Hi,
You've been given a number of answers, all of which 'may' be correct for a +40 year old car that's been buggered up by any one of it's previous answers.
tmg513 probably gives the best answer.
In the 1970s Morgans used their own machine shop, by and large the thread forms and hardware they and the factory used were imperial, BSW (British Standard Whitworth), BSF (British Standard Fine) or BA (British Association) the fittings for grease nipples would have been BSP (British Standard Pipe).
Note the clue in the letter B.
For all these threads (except BA) the angle between the threads is 55 degrees. For the BSP fittings on grease nipples and radiators, the tapped / female thread is parallel whilst the male thread on the nipple is tapered. This means that measuring the diameter /size of the thread is problematic as the diameter varies.
In the period American SAE threads with a 60 degree angle and form are found on bought in assemblies such as the engines and gearboxes from Triumph, Rover and Ford.
The best way to determine the the thread size is to use a thread gauge for pitch and thread angle. For completeness;
1/8 BSP 28tpi (teeth per inch) 55 degrees thread form
1/4 BSF 26tpi 55 degrees; and then onto our colonial cousins,
1/4 UNF 28tpi but 60 degrees
1/8 NPT 27tpi but 60 degrees,
As grease nipples are tapered any fitment relies on interference, thus with such similarities a mismatch will appear to work.
All of which is jolly interesting but doesn't help you, so back to tmg513's suggestion and go down to your local agricultural merchant with the offending part.
