Right Said Thread
Discussion
After joining the local target shooting club, I'm going to be allowed to 'adopt'
one of the club rifles. I want to put my telescopic sight on it to do bench rest
but the gun has no dovetail mounts.The barrel top is plain but for two tapped holes.
I guessed the holes to be 3/16 inch diameter but as for the threads? Not a clue.
So I devised a cunning plan. There's another, identical,rifle, set up for prone shooting.
It has peep sights fitted but it also has scope blocks. I figured that, if I removed one of the screws
from a scope block, it'd tell me what I need to know. I'd measured up and found that on both guns,
the tapped holes are set at very similar distances apart. I'm certain they aren't factory additions.
So, I removed one screw and put my calipers to work. The bolts are indeed 3/16 / 0.135 Inch and
are 1/4 inch long. Here's the rub. To find the threads per inch figure, I took these pics...
The red lines are my doing and the calipers are set at 0.125 inch, or 1/8 inch. Sadly, getting the TPI
figure this way isn't working. In theory, counting the thread and multiplying by eight should work.
Here's the rub though. Are BA? BSF? UNC? UNF? US Machine or...? All I know is they aren't British Cycle - the TPI count is too high.
I thought of buying a stack of set screws via eBay and doing trials but I can only find them by pack size and that'll get expensive.
The floor is open then. In case you're wondering, the guns are both BSA Martini International Mk.ll.
one of the club rifles. I want to put my telescopic sight on it to do bench rest
but the gun has no dovetail mounts.The barrel top is plain but for two tapped holes.
I guessed the holes to be 3/16 inch diameter but as for the threads? Not a clue.
So I devised a cunning plan. There's another, identical,rifle, set up for prone shooting.
It has peep sights fitted but it also has scope blocks. I figured that, if I removed one of the screws
from a scope block, it'd tell me what I need to know. I'd measured up and found that on both guns,
the tapped holes are set at very similar distances apart. I'm certain they aren't factory additions.
So, I removed one screw and put my calipers to work. The bolts are indeed 3/16 / 0.135 Inch and
are 1/4 inch long. Here's the rub. To find the threads per inch figure, I took these pics...
The red lines are my doing and the calipers are set at 0.125 inch, or 1/8 inch. Sadly, getting the TPI
figure this way isn't working. In theory, counting the thread and multiplying by eight should work.
Here's the rub though. Are BA? BSF? UNC? UNF? US Machine or...? All I know is they aren't British Cycle - the TPI count is too high.
I thought of buying a stack of set screws via eBay and doing trials but I can only find them by pack size and that'll get expensive.
The floor is open then. In case you're wondering, the guns are both BSA Martini International Mk.ll.
Celtic Dragon said:
Have you got a prototype engineering place or car restoration place that could have an idea about it.
Off the cuff whilst the part is imperial, has it been threaded as an M5?
I moved here very recently so I'd have to scout around for an engineer/restorer.Off the cuff whilst the part is imperial, has it been threaded as an M5?
The opinion at the rifle club is that the drilling/tapping would have been done
a long time ago so it's unlikely to be metric.
It seems the best plan is to just keep getting and trying likely set screws..
Maybe I missed something in the description, but I assume you'll be buying some new scope blocks. (Otherwise just move the blocks and matching screws across).
Whenever I've had to deal with fitting imperial machine screws (e.g. my old Wheelhorse), it's a nightmare of trial-and-error, and I keep meaning to buy a thread gauge. I now have a labelled set of the most common sizes I come across, for comparison. You can buy small packs of mixed nuts, but they're expensive in the UK.
Otherwise, is there a model railway group nearby? Or a car restoration place? Or even a hydraulic hose shop (common in rural areas)? They will have lots of assorted nuts to try on it (gently) for a few beer tokens.
Whenever I've had to deal with fitting imperial machine screws (e.g. my old Wheelhorse), it's a nightmare of trial-and-error, and I keep meaning to buy a thread gauge. I now have a labelled set of the most common sizes I come across, for comparison. You can buy small packs of mixed nuts, but they're expensive in the UK.
Otherwise, is there a model railway group nearby? Or a car restoration place? Or even a hydraulic hose shop (common in rural areas)? They will have lots of assorted nuts to try on it (gently) for a few beer tokens.
t to your rifle. 
Thank you, gents for your replies.
I thought of employing a pockle flange but as they're made of unobtanium,
sourcing one is an issue. I could try a Montefiore clamp but they're just as rare.
All joking aside, I can't retap because it isn't my rifle and, not yet
having an FAC, I can't bring it home to work on it.
I have a riser rail with an 11mm dovetail on top. This is long enough to allow
some fore-and-aft adjustment to get the eye relief right. The existing tapped holes in the barrel
are spaced 22mm apart at the breech end, 15mm apart at the muzzle end. However, thay're spaced like this...
...which my scope mounts won't stretch to. So I'll mount my rail using the breech end holes.
I thought of adding a Picatinny rail but I rejected it on two counts. 1: My match scope mounts cost
well over £34 and I want to use them, and 2: My scope has a big objective lens (55mm). A Picatinny rail would
srve well enough but the riser I have leaves 5mm clearance between the fore end of the scope and the barrel.This is fine by me.
Re . the thread from, I think I have the answer. I found a firm on the web offering single set screws at up to £1 apiece plus P&P.
Rather than order many test screw at minimum pack size, I felt I could use this place so I wrote an email.
The reply came this morning ,saying, "It looks like 2BA to me." I tried calling but I went to voicemail.The chap sounded mature
and he supplies all manner of fittings to model engineers.
On the strength of this, I've ordered some 2BA screws so the guy's experience will be put to the test on Wed.
Meanwhile...
I thought of employing a pockle flange but as they're made of unobtanium,
sourcing one is an issue. I could try a Montefiore clamp but they're just as rare.
All joking aside, I can't retap because it isn't my rifle and, not yet
having an FAC, I can't bring it home to work on it.
I have a riser rail with an 11mm dovetail on top. This is long enough to allow
some fore-and-aft adjustment to get the eye relief right. The existing tapped holes in the barrel
are spaced 22mm apart at the breech end, 15mm apart at the muzzle end. However, thay're spaced like this...
...which my scope mounts won't stretch to. So I'll mount my rail using the breech end holes.
I thought of adding a Picatinny rail but I rejected it on two counts. 1: My match scope mounts cost
well over £34 and I want to use them, and 2: My scope has a big objective lens (55mm). A Picatinny rail would
srve well enough but the riser I have leaves 5mm clearance between the fore end of the scope and the barrel.This is fine by me.
Re . the thread from, I think I have the answer. I found a firm on the web offering single set screws at up to £1 apiece plus P&P.
Rather than order many test screw at minimum pack size, I felt I could use this place so I wrote an email.
The reply came this morning ,saying, "It looks like 2BA to me." I tried calling but I went to voicemail.The chap sounded mature
and he supplies all manner of fittings to model engineers.
On the strength of this, I've ordered some 2BA screws so the guy's experience will be put to the test on Wed.
Meanwhile...
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