Diff filler/drain plug thread

[PaulCH]

I have my diff on the bench to replace the oil seals, but not a full rebuild as it was working fine other than the oil leak. I would like to install a drain plug in the diff and replace the filler plug with an allen key plug at the same time. I have purchased the filler plugs from one of the main suppliers (x four to do the gearbox and engine in due course). I know the thread size has been much discussed over the years, but I still see contradictory messages. Several comments say the thread is 3/8 BSPT (which has 19 per inch), and others say it is a thread with 18 per inch. I have both NPT and BSPT taps in 3/8 inch. Holding the new plugs against the NPT tap seems consistent with the thread pitch, but a number of comments say the NPT thread is not compatible. Can anyone advise with confidence what tap I should be using? Thanks
Paul

[stag8manod]

Hi Paul, I would suggest you get the plugs checked with a thread gauge, Also the BSPT plug has a 55 degree thread angle and the thread form is radiused top and bottom. The NPT plug has a 60 degree thread angle and the thread form is not radiused at all.
Cheers Ian A

[new to this]

I fitted a drain plug in mine it came with magnet built in to the plug,but mine a metric thread

[Phil S]

If all else fails drill and tap a piece of scrap for a test fit

[barkerwilliams]

I would ask the supplier of your plugs.

Alan