SMLE Bolt Head Extractor Screw Thread

All,
I've just received two SMLE bolt heads. Both are marked M261 and have an S stamp as well (S for spares?....and are long length....0.640"). They are of the malleable cast iron type (I think....not the more modern looking silverish type with the M stamp) and have the 0.020 chamfer only on the left edge of the bolt face. I think these are WW1 era bolt heads and were replacement parts (S stamped).

Here's the question:
What is the extractor screw thread supposed to be on these bolt heads? None of my standard (BA5?) cheesehead extractor screws from other SMLE or No1 bolt heads or new ones from the spares box will thread into these two boltheads...they bind up after a thread or so of engagement. Are these boltheads supposed to have 5BA threads for the extractor screw? As an aside, I gently tried a standard 4/40 screw and it theaded (loose engagement) and did tighten up. Do I simply need to chase the threads with a 5BA tap or are the threads different and I need a different extractor screw?
Thanks!
GC

The extractor screw thread for the No 1 is normally .126" diameter by 42 tpi, an "Enfield special" size. Some late WW2 BSA boltheads are believed to possibly be 5BA.

The No 4 and No 5 are 5BA.

There were many(?) threads on the old No1 rifles that we were permitted to chase out with BA taps in order to utilise standard stocks or BA No4 screws. You could have hit on one there as I don't have the EMER handy

Quote:

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Originally Posted by Peter Laidler

There were many(?) threads on the old No1 rifles that we were permitted to chase out with BA taps in order to utilise standard stocks or BA No4 screws. You could have hit on one there as I don't have the EMER handy

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Thanks Peter & Maxwell Smart.

Enfield "Special": 0.126" x 42 TPI (Enfield form).

BA (5BA): 0.126" x 43.1 TPI

Bruce_In_Oz,
Thanks.
Well I've ordered what are listed as SMLE extractor screws and that do not have a note that says "for both No1 & No4 rifles". I'm hoping for the correct screws but won't be surprised if they turn up as 5BA. And, just in case I've asked my Dad to send up his BA tap set;)
GC