Oil Tank filter thread?

Mar 28, 2018
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Can anyone advise the size of nut that screws to the oil tank filter ( oil feed ) that clamps the oil pipe tail on an N15 ( or Matchless G15 )
The outlet thread is 20 tpi and 5/8" od which according to my reckoning makes it a UNS thread form ?.
The sizes of the flow stems that I have found seem to bear no relationship to the thread size, instead using the size of the hole in the nut.
I have found a flow stem that appears to relate to the correct size for the oil pipe, but the thread is very loose.