BSA Standard and Light Pattern Fasteners  

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mrben0
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Ok, so 9 times out of 10 the screws on these old gun are somewhat mangled, and this really makes even a nice gun look tatty.  I am thinking about turning up a few replacements for my guns.  Can anyone tell me what the threads are? they are obviously not Metric, so I am think probably BA, BSF or even UNF  can anyone cast any light on these please.  also the piston seal screw what thread is that?

Many thanks in advance

John

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Posted : 7th April 2018 09:02
Garvin
(@garvin)
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mrben0
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Thank you very interesting.

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Posted : 7th April 2018 17:50
evert
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Posted by: Garvin

John I suggest you look at these threads for discussion of BSA screws, which were proprietary. 

Not trying to argue, but it think I may have identified the correct thread standard for the BSA Standard/Light pattern washer screws, or at least something that is a close substitute. 

I have two screws in front of me; a somewhat crushed one from my Light pattern, and a new fresh one from Knibbs.

The crushed one has some quite complete threads, which measure 0.1635" x 36TPI

The new Knibbs made one measures 0.1615" x 36TPI

On the Norwegian machinist FB-forum someone posted a large thread dimension Excel file some time ago, and checking that table I found a thread that seems to be almost identical is the BSA washer screws; the table calls it ASME 8-36, and the described dimensions in the list are 0.164" x 36 TPI. 

This table describes allowed minimum and maximum major diameter for 8-36 UNF close to the variation between my screws, at 0.1632" maximum, and 0.1577" minimum.

I'll try so find some 8-36 screws and see how they fit in the real world. 

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Posted : 7th July 2018 11:33
mrben0
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Very interesting, look forward to seeing your findings.

 

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Posted : 8th July 2018 16:26
evert
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After some more googling, I found that some of Leupolds scope mount screws are 8-36, which means that I already had some 8-36 leupold screws in the basement! Measuring the Leupold screw gives me .161" diameter, and checking it against the Knibbs screw reveals that the thread pitch is VERY close, but not exactly the same. However, the threads are so close I think the screw will fit the pistons without trouble, I will test the fit and report my findings the next time I have one of the BSA's apart.

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Posted : 9th August 2018 22:01
  
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