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    Contributing Member Rockandroll's Avatar
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    English or metric. How to tell?

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


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    Really Senior Member Bruce_in_Oz's Avatar
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    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 "Britishicon" BA thread series is actually derived from the old Swissicon "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.
    Last edited by Bruce_in_Oz; 02-25-2021 at 06:47 PM.

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    Contributing Member Flying10uk's Avatar
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    Quote Originally Posted by Bruce_in_Oz View Post
    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.

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    Contributing Member ssgross's Avatar
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    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.

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    Really Senior Member Bruce_in_Oz's Avatar
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    Yep! That Zero BA diameter detail seems to have missed "publication" in my first post.

    The other very Britishicon" 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 Frenchicon 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 Japaneseicon 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.
    Last edited by Bruce_in_Oz; 02-26-2021 at 05:06 PM.

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    Contributing Member Rockandroll's Avatar
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    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.

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    Contributing Member Flying10uk's Avatar
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    Quote Originally Posted by Rockandroll View Post
    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.

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    Advisory Panel browningautorifle's Avatar
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    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.
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    Regards, Jim

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    Really Senior Member Salt Flat's Avatar
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    deleted -due to duplicate info posted 1 minute after BARs post. My timing has been like this lately- Go figure?
    Last edited by Salt Flat; 02-26-2021 at 11:51 PM.

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    Contributing Member Rockandroll's Avatar
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    Thanks guys. I’ll dig my mags out of long term storage and check them.

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