BSA rear guard screw size? Superstar, Meteor etc

[Phil Russell]

Well, thank you all for taking such trouble. I appreciate it. I wonder which thread the Knibbs part is.
I am warming to the idea of retapping the thread now....
Cheers, Phil

[evert]

Well, thank you all for taking such trouble. I appreciate it. I wonder which thread the Knibbs part is.
I am warming to the idea of retapping the thread now....
Cheers, Phil

Or you could get a more recent supersport/lightning rear block, that is threaded M5. Then you can change back if you want to get the gun original again.

[Phil Russell]

Or you could get a more recent supersport/lightning rear block, that is threaded M5. Then you can change back if you want to get the gun original again.

Good point. But then again, the gun has seen a bit of attention from previous owner(s) so I do not think it needs to be in absolute original condition ... that would be a tad difficult to arrange.

Cheers, Phil

[evert]

Measuring the screws and relating to the BA thread system got me confused, so I had to learn more about it.

Its a British standard, so I expected the pitch to be imperial - but none of my imperial thread gauges matched the pitch of the early Supersport/Superstar rear screw. (clearly not a 30TPI, almost a 32TPI). The closest thread gauge I found is a 0.8 mm pitch - on a British thread system???

Time for some googling!

This page cleared up my confusion:
https://journeymans-workshop.uk/baandme.php

So, the table lists the 2BA as .185" diameter x 31.4" TPI!!! That does not really sound logical..

But the text goes on to explain:
"British Association threads were a metric thread system devised for small screws. Originally included even smaller sizes intended for use in watchmaking but these have been superceded by specialist horological thread series. Not metric like you might expect, but with diameters determined by a factor proportional to a power of the logarithm to the base 10 of the thread pitch in millimetres. Hence the rather strange mix of metric and imperial in the above table."

Aha! Dividing 25.4mm (1") by 31.4 TPI gets you 0.809, so the close to 0.8mm pitch starts to make sense.
The 4.6mm I measured with worn calipers is close to the listed .185", which translates to 4.7mm.

Its a confusing (when compared to more recent thread standards) system, but the 2BA really is close to a M4.7x0.8 thread.

Which is good for re-tapping to a M5x0.8 thread!

[Phil Russell]

Having stripped the relevant block out, I find a 2BA screw fits. Why a 2BA would not fit with the block assembled on the rifle in the first place I do not know. if this thread is really 2BA then it could explain the untidy appearance of the guard screw fitted. Maybe this screw was forced into a 2BA thread???
Whatever ... onwards.
I continued my look at the rifle by stripping it. Not too bad really but the spring guide was a pretty loose fit in the spring and the large circular end broken off. New guide is being made. Small O ring breech washer was somewhat damaged BS 010 seems to fit OK in the recess but I do not have the larger O ring. I think it is OK though. Mainspring seems fine... must measure it, as it the piston seal. I will also add a piston sleeve.
So next ... trigger reassembly. Why do I expect coffee to be in demand?

At least it is raining here so I would not be outside anyway.
Cheers, Phil

[evert]

Having stripped the relevant block out, I find a 2BA screw fits. Why a 2BA would not fit with the block assembled on the rifle in the first place I do not know.

Some Supersports/Lightnings I have worked on have had alignment problems with the holes in the rear block and the holes in the cylinder.

[Thegreg]

Some Supersports/Lightnings I have worked on have had alignment problems with the holes in the rear block and the holes in the cylinder.

The alignment issue is most likely caused by the opposing cap head screw which fastens through the Maxigrip rail, it makes the back block camber over when the rear guard screw is removed, sometimes chewing the thread too. A good tip to avoid this is remove both screws and reinstall simultaneously.

Cheers
Greg