Hi again Kerryo,
Now that you have shown a photo
it is obvious the original cause of the problem.... The socket for the plug
shown, is a parallel thread... designed to take a pipe fitting, which would also be parallel. At some time, a standard plumbers tapered thread plug has been fitted, this will strain both the thread and the casing.
If you are lucky, the short tapered plug, will have only damaged the outer part of the base plate... Remove the tapered plug and discard. Examine the internal depth
of the threaded section. clean very thoroughly with a solvent (clean diesel
or similar) if the crack does not show all the way through, purchase
a parallel Brass or preferably Bronze plug that is at least the depth
of the threaded portion, and fit with PTFE TAPE. not over tight, but very firm.
This should cure the problem.
If the crack is all the way through, You will have to dismantle the base plate completely, clean throughly with a spirit solvent, and try and find some one to Braze or silver solder the crack and re cut the thread.
There is no other positive way. You can not weld the cast steel
of the base plate. Unless you are extremely lucky to have one of the rare Aluminium units, which can be welded with TIG.
The thread is a Standard Pipe Thread, if you have difficulty obtaining a suitable plug, you can use a common male to female pipe reducer with plug.
Beware Hydraulic fittings, they can be BSPT but are more commonly a different and finer thread. Let us know the result. h the boat