Have a look at this YouTube video - Here
which shows one way to line bore the timing side bush.
Note that careful measurements are made with the cases assembled to check the drive side housing is Concentric
, then the drive side is removed and the timing side is also checked for concentricity.
By using a mill with the cases bolted down the result should be that the timing side bush is perfectly Concentric
with the drive side once bored.
There are other ways to do this but using a solid mill seems best.