SAN 553 malfunction and fix | SIG Sauer 556 Forum

SAN 553 malfunction and fix

Discussion in 'Technical Discussions & Troubleshooting' started by CGRBB, Nov 25, 2019.

  1. CGRBB

    CGRBB Active Member

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    I have had a SAN 553 rail model for about four years now. I purchased it new utilizing GunBroker. It is an Exeter import, not a JDI gun. It is a registered SBR, and is equipped with a Swiss folding/collapsing stock. When I first bought the gun, I almost immediately purchased a SAN four-position gas valve, as I intended to equip it with a suppressor occasionally. I initially shot the gun unsuppressed, and noticed some relatively nasty malfunctions from time to time. I didn't bother looking into it much at the time, chocking it up to break-in finickiness. The silencer I intended to use on the gun soon received its stamp, and I fastened it on. I was oblivious at first, but eventually noted that the gun no longer experienced any malfunctions. I suspected some kind of under-gassed situation, but hoped that perhaps the gun had worked out its teething issues during my shooting with the suppressor. Sure as shit, as soon as I remove the suppressor, I experience a malfunction within 10 rounds. It is also worth noting that I did not use what would be the "suppressor" setting on the four-position gas valve while shooting with the silencer, as I felt the ejection was rather weak. At the time, I was not in the position to diagnose and/or repair the gun, so I re-attached the suppressor, and it lived that way.

    Fast-forward three years. I decide I am going to address the problem with this gun, as it is the only one of my Sigs that has issues, and it especially bugs me because of its SAN origin. I remove the silencer, and fire thirty rounds. Five of those rounds are involved in the same type of malfunction I experienced years before. It is worth noting that I am shooting factory-loaded 55gr 5.56 brass-cased FMJ ammo, to eliminate that as any kind of contributing factor. I have also re-installed the factory 2-position valve at this point, to eliminate the four-position valve as a potential culprit. The malfunction is always the same; the fired case fails to completely eject, and ends up jammed in the ejection port backwards, as the next round is fed into the chamber. Considering the ejector engagement is fine, I conclude there must be some kind of slight under-gassing problem. I disassemble the gas components of the rifle, and take the following measurements with a digital caliper. I used drill bits from a quite thorough set to measure the ports, and then took a measurement of the shank of the bit with the calipers.

    Small (normal conditions) valve port - 1.1mm or .0435"
    Large (adverse conditions) valve port - 1.25mm or .0495"
    *The valve is marked 1.2 and 1.3

    The port in the barrel measured 2.2mm or .0860" and the port in the gas block and gas tube are both visibly larger than the barrel port, so I didn't bother measuring them. The valve is certainly where the problem lies.

    I reassembled the gun and re-attached the silencer. I wanted to see which of these two ports the silencer seemed to prefer, as my plan was now to make this two-position valve into a "suppressed/unsuppressed" valve. The gun functioned just fine on both settings, so I decided to designate the smaller of the two as the suppressed port. I removed the valve, clamped it in a small vise, and put it under the drill press. The large port was aligned under the chuck, the vise was clamped to the press, and I picked a bit that looked like it would be a small step up. I think we all know these valves are hard. Needless to say, the bit I used probably ain't worth a damn anymore, as it was not a proper machinist bit. But, the port had been enlarged. I cleaned up the inside of the valve with a quick touch of a dremel, and back into the gas block it went. At this point it was pouring rain outside, and my post-repair testing was abbreviated. I fired 15 rounds unsuppressed using the new port, and experienced no malfunctions. Considering my previous malfunction rate was nearly 20%, I figured this was a satisfactory performance, and probably not just chance. More testing will of course be performed at the next opportunity, but I was eager to share my small project on the forum, so I will just update this post later with applicable follow-on info.

    The new large (now "unsuppressed") port measures 1.53mm or .0605"

    It certainly bugs me that a gun from the much-lauded SAN factory gave me such trouble, especially considering the rich history of the 552/3 model. Maybe someone with more insight can provide additional info as to why this was.
     
    Last edited: Nov 30, 2019
  2. CGRBB

    CGRBB Active Member

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    F54FD9FD-FA6E-4373-96C9-9E1EB3E48AC7.jpeg 08380562-E594-4506-BC36-837E7B69A2D4.jpeg Pictures of the persistent malfunction
     
  3. rambo

    rambo Member

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    I don't have an answer for you but I'm wondering if you also measured the 4-position to see how it compared.
     
  4. CGRBB

    CGRBB Active Member

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    The gun never ran correctly with the four position valve either, so I opted to do all measurements and modifications on the less pricey two position valve. The four position valve only serves to give a smaller (intended for silencers) and off (rifle grenades?) setting. If my memory serves, the two normal ports are the same as the two ports on a regular valve. Certainly wouldn’t hurt to check them though. I will see if they differ at all.
     
  5. CGRBB

    CGRBB Active Member

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    Upon measuring the 4-pos valve, it seems that it is a bit different, even though it is marked the same (1.2 and 1.3). The “normal” port measures 1.26mm or .0495”. I didn’t have a bit that fit right in the “adverse” port on this one. I had one that was barely on the verge of sliding in, but wouldn’t actually go. It measured 1.5mm, so the port is just a touch smaller than that. Interesting to see the slight variances. Sadly, I still experienced that 20% malfunction rate, even on that ~1.5mm port on the 4-pos.
     
    Last edited: Nov 30, 2019
  6. jb76

    jb76 Member

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    Did you measure the Piston head compared to the interior of gas valve? Just wondering. Maybe there was a piston head revision? Creating too much gas blowby? The us full length sigs had the same piston size as the 551's as those has parts were interchangeable. But the short 556 patrol gas system had no interchangeability with the 552/553 systems.( Smaller piston head)
     
  7. CGRBB

    CGRBB Active Member

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    Not to mention the length of the shorter gas systems differed USvsSAN. I would have nothing to compare the OD/ID figures to, as I do not have another SAN 553. Fitment compared to a 551 or US short gas would be inconclusive due to the gas system lengths, etc.
     
  8. CGRBB

    CGRBB Active Member

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    I have 552 components, but feel that would also be inconclusive, due to the fundamental differences in the layout.
     
  9. Bosley

    Bosley Active Member

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    Have you tried over-lubricating it, running it real wet to see if that makes any difference?
     
  10. CGRBB

    CGRBB Active Member

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    I never considered that. It may have provided some trivial results, but the problem still would have needed solving.
     
  11. hooch

    hooch New Member

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    Even on a piston upper, especially if this was a form 1 DIY sbr, dwell time and reciprocating mass could be the main issue
     

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