12mm hex fitment. However the centre hole is quite large, I could just use metal washers as spacers but these Tamiya bushes fit perfect. So I might use them or 4 bearings of the same size, might confuse some people because it'll look like free spinning wheels on all four corners.
The combination of different inserts, rims and tyres is making fitting the beadlocks very awkward. Inserts are holding the tyres away from dropping into place.
The ribs on the insides of the tyres weren't helping, seemed to be snagging on the edge of the inserts. Didn't want to cut them so instead I turned the tyres inside out and cut the ribs off.
Seems lots better, still very hard the get the tyres lip to drop into the beadlock. After rebuilding one a few times I think it has now worked, from what I can tell it looks like the beadlock is secure all the way round.
I think wiping a bit of oil onto the rubber would help lots, but then I don't want to make it easier for the rubber to slip out in use. Maybe a mix of water and washing up liquid would work, it might dry sticky.
Anyway going to persist tomorrow and try to get the other three wheels finished.