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>T160s were modified at the factory about half way through production but as stu says not consitently, for instance my late T160 had evrything at 3/8 diameter until we looked at the oil pump backing plate which was still at 5/16 (it was the original fitment)!

3/8" is the feed spigot OD (the upper one) or at least, that's what it is on my NN067xx T160 which is a reasonably late model.

The ID best as I can measure due to its position is approximately 0.300". 

According to the Jack Shemans Service notes (published in Classic Mechanics issue 12, April/May 1986, reproduced in the TR3OC Service Notes, page 7, chapter 7) and which I have quoted the relevant section from, below:

"Jack Shemans:

A useful modification to the oil feedway involves removing the pipe, which has 1/4 inch bore, opening up the hole, right through to the oil pump housing and fitting a pipe with a 5/16 in. bore. This modification, a spin-off from the racing days, helps the oil get round more easily from cold and was incorporated in later models of the T160."

Therefore, 5/16" through to the pump would seem to have been the enlarged size.