles to slip into 3/4" x .062 tubing. This tubing is about .002" undersized to accept the axles. A better choice would be 3/4" x .059 tubing. I didn't want to order more tubing so the axles spent some quality time on the belt sander as I carefully reduced the diameter about .002". They now slip into the .062 tubing.