The kingpin is 1" imperial.
It is reamed with an adjustable reamer. Do not attempt to ream with a fixed 1" reamer because the tubes that carry the bushes are welded to the subaxle forging and will always be slightly out of allignment. The bushes are supplied slightly undersize so that the adjustable reamer can accommodate the mis-alignment without having to go over-size.
Hope that this helps.
