English or metric. How to tell?

[Rockandroll]

Title tells it all. How does one tell the difference? Thanks

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[Bruce_in_Oz]

Just about every part is different; sometimes subtly, often not.

Manufacturer's marks for starters. Then the obvious stuff like furniture, muzzle devices, gad blocks, sights, dust covers, gas plugs, extractors, cocking handles, butt-plates, etc.

One interesting thing, however is that during the development of the "inch" pattern, a lot of effort went into retaining a lot of component dimensions. You can plug an Inch "upper" assembly onto a Metric "lower" / TMH and it will work. The threads on the screws are different, but VERY close. The rear-sight windage screws are ALMOST identical; not surprising, as the VERY "British" BA thread series is actually derived from the old Swiss "Thury" metric clock-makers thread system. For example, Zero BA has the same pitch as M6, but EVERYTHING else about it is different.

NEVER try to force ANY thread when assembling either system. If the mating surfaces look unmolested, and the right size, but do not run all the way relatively easily, STOP. You may have the proverbial "mixed bag". Breech thread? 16 TPI and 1.5mm pitch are "close", but I'd advise against trying to fit one into the other, especially with the aid of valve-grinding paste and / or big wrenches with four-foot "cheater bars".

There are several good books that detail all of the differences, but original "works" drawings are MUCH harder to find. There is also a "spectrum" of web-sites that deal with these matters to varying degrees of competence.

The whole headspace / locking shoulder caper is the big choke-point, even if you have all parts of the correct pattern. If your assembled rifle fails headspace, ( and this is NOT the same as SAAMI "sporting" spec), you need to find a gun plumber who is FAL / L1A1-savvy AND who has the tools and gauges (and a goodly collection of the full range of the shoulders), to go with it. I understand that there are a couple of such gentlemen who frequent these pages.

[Flying10uk]

For example, Zero BA has the same pitch as M6, but EVERYTHING else about it is different.

Yes, but in practice the two threads will fit together fine, without needing to be forced together. When I last measured the external diameter of a 0 BA thread it was 6mm diameter.

[Bruce_in_Oz]

Yep! That Zero BA diameter detail seems to have missed "publication" in my first post.

The other very British" thing about BA threads is the size relationship:

To get the dimensions of a size say, 1 BA, multiply the previous larger size (Zero) buy 0.9. Ditto all the way down.

Had to be something to do with manual gearbox lathes that could be adjusted in those multiples. For us mature chaps, the original Meccano screws and nuts were all BA, Apparently, when the French took over, they changed to the nearest metric thread.

And then, there are "Enfield" screws; bearing no resemblance to any other on the planet. From the Martini Henry to the last No1 Mk 6, and including the P13 / P14, all were held together with "Enfield Specials", except the Lee Enfield stock bolt, which is 7/16" Whitworth. Then, there are the VERY British "Admiralty" threads, as used by the entire Japanese civil and military ship-building industry until fairly recently.

Mausers up to the M98 and the Type 30 and 38 Arisakas are full of Imperial threads, not so much the Type 99.

There is also a range of "aerospace / aeronautical" screws that have a simi8lar "form" to BA; apparently a good thing when bolting together components made from Aluminium alloys.

[David TS]

The other very British" thing about BA threads is the size relationship:

To get the dimensions of a size say, 1 BA, multiply the previous larger size (Zero) buy 0.9. Ditto all the way down.

Bruce, that's not actually correct.

1BA is 88.333% of 0BA diameter, 2BA is 88.679% of 1BA diameter, etc - and it doesn't follow in a linear scale on pitch either.

BA is a weird thread, as despite the 'British Association' name, it is actually a Swiss thread, and metric thread form.

---------- Post added at 05:50 PM ---------- Previous post was at 05:46 PM ----------

Yes, but in practice the two threads will fit together fine, without needing to be forced together. When I last measured the external diameter of a 0 BA thread it was 6mm diameter.

M6 coarse and 0BA are indeed 6mm dia x 1mm pitch.

HOWEVER, interchange them at your peril, they couldn't be more different.

M6 coarse is 60 degree thread form, 0BA is 47.5 degree thread form.

Interchanging them is NOT a good idea!

[Flying10uk]

M6 coarse is 60 degree thread form, 0BA is 47.5 degree thread form.

All depends on how much torque is put on the nut/thread.

If memory serves me correctly 2BA will fit 3/16 B.S.F.

---------- Post added at 12:51 AM ---------- Previous post was at 12:43 AM ----------

A story that I heard was that BSW dropped a spanner size on all it's threads, making it the same as B.S.F. spanner sizes, during WW2 in order to save steel. How true and how much steel was saved I don't know but it does give one explanation as to why one encounters 2 sizes of B.S.W. nuts/bolts.

[ssgross]

well...since no one gave the smart a%* answer, I will
if the number has an abbreviation with an m in it, it's metric. all else is standard.

[Rockandroll]

Well, I got what I got. The only thing going into the future is, will any mag fit (inch or metric) and lock into this firearm? Thanks.

[Flying10uk]

will any mag fit

No, but metric mags are said to fit imperial rifles but not the other way round. I've never tried it.

[browningautorifle]

That IS correct. Metric mags have such a small front projection, just a small sliver. Inch mags have a large steel nose projection to lock them in place. You can put a metric mag on an inch gun and it will stay but could come loose. An inch mag will NOT fit a metric gun without mod of either the mg or the upper.

Inch mag left, metric mag right.