I concluded that the torque required to operate the valve repeatedly could potentially loosen the pipe where it screwed into the boiler. In the worst case, it could damage the boiler's thread.
I decided that a heavy duty steel nipple would do the job well and be much stronger as well as being shorter. However, when I came to try such a nipple, it went up to the end of its tapered thread in the boiler's hole. Thus, I was not confident it could seal properly.
The Heritage Railway Association (HRA) have published a document on boiler plugs. They are very concerned that the male and female threads for boiler plugs should be compatible and not a mixture of seemingly well fitting threads. Whilst this blow down valve is clearly not a boiler plug, it does fit into a boiler plug hole with a tapered thread and I have ensured that the threads will be the correct fit.
Sentinel used 1.25" tapered BSP threads for their boiler plugs although they made the taper longer to allow for wear due to regular removal and replacement. Now for the arithmetic:
|Plug + Nipple = Plug-nipple!|
I have yet to check the fit but I'm sure there will not be a problem initially. I say initially because if wear does take place, the handle of the blow down valve will rotate with each tightening and not so easy to operate. Some care will be required.