Hopefully, this post is a definitive article on the thread type following the previous Technical post titled removing diff filler/level plug which has attracted over 2,100 views to date and prompted a discussion on the type of thread used on the plug. To avoid people having to trawl through that post and get a simple answer to the thread used, I have created this new post.
The previous thread, (no pun intended!), highlighted the issue of finding a suitable tool to fit the male square drive on the standard differential plug. Should anyone wish to change this plug, particularly the filler/level plug - to a different form of drive, such as an Allen key entry or Hexagonal head or just get a better more accurate square drive, they now have the thread size to use. In my research, I have looked at several other car forums who use the Salisbury 4HA differential and this question has also been raised there.
It has been an interesting (!!) journey and I have found a definitive catalogue in New Zealand on most Sump and Differential Drain Plugs and washers used worldwide. I draw your attention to pages 11 listing the Pipe Plug 3/4 -14 PTF DP 3003 and Page 24 showing the details and photo :- http://www.acl.co.nz/wp-content/uploads ... alogue.pdf
I also received the following in an e-mail from S.N.G. Barratt. "The threads in the differential casing of these Salisbury axles are 3/4 BSP, the standard iron square drive plug (3823 which we supersede to the later 607172J) is 3/4 X 14 BSPT (tapered thread). It is interesting that the female plug used in XJS and some later differentials (JLM632) is also listed as 3/4 BSP. Unfortunately, we supersede this part also to 607172J so I cannot check to see if JLM632 is also a tapered thread "
Examples of Plug designs :-
A= Standard Jaguar Black Iron plug, (removed from spare differential), around £3.00. Note the drive is not exactly square, but more a diamond shape which could be giving the issues on finding a suitable tool to undo it, as highlighted in the previously mentioned post.
B= 3/4" BSPF Nickel plated Allen key drive plug with 10mm Allen Key entry from eBay at £4.07. There is even a larger 17mm Allen key drive plug in alloy on Ebay at £7 as Subaru use this thread in their differential (albeit Japanese JIS Tapered Pipe Thread, which is surprisingly the same as British Standard Pipe Taper).
C= 3/4" BSPT Black Iron plug as available on such sites as eBay at £1.87. (You equally could go to your local plumbers and buy this item in Black iron or Galvanised for around £1.50). Note, this has a perfectly square drive which a 9/16 inch A/F spanner fits exactly.
D= 3/4" BSPF Stainless Steel Hex Head hollow plug from eBay at £2.35.
Note:- B, C and D are longer plugs and being Taper Threads, seal at exactly the same point as the shorter Plug A, they will protrude further out from the casing - not that these plugs are visible when in the car.
I don't want to be drawn on it, but the more US orientated NPT, (National Pipe Thread) at that diameter is also 14 threads per inch and although the thread profile is different at 60 degrees rather than the 55 degrees used on the BSP thread, this potentially could fit also at this particular inch size. Without getting into the details of using dissimilar metals, potential galling of stainless or alloy into other metals, high pressure sealing and the interesting Interference fits of specific fuel threads discussed in the previous post, we are talking about a filler/level plug operating in a vented non-pressurised environment. I would recommend a small amount of thread sealer for a complete effective seal and the choice is yours if you choose to change your plug.
Now back to more serious car stuff - Have fun !!
Dave
