It just moves them a tiny bit, more of an angle that slots. I know it seems wierd but it also seems to happen far more on prints other than the ones bought from me so it has to be some sort of print issues that is very slight.
Mine were also full-threaded, does not change anything. 
Thanks Philipp! Good to know that using fully threaded bolts can be taken out of the equation…
I’m away working at the moment so can’t try as Ryan suggests and apply tension to the clamp using the tension bolt first. (I had previously attempted that but it didn’t seem to have much of an effect which is why I was considering what else might be causing the problem.) I’ll have to try it again when I’m back home.
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Update: 25 July 2023.
I had another go of applying tension to the clamp bolt (while the other two attaching bolts are loose) to set the mount bolts correctly and close the gap between the tubes and the bearings. It did lessen the gap but I was putting on more tension to the clamp than I was comfortable with. (The clamp was squeezing the bearing to the point where it amost was jamming it.)
I’m still not happy with the gap between the thread of the bolt and the bearings inner race as this introduces a lot of play, and I also suspect my cheap bearings from Aliexpress may also add unnecessary play. Might have to source some better quality bearings and different bolts
As my tubes are 25.4mm, I’m using the “J” version 3D printed parts…so I decided to re-print the core clamps but this time print the “F” version which are for 25mm tubes, to see if the .4mm difference would help close the gap.
I’m happy to say it does. With just a small amount of tension on the tension bolts (and sometimes none at all) the gap completely closes up between bearing and tube…or does it???
I found that if i rotated the tube, in one positiion there is a small gap and hence some play, and in another it is too tight and the bearings seem to be under load. That must mean my tubes aren’t perfectly round! Grrrr
I don’t know why this isn’t noticable for the trucks on the side rails as all the tubing was cut from the same 6m length of stainless tube, maybe it has something to do with the position of the bearings on the trucks and how they connect with the rails.
Anyway, I’m hoping when I assemble it now, I can rotate the core tubes to a position where the fit is nice and snug with zero play and then lock them to the trucks so they don’t turn.
Progress I guess!
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Hi Mark_Kiwi , if you are ever in Whanganui you are welcome to call in. I have completed Primo and LR3. I also have a ZenXY underway.
Paul
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Thanks Paul,
I do occasionally get down that way with work…more New Plymouth or Palmy but in the general area 
so I’ll keep it in mind. Thanks!
Mark_Kiwi, where are you based? I am in the Waikato and thinking of embarking on the same journey as you.
Hey Don,
I’m in West Harbour in Auckland but did source my SS tube in Hamilton.
Started assembling but suspect my cheap Chinese bearings are no good because I was having issues trying to square everthing…so have put everything on hold until I source some more.
Also had to put everything on hold because my wife has had hip replacement surgery.
Currently my machine is under a sheet in the garage waiting for me to get back to it…best I pull finger and souce those bearings.
I’m thinking I might disassemble everything and start the assembly from scratch. Maybe my rails aren’t perfectly square, I need to get that perfect and try assembling it again.
Good luck!
Mark