Sig-Sauer Mosquito: Hammer Spring Replacement


Sig-Sauer Mosquito: Hammer Spring Replacement

So you are ready to lighten up the double action on the trigger?  Here we go.  We are going to use a hammer spring from Wolff and since they don’t have any for the mosquito, we are going to use one for the P226 and similar *WITH* plastic hammer spring base insert.  Note there is nothing plastic on the spring, so don’t be confused.  The lightest I recommend is 17#.  Anything less will produce light strikes.  First, take apart the gun according to my previous post.

Wolff Hammer Spring:



Pull out the hammer strut.  Where the brass ring touches the hammer strut needs to be trimmed back, since the new spring is longer and this prevents the spring from being fully compressed.



chuck up the strut in a bench vice.

Slowly trim the hump back.  I used a dremel, but if you do go slow.



Now flatten it out with file and clean it all up!

And here is the finished product and you are ready to reassemble with the new spring:

I used a zip tie to keep it in place:

That’s it.  Put it back together and enjoy your lighter trigger pull.  My wife has used her gun in about 4 steel challenge competitions so far and it runs great. For practice she uses the Remington Golden’s.  The Remington’s have occasional failures, but as far as I can tell most of them are due to broken primer rings. For the matches she uses the Mini-mags without a single failure.


Good Luck!


19 Responses

  1. Stephen says:

    Hi Rob, i just read over both of your pages here and have a couple of questions. First Did u measure the trigger pull before and after this mod? Second What is the weight of the original hammer spring?

    I bought my mosquito last summer and enjoy shooting the gun. its a fun plinker at the range and has helped me get more steady and accurate as i have practiced with it. what has annoyed me with the gun is the insane weight of the double action trigger pull. I looked it up its something like 13lb 8+ounces and 6lb 4+ounces for the original setup. Just out of curiosity does your wife enjoy to shoot this gun more now that you did the trigger job and hammer spring replacement?

    Thanks Steve

    • Rob says:


      I did not measure the triggerpull before the mod. The main reason I jumped into this project was that the gun was getting really bad. The triggerbar was grinding into the frame (I did not know that before I got started) and they cycling of ammo was also not reliable. There were “waves” that were worn into the frame by the trigger bar. I’m sure that this probably not an issues for folks that just shoot the gun occasionally. After the mod my wife noticed a difference in smoothness and that is all that mattered. The double action triggerpul is still not as light as I would like it and is in the 12-13# range, but my wife never shoots it in double action so it is a mute point. The single action is just about 4# now and feels really good. Like I stated my wife shoots steel challenge with it and has no complaints.

      I tried to help out the double action more, but I just could not drop the pounds without sacrificing reliablity.

      Take care!

  2. Matt says:

    Thank you for this. It is absolutely amazing with great detail. Do you take these projects on for other people as well, or do you just “show us the way?” Either way, this was very helpful.

  3. Andy says:

    Thank you for sharing it! It is very helpful and the trigger in Mosquito is awful.
    Greetz from Switzerland!

  4. Ray says:

    Thanks for posting this! Would it be possible to list the exact Wolff part number here?

    • Rob says:


      This is the one that I used:


      Take care,


  5. jerome a. majesty says:

    I have 1 of these monsters. your detail is great. how much did you grind off since shoulder is small?

    • TM says:

      The short answer is enough to be able to cock the hammer back. The longer answer is, there is not much that needs to come off. I basically took it back until it started to angle down. If you are not comfortable with that approach you can start by clipping coils off the spring instead. Just make sure you buy an extra spring if something goes wrong or the spring tension is reduced too much. Good luck!

  6. robert says:

    STOP Don’t buy this gun!!!! I bought it for a Christmas gift for my wife at Gander Mountain. She can not pull the triger, so I tried to trade it in on something that she could fire. Guess WHAT? After paying $449 they would only give me $125 on a trade in. Great store!!!! (HA) I only shot it a few times. Never again will I deal with them or buy another Sig product. Even called the factory and the guy said thats the way we build it, live with it. Both are a RIP OFF. This was Mosquito. Junk, found out it is not manufactured by Sig.

    • Dr T says:

      I have a Walter p 22 and it has been far better and easier to shoot. Trigger was 10 times better. Wish I never bought the Sig. I Have several other real Sigs and they are only standard issue quality triggers made for the average shooter even with the SRT it’s only just acceptable

  7. Cole says:

    If I decide to do this modification, it will be the third major mod to make my gun shootable. I just recently went shooting and my slide has been sticking in the back position nearly every time I fire a round. I saw that the slide was all scratched up, so I ended up sanding down the plate on the slide, as well as the portion of the hammer that touches the plate. I made them both completely smooth with sand paper and then used my dremel polishing wheel to make it like a mirror. It worked for about 4 clips, but then started sticking again. Turns out, the crease down the center of the hammer that looks like it is only on the outside, continues within the metal on the inside of the hammer. The metal started peeling away again, creating a sharp edge on the hammer, digging into the slide, and causing it to stick again. I will try this spring replacement and will re-sand and polish the slide and hammer again. Hopefully this will fix the issue. The last thing I can think of is digging a trench down the center of the slide with my dremel. This would prevent the the sharp center of the hammer from digging another trench. If this fixes the issue, I will try to document and post my findings. Thank you for this post!

    • TM says:

      Cole, this gun has lots of Mim parts and I also polished the hammer, however i would try something with a light grit ( like 800 or 1000). Then keep the slide and hammer sloppy wet wit FP10. This gun is my least favorite and I’ve not let my wife buy another sig since.

  8. will says:

    HELP!!!!! You see the 2 spacer washer that go on each side of the hammer? I have 2 tiny little ones like those that i think goes on each side of the silver safety bar lever thing that sits inside the gun next to the hammer that sits on a pin, no diagrams and no other vids have showed these washers im pretty sure they go on this pin for that safety lever but does 2 go on one side or 1 on each side? or should i take them out? also in the sig parts diagram it shows 3 different hammer spring washers and there was only 1 little brass one on mine

  9. Mike M says:

    I did the replacement of the main spring with the Wolf P226 spring you noted in the second post, the trigger is markedly lighter on single action. You don’t say anything there about trimming the spring length. The stock one I took out was about 1 1/8 inch and the Wolf spring was about 1 3/8 long. I could not get it in untrimmed (even with the dremel notch to the main spring guide rod.) so I trimmed it to 1 1/4 long. I get light strikes about 7 % of the time using Rem ammo. Any ideas what I should try? I was foresighted enough to have bought two springs. I’ll try CCI mini mags this weekend. OBTW your directions were great !

  10. […] Now that’s it for tonight, reassemble and try it out at the range.  In my next post I’ll show you how to use the 17# hammer spring from Wolff on your Mosquito. […]

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