TR's 2-4 use a combination of a solid pipe 202973 from the oil filter housing, a flexible hose 102238 at the bulkhead and solid pipe 200646 to connect to the oil pressure gauge.
Triumph listed an alternative nylon pipe 138308 for later TR4's and all TR4A's. This pipe, which has a Smiths gauge fitting at one end and a 3/8"UNF male fitting at the other, takes the place of solid pipe 200646 and the flexible hose 102238 mentioned above. 138308 can be used on TR2-early 4 as well as late TR4 and TR4A.

Triumph listed this same pipe 138308 for TR250 and carburettor TR6 however, this is odd as the connection at the engine end fits directly into the block with a male end, which should be a taper thread 1/8" NTP.

We will never know how it came to be that Triumph generated this confusion over the 138308 pipe in as much as 138308 is used on the TR4A and in that application has a 3/8"UNF male end but in the TR250 and CC/CF TR6 application an 1/8" NTP taper end is required to screw straight into the cylinder block.

To overcome this confusion we have created a new part number 138308SS, which is a stainless steel braided pipe that comes with both 3/8"UNF male and 1/8" NTP ends. This stainless steel braided hose is much safer than the standard nylon part as a nylon pipe could be prone to failure for any number of reasons. See below

For reference All TR5 and CP/CR Petrol Injected TR6 use pipe 149867 which has a Smiths gauge fitting at one end and a female swivel end at the other. This fits onto a male/male union 143943 which in turn is screwed into the block. A stainless steel braided version of 149867 is available as 14867SS too. See below