Per this thread, some people are reporting 2 issues with half shaft supplied by Factory Five with the complete Mark IV kit:
1. The half shafts are too short by approx 9/16″
2. The splines on the end that goes into the diff are not machined long enough for the shafts to seat properly and the circlip to engage before the spline’s shoulder bottoms out
For those that had issues, the splines are only 1 1/2 ” long, and the fix was to use a Dremel to lengthen the splined area to 1.6″ inches so that the shafts would not botton out before going all the way into the differential housing and locking in.
I carefully measured the lengths and confirmed mine both appeared to be within spec. I’ll know better once I’ve test fitted them.
Update: I was able to install mine and confirm positive engagement of my c-clips. I’m not sure why but mine weren’t an issue.