Interesting problem - looks like the bolts that came with the #Frankenbot to secure the hot end and extruder to the stepper are 1-2mm too long. I thought it was a bit stiff trying to get the bolt in far enough to secure the hot end. I had it barely snug and realized something was wrong. Check the stepper assembly bolt- I was backing it out!
(I loosened the supplied bolt and removed the hot end for the picture)
So guess I’ll be trimming the hot end assembly bolts by a smidge.
I checked all 3 and they are all about 2mm too long. I have some similar bolts from my Simple Maker Kit (that I’ve been upgrading) and they are either the same length (ie: too long), or too short. So I took my little rotary tool and nipped them. All good now.