Flash can only gets part way down threads

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  • Nickberg500

    Ultimate Member
    Sep 20, 2019
    1,064
    North of Baltimore County
    Bought a 9mm upper recently, the seller thought it had a 1/2 x 36 thread. Just tried to put a 1/2 x 36 flash can on it, and it only goes on a few full rotations before it binds up. I tried cleaning the threads (they were a little dirty) with ballistol, that didn't seem to help.

    What does this mean? Are they not the correct threads? Am I doing something wrong? Pic attached.
     

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    Joseph

    Ultimate Member
    MDS Supporter
    Oct 13, 2009
    2,760
    Clinton MD
    The difference between 36 and 28 should be evident just by counting the threads over a certain distance and extrapolating to come up with threads/inch.
     

    4g64loser

    Bad influence
    Jan 18, 2007
    6,381
    maryland
    My colt SMG barrel is 36tpi but other, newer, pcc barrels are 28tpi. Story is colt went 36 to keep idiots from putting m16 muzzle devices on the 9s and thus damaging them. With the proliferation of 9 ars, I doubt most manufacturers are as concerned.

    If you don't have a thread gauge (many you find don't go that fine anyway) you can either count and extrapolate as mentioned above or buy any 1/4-28 (UNF) bolt that the hardware store sells and use it as a gauge.
     

    Nickberg500

    Ultimate Member
    Sep 20, 2019
    1,064
    North of Baltimore County
    The difference between 36 and 28 should be evident just by counting the threads over a certain distance and extrapolating to come up with threads/inch.
    9mm handguns are 1/2x28. **Most** 9mm AR's are 1/2x36.
    My colt SMG barrel is 36tpi but other, newer, pcc barrels are 28tpi. Story is colt went 36 to keep idiots from putting m16 muzzle devices on the 9s and thus damaging them. With the proliferation of 9 ars, I doubt most manufacturers are as concerned.

    If you don't have a thread gauge (many you find don't go that fine anyway) you can either count and extrapolate as mentioned above or buy any 1/4-28 (UNF) bolt that the hardware store sells and use it as a gauge.
    I'm measuring 12 threads over 0.343"... Which comes out to 35 TPI... Which sounds on the money with the 1/2 x 36 I ordered. Maybe they sent me the wrong part?

    Also I tried a spare 1/2×28 adapter, it started binding after a few turns.
    d2fbd17a032a3cdf8250fe314cf02790.jpg
     

    Bertfish

    Throw bread on me
    Mar 13, 2013
    17,606
    White Marsh, MD
    Try to measure the TPI on your muzzle device. Maybe measure the thread length then count them with a pick by feel? Hard to look inside the internal thread I know
     

    Nickberg500

    Ultimate Member
    Sep 20, 2019
    1,064
    North of Baltimore County
    Try to measure the TPI on your muzzle device. Maybe measure the thread length then count them with a pick by feel? Hard to look inside the internal thread I know
    I'll give it a try tmo... I tried putting the supposedly 36TPI flash can on my 1/2x28 AR pistol and it went all the way down. I think the vendor sent me the wrong flash can.
     

    Growler215

    Ultimate Member
    Dec 30, 2020
    2,170
    SOMD
    It's possible they sent the wrong part. You can probably check the TPI on the flash can. Maybe make an impression on wax paper or aluminum foil by pushing down with a pencil and count how many in 1/2 inch.

    From you last pic, the threads on the barrel don't look too bad, although there seems to.be a metal shaving or something on 4th thread.

    At any rate, you can get a 1/2-36 tap and die set on ebay from a US stock seller for $12.50 with free shipping to chase the threads on the barrel. If they're galled because someone over-torqued a previous muzzle device that may be your only choice.
     

    Gcs7th

    Ultimate Member
    Jan 26, 2012
    1,280
    AGC
    If you take another .5x28 barrel and touch threads they should pretty much interlock. I’ve had a few barrels where the threading was exceptionally tight.
     

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