Rad drain tap hole thread

[Steve Bryant]

That could well be a possibility Ruairidh.

[Ruairidh Dunford]

That is why several of us suggested it to you.

[Steve Bryant]

To my knowledge the rad is as made originally so I would have to think that the hole and threads are as originally made also.  Probably the best plan of action would be to turn up a thread cleaning tap to match the thread form and TPI and diameter of the drain tap and carefully run that through the hole until the correct fit is achieved.

[Bob Culver]

Nothing is impossible. Radiator and the various cocks could be various threads, including 1/2 UNF. TPI can usually be established with care. A taper tap run through will cut too large for a parallle fitting.

[Reckless Rat]

Have you not got one of these?



Even if it won't fit inside you can take a cast with plasticine or silmilar

Photo shows a metric gauge but you'll need an imperial one.

[Steve Bryant]

Reckless, thanks for the suggestions. Yes I have several thread gauge sets, but you gave me a great idea about taking an impression from the hole itself so that I can check the TPI then make a cleaning tap to do the honors.

Stephen

[Hugh Barnes]

and perhaps check that it's the same thread as the drain tap....

[Steve Bryant]

I'm hoping it is Hugh.

[Dennis Nicholas]

Another possibility is to use soft wood dowel screwed into the hole to get the thread size/type

[Bob Culver]

I have found it more convenient to count hthreads in say 1/4 inch using the ever handy vernier gauge. 
Probably not applicable here but poor threads can sometimes be improved by sawing slots in a bolt, preferably tough stuff, as a tap.
From various sources charts are availablae of threads in progressive sizes with tpi/mm pitch, tap sizes etc. Very helpful for identity and for figuring remedies.