Morris Minor Forum

Another post drifted into British Pipe threads versus National Pipe thread. (my fault) Since there are so few pipe threads onboard, I don't think this subject has ever been flushed out. So far I've identified that the oil pressure sender and the brake switch are 1/4 pipe with 27 threads per inch (TPI) which is National versus British pipe threads. That means the entire engine has common threads for us on this side of the pond. The other pipe threads, the gearbox and differential fill and drain plugs are not quite as straight forward. I had some loose gearbox plugs but no rear axle plugs and they are 14 TPI as is 1/2 National AND British Pipe thread. However, the diameter is quite a bit bigger than National Pipe. Comparing the to thread systems is not easy. The smaller sizes have different TPI with British having one more. From 1/2 to less than 1 in. they both are 14 TPI. After that the thread counts departs until super big pipes. (two feet) Going back to those 14 TPI sizes, National has 1/2 and 3/4 but British also has 5/8 and 7/8. Now if you look at pipe diameters over the various sizes, they constantly change which is the bigger of the two. At this point I know the gearbox plugs are not 1/2 National pipe threads but may be 1/2 or 5/8 British Pipe threads. Possibly it's 3/4 National. I'll look around to see what I have to verify and find some differential plugs.

I think that plug as you said is 1/2-14 NPT.....however the difference is in the external wrench you use on the plug.....later plugs are SAE where as I have read the original use the British wrench sizes......I think the common replacement available today is 1/2NPT with SAE wrench specs....

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I found a list that identifies the gearbox plugs as 1/2 British Standard Pipe (BSP). It did not include the differential plugs. Yes, the gearbox plugs use a 3/8 Whitworth spanner, the same as lug nuts. While the lug nuts are 3/8-24 UNF (Austin hubs), the Whitworth hex on the gearbox plug help identify the BSP threads. It's odd that the gearbox was introduced with the only British thread, but it's also odd that they would have retooled it later to 1/2 NPT but it is a secondary operation to tap the holes. The hex is part of the plug and likely would be different.

To further complicate matters there is also British Standard Parallel Pipe. That means the threads are parallel/straight like a bolt and not tapered like BSP and NPT. The BSPP uses a washer for sealing whereas the tapered pipe relies on an interference fit of the threads to seal. Even though the thread diameter and pitch may be the same, the BSPP and BSP should not be interchanged.

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Charles, I learned that too the other year and it turns out National Pipe also comes straight. There is also British Standard Brass thread which is 26TPI for all sizes. We might have that on the fuel system.