Pipe taps are tapered, and only one is used to tap a hole. Are you sure this isn’t a 1/4NPT thread? Everything else on an XK engine is ANF or UNF threads, no BSF or BSPP except the SU carbs.
There are BSP threads all over the place–like the hose barbs out of the manifold and water pump, and the large freeze plug centre of intake side that accepts a block heater.
Regarding taps, yes the tapered holes like BSPT use tapered taps. You have to be careful as to how far you go. To create a tapered hole before tapping, one uses a drill followed by a pipe taper reamer–specific for the pipe size. IMHO.
Rob not all pipe taps are tapered, I have a 1st 2nd and finish marked 1/4 BSPP
The 2.4/3.4/8 crank plugs are 1/4 BSP. The other BSP threads are the heater connection spouts
and the adapter for the water gauge bulb, both 3/8 BSP.
Peter B.
Jim , so you agree thatt the crank is BSPP, so being 1/2 correct pehaps some cream on the Humble
pie !! I find that, sometimes, when using new bungs they" lock up " before full insertion so I allways run a finish tap to clean the threads in the crank and ,as allready stated, Loctite the bungs.
The factory, I think, used just a 2nd tap which of course results in the innermost threads forming
a taper for the bung to bite into.
How much taper is there on your new bungs?
Peter B.
I was thinking internal parts, crankshaft & camshaft bearing caps, oil pump, etc. all ANF and ANC.
Specified dimensions of 1/4-19 BSPT and 1/4-18 NPT are really close.
I have no way of checking what the thread is , even the supplier didn’t know! .My plugs measure 12.58 / 13.28 diameter x 12.85 long . that gives an angle of 1.56 ° which is about right for 1/4 BSPT at 1.47° .
I came across this on the net
I didn’t imagine this question was so complicated.
Jim
I have ordered a 1/4 BSPP tap . I can run it carefully down a good hole to check for T or P
Can you check if the threads in the crank are tapered using an internal vernia before ordering the BSPP tap which will the wrong one for a BSPT.
I could only measure the opening , I have nothing suitable to measure down the bore . My money is on it being P , maybe my turn for him humble pie .
A cheap set of verniers would have the means to get far enough in
to double check.
It would be well to check the thread pitch on the old plugs with a thread gauge.
One could also check for taper on the old plugs by holding two straight edges on it and see if they are parallel or converge.
OK I was wrong !
I think this is a conclusive answer .
1/4 BSPT Plug ( Tapered )
1/4 BSPT Tapped hole (Tapered )
I will put some more detail in a video later in XK section .
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