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Thread: Phosphate under gas block

  1. #21
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    Most here know I don't really care very much about meeting specs exactly... I care about the specs that matter, and ignore those that don't. This is one of those that, for the most part, doesn't matter.

    I have worked on lots of ARs... The only time phosphating under the FSB has ever made the least bit of difference is when replacing the FSB with a gas block that has a different barrel footprint. Sometimes we'd have to Alumablack the barrel in the area that used to be covered, but no longer was.

    For that reason, I prefer them to be phosphated under the FSB... but that's a cosmetic issue, not a functional one.

  2. #22
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    When I get home I'll check the barrel and FSB prints and see what the post Park finish ream diameter is on the FSB bore compared to the post Park diameter of the seat.

    ETA: print doesn't state that the dim on the barrel is after Parkerizing, I doubt it is, but the seat (M16A1) is called at 0.6238" +/- 0.0005" and the hole in the FSB is 0.6249" + 0.0005" after Park and hone. . Which makes the maximum allowable mismatch 0.0021". How much thickness can phosphate be expected to add?
    Last edited by Tweak; 12-04-11 at 19:42.

  3. #23
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    Quote Originally Posted by Tweak View Post
    When I get home I'll check the barrel and FSB prints and see what the post Park finish ream diameter is on the FSB bore compared to the post Park diameter of the seat.

    ETA: print doesn't state that the dim on the barrel is after Parkerizing, I doubt it is, but the seat (M16A1) is called at 0.6238" +/- 0.0005" and the hole in the FSB is 0.6249" + 0.0005" after Park and hone. Which makes the maximum allowable mismatch 0.0021". How much thickness can phosphate be expected to add?
    Quote Originally Posted by Clint View Post
    M Phosphate reference thickness is .0002"-.0004" buildup.
    And the minimum clearance is .0006".

    This is pretty much a RC2B fit plus a .00035" coating allowance for the barrel.

    The "B" fit knocks a grade off the hole diameter (~.0001) to allow for tool wear.

    And then the shaft tolerance range expanded from only .00025 to .001", probably to lower manufacturing cost.
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  4. #24
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    Phosphate is 0.0002 to 0.0004 per side. So on the barrel, it will add to the diameter by 0.0004 to 0.0008.

    The "cost" of this is that it adds in a variable which can vary by 0.0004 on top of the barrel and gas block machining variance. So you have to allow for that in the barrel, and have to design in what amounts to a larger gap in the end a greater percentage of the time.

    Now you may say "Well, it seems to work," but Colt allows for rifles to be from 700 to 950 rpm cyclic rate before they ship. No doubt, some of this wide cyclic rate variation is due to varying amounts of gas leakage.

    One could argue that by not putting Park under the gas block, the production rifles would have less gas leakage on average, and hence less variation in cyclic rate. Whether this is actually a significant factor, I don't know - but if an engineer decided not to Park under the gas block for this reason, I would think that is a reasonable decision.
    Last edited by rsilvers; 12-04-11 at 21:44.

  5. #25
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    Clint,
    Most of that went right over my head.

    Back in '98 when I first brought up BFI's lack of phosphate under the FSB it was to illustrate their lack of compliance with the mil spec when they were, at the time, climbing high on the milspec wagon. Funny to see how many have run with that in the intervening decade.

    Parkerizing the FSB in place is faster and less prone to create cosmetic flaws than Parking the parts separately and then mounting the FSB. But, at best it shows a lack of adherence to the spec and at worst leaves bare, inaccessible steel that has been exposed to water, heat, and acid through the Park process. Little shortcuts like that add up to a less expensive product.

  6. #26
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    .

    Just wondering...how is that small little area on the barrel under where the FSB will reside not park'd when the rest of the barrel is?

    Is it park'd with the FSB in place, is it taped off, milled off, ...?

    .
    Last edited by ucrt; 12-04-11 at 22:48.

  7. #27
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    Parkerizing doesn't creep very much and doesn't like to stick to un prepped metal.

  8. #28
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    What is really accomplished with the tighter fit? Has testing shown any repeatable results that are desirable, or even noticeable?

  9. #29
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    This whole discussion is missing the point. Why does Colt, for example, have park under the FSB?

    BECAUSE it was assembled correctly. The pin holes are reamed after the barrel nut is torqued and the barrel is in place. This makes the FSBs truer on a factory Colt gun than the aftermarket stuff.

    The corrosion protection is largely negligible... and this gas leakage notion is absurd.
    Last edited by markm; 12-05-11 at 10:51.
    "You people have too much time on your hands." - scottryan

  10. #30
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    Quote Originally Posted by markm View Post

    The corrosion protection is largely negligible... and this gas leakage notion is absurd.
    Let's talk to the gas leak for a moment. How is the seal at the gas tube? What happens when we weld the gas block to the barrel? Any gains? What sort of ammo does one need to shoot to reap the benefits of the better seal at the gas block? Why is there a range on the acceptable cyclic rate? Is it because of the gas block seal or because of the variances in ammo, variances in locations where the gun will be shot, variances in the acceptable gas port size, variances in the carrier/gas ring interface? Tolerances stack fast and the cyclic rate is the very end result of a pile of stacked tolerances.

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