Thanks for the reply Stefan, the radius was on my mind, but as I need a spacer washer between the axle and the bearing face to minimise the possibility of the outer race of the bearing rubbing on the axle I could not easily accommodate it. The Gemini front stubs also are not radiused to the larger dimension as the bearing seats onto this section so this gives me a little hope. Driven, they will be using the Kart hub as an inner support on the 30mm section and both bearings as an outer. Undriven they will be on the 17mm bit solely but without the torque forces from the axle so like the front to an extent, but as you say - with the extra load and no springs.
I worry a little about the stress on it too. But the axle was almost for nothing, with the hubs and sprocket / brake carrier and in the interest of experimentation I think I will give it a go. If it fails, it could be painful. there are a number of "front wheels coming adrift on a cyclekart" videos around so I think this is a possibility, although most of them seem to blame bearings binding and the spindle twisting off the kingpin carrier.
The axle was not apparently surface hardened as it turned on the lathe the same all the way through
Jim