A mate bought the correct size range piloted adjustable reamer from L & P Tool makers in Victoria.
It was over $ 400.
You do not need to line bore the main.
You can set the cases up in a mill
Dial in the crank case mouth then dial the outer race of the primary side in.
Remove the primary side and bore the timing side
There are details in a thread on britbike
http://www.britbike.com/forums/ubbthreads.php/topics/762502/1928-ariel-model-c#Post762502.
MM is a bit anal and an Physics professor so working in .00001" is second nature to him and not necessary or even desireable on our steeds.
As for our one, I cast up a plug of fiberglass body bog in a spray can lid.
This was then turned down to be a tight push fit into the primary side main outer race.
A word of warning here, the new bearing race was a lot bigger than the old one , so do it first.
A hole was then drilled then finish bored in the plug to give < 0.001" play between it & the reamer pilot.
Note the plug was mounted in the lathe and the case offered up to get the outside dimeter.
The plug was then drilled & bored to size.
We did this to avoid having to accurately remount the plug to account for the run out in the lathe chuck.
When pressed home into the bearing race, the pilot hole did close up a bit tighter which made the reaming a little difficult but the end was a main bearing with .0005" running clearance on the very first time we did one.
Fiberglass plug was chosen because I had a big tin of filler going off & it is dead easy to machine to a fine finish by a dirt driveway mechanic.
The offical BSA tool, indexes the bush from the out side of the timing side crank half and you could not set up a system more prone to operator error than that.