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Topic starter
Greetings ,
I am sure many - whoever has mounted a scope will know that the screw thread in the HOWA action is a..
#6-48 x 1/8” or an #8-40 UNC
See here .. Screw Size Guide:
My question .. seeing as we live in a metric World
Seeing as a #6-48 is 3.51 metric equivalent would it be a problem to drill and tap your receiver to an appropriate metric size - thread pitch ( M4) cap screw ?
Easy enough to find these at a good Bolt and Nut place
What say you
Posted : 24/02/2026 9:21 pm
Topic starter
Greetings ,
My question .. seeing as we live in a metric World
What say you
NOT so easy
4mm drill bit size is 3.3mm = special order
3.3 does not remove ALL the old thread (3.51)
SO
4mm iffy
No one keeps American UNS ( non standard ) UNF .. maybe ?
#8-36 IS standard UNF - where to find ?
5mm is much too big
Watch this space
Posted : 25/02/2026 8:40 pm
Posted : 26/02/2026 12:13 am
Topic starter
There is also BA British Association Imperial small cap-screws with fine pitch designed for this type of application ( engineering )
British Association screw threads
However
Also NOT available in SA - at least as far as I looked
NOTHING Imperial in SA
Metric is useless - no half sizes or in-between for small fine thread cap-screws
In the end I found some #6-48 Torx cap screws locally - someone obviously imported some from the US
Posted : 27/02/2026 10:17 pm
Topic starter
https://www.360arms.co.za/shop/warne-scope-base-mount-6-48-gunsmith-screws-long-zba938/
R 180.00 Incl. VAT
For four = R45.00 EACH - IF my arithmetic is correct
Posted : 27/02/2026 10:19 pm
If you are desperate then that price might not feel that bad. Have you tried bolt and nut companies?
Posted : 28/02/2026 4:25 am
try Temu - Take a lot - Amazon ?????? or some of the on line stores. If you know what it is called correctly, you can order it.
I am who I am - I am not who you want me to be.
Therefore I am me.
Posted : 28/02/2026 12:42 pm
Topic starter
If you are desperate then that price might not feel that bad. Have you tried bolt and nut companies?
Yes
UNOBTANIUM ...
NO-one appears to bring in Imperial US or UK any-more
I found a few so sorted for now..
Posted : 04/03/2026 11:39 pm
Topic starter
try Temu - Take a lot - Amazon ?????? or some of the on line stores. If you know what it is called correctly, you can order it.
Yes can do that - but what a bother
There is another SA supplier .. at R22.00 per cap-screw
Maybe one should buy a couple .. just to keep on hand
Posted : 04/03/2026 11:41 pm
If I am ever in the states I might just have to pop into a hardware store and buy a fortune's worth to sell back in ZA to pay for my flight... or make lots of friends 8)
Posted : 06/03/2026 4:16 am
Topic starter
If I am ever in the states I might just have to pop into a hardware store and buy a fortune's worth to sell back in ZA to pay for my flight... or make lots of friends 8)
IF you do ...
Get ...
#6x48 UNS - the most common one
#8x40 UNS - the newer upgrade ( bigger )
#8-36 UNF - a possible for a re-drill
Also make sure to get the correct drill bits and associated TAPS
Just get very long ones - easy enough to cut to size
Posted : 06/03/2026 9:05 pm
Greetings ,
My question .. seeing as we live in a metric World
What say you
NOT so easy
4mm drill bit size is 3.3mm = special order
3.3 does not remove ALL the old thread (3.51)
SO
4mm iffyNo one keeps American UNS ( non standard ) UNF .. maybe ?
#8-36 IS standard UNF - where to find ?5mm is much too big
Watch this space
I play around in my garage with making stuff and fixing stuff. A lot is with screws of thin diameter, M3-M5. Tapping and threading. I still haven't seen a 3.3mm drill bit off the shelf, but I have found the following experience:
With soft materials, like aluminum, or plastics, I drill a 3.2mm hole, and tap an M4. Its a bit of a squish to get started, but it works. You can work a 3.2mm drill around a bit to make the hole closer to 3.3mm
With harder materials, mild steel, and steel with a bit of carbon I drill a 3.0mm hole, and then ream out to 3.5mm, so there is no extra scraping and rubbing to make the 3.5 hole bigger than it should.
1) the 3.5mm m4 tapped holes still manage to hold an m4 screw very well, even though the pitch is 0.5mm instead of 0.7mm . I have not had any of them pull loose and strip the threads.
2) It is a lot easier to tap an M4 with a 3.5mm hole, and there is less chance that you are going to break your die off in the hole and cause even bigger problems than you had. I imagine gun metal often has quite a bit of carbon in it, so this is a real threat.
If a perfect thread is important, I would perhaps look at an M4 fine thread. This taps into a 3.5mm hole. Here the drill bit is easy to find, but the matching dies and screws are harder to find.
Posted : 06/03/2026 11:51 pm
If you go the M4 fine route, assuming you can find the screws, you won't have residual thread from the #6 hole as the hole is 3.5mm, and the fine thread has some advantages over the standard coarse thread.
1) handle about 12% extra tension - a slight gain
2) less prone to working loose through shock/vibration than coarse thread
3) it works better on thin materials (some rifles don't give you much metal to work with.
Posted : 07/03/2026 12:08 am
Topic starter
Thanks
Very helpful
For "DIE" read "TAP" ... ?
Basic Metric Thread Chart
https://fullerfasteners.com/tech/basic-metric-thread-chart-m1-m100-2/
Posted : 07/03/2026 7:12 am