Hi Austinpike,
From the dimensions you have quoted, the thread is probably 1" x 20 tpi cycle thread (either British standard cycle thread or CEI). The only difference being the included angle of the thread - British standard had 55 degree and CEI had 60 degree. These threads were in common use in the cycle industry in the early to mid 1900's - with the smaller diameters available in both 20 and 26 tpi - most larger diameters were 20 tpi (as on steering head bearings). Then, in the late 1900's, even the cycle industry went Metric. The main reason for using such fine threads was that with the better mechanical advantage of the fine thread, sufficient pressure could be applied with the relatively small and crude spanners that were usually included in the toolkit. Where alloy castings are concerned, though, a coarser thread is preferred as it would be much less prone to stripping. Regarding fitting a 'Helicoil' or similar insert, I would be surprised if they are available for this size of thread, as there is probably little demand for them. You may have to enlist the help of an engineering firm to make a special oversize tap and a new plug.
Eddie.
