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Funky screws in Mercury 20hp carb.

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  • Funky screws in Mercury 20hp carb.

    Hi. Today I am working on a 20HP mercury boat motor. Good compression, spark, won't cough, got carb kit after seeing carb was filthy inside and a light cleaning didn't make the motor run.

    Fun starts while trying to get new screws as most where VERY tight and a few had the heads half strip when removing them

    1980's American engine (At least I assume), everything is imperial.. So I go down to lordco and ask for some #6 machine screws in course and fine.

    Go to compare #6-32 screw with thread.. the 32 is wayyy too course. Find a metric M4.. and it fits.. but of course they don't have metric *screws* just bolts.

    Ok so ID'ed one. Next was the 'fine' screws... Try and screw a M3 nut I have (At least I think its M3! bin was marked M3) I had at home and it does not go on. Measure threads... None of my gauges fit, but the 40TPI seemed to come closest, screw thread appears a little wider, maybe 42?

    OD is 3.35mm or 0.132", Pitch of 0.604~ or 42

    Have I found some kinda crazy 3.5mm screw?? Am I doomed to walk the earth forever more in search of 3.5mm screws?
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

  • #2
    No idea.. but if it is a M3.5-0.6 screw - they are easy to find even here.

    Maybe someone like me put them in years ago when the right screw wasn't in my box

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    • #3
      6-40 extra fine thread

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      • #4
        Looking at ebay I can get 10 SHCS M3.5x12 for $12 after shipping to canada.

        Or 100 of em in cheese head, made outta cheese metal, guaranteed to cheese me off when they strip just inserting the things, for $6 from china.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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        • #5
          #6-40 is fine thread. #6-48 is extra fine.

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          • #6
            can you re-touch up the heads of the old screws now that they are free'd up?

            even if you have to put a huge flat blade slot through them or something?

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            • #7
              Originally posted by Rosco-P View Post
              6-40 extra fine thread
              Hmm, my spec sheets say those should be 0.138", or 3.5052mm.. Very close indeed but rather larger then mine. and mine didn't really mate up with 40TPI, my cheapo thread gauge might be off but it was at least a half tooth off by 1/2" suggesting at least 41~ tpi.

              M3.5 are 3.354mm to 3.479mm

              I am suspecting metric a little more because the other screws in the carb are metric. Although I have no idea what an American motor is doing with a metric carb.

              Just don't tell me I need british gas to use a metric carb
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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              • #8
                Originally posted by A.K. Boomer View Post
                can you re-touch up the heads of the old screws now that they are free'd up?

                even if you have to put a huge flat blade slot through them or something?
                My bigger concern is some of them where so tight they felt like they could snap off at any moment, some even had a lot of torque after a turn or two before they loosened and iv had heads suddenly strip/snap in half after that much force for several turns. I don't trust screws after that much torque has been applied, especially such small screws. I could just barley remove them with a normal screwdriver (I hope to never use my manual impact on a carb!). they are also very rusted and this is a boat motor so I would like to go with new.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  Mikuni carb?

                  What's the carb manf and numbers?

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                  • #10
                    I have just sold my Mercury 2.2, which I bought in 1989. It is actually a Tohatsu 3.5 with an aluminium disc with a small slot in it between the carb and the cylinder, to reduce the power.
                    So not all Mercuries from the 1980s were made in America, but rather in Japan, and it would be quite normal for them to have metric threads.

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                    • #11
                      Originally posted by lakeside53 View Post
                      Mikuni carb?

                      What's the carb manf and numbers?
                      No idea. has a circle mark with.. X diagonal from a K, if you hold it upside down..
                      Another circle mark with a P and two dots. then a 1 next to that.

                      Numbers on the outside of the carb are: WMC9-1785. There are some numbers on the inside of parts (float bowl and butterfly flap) but I kinda doubt those are useful.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

                        I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.

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                        • #13
                          Originally posted by RandyZ View Post
                          Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

                          I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.
                          Not sure there are any mercury dealerships near me.. Nearest boating place is a 30 minutes drive away, although I do need go near there soon to drop some stuff off at recycling.
                          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                          • #14
                            Originally posted by RandyZ View Post
                            Why don't you just go down to a deal and order the screws you want? Probably cheaper than driving all over looking for odd balls.

                            I worked at a mercury dealership last summer, so as a word of advise, bring your serial number of the motor. The way Mercury made changes on the fly, the only way to get the proper part is by the serial number. Mercury's computer system needs the number to even open the search window.
                            What Randy said.
                            Eight different carbs types where used for the 2-40 hp Mercs according to my Mercury shop manual. Manufacturers include Tillotson, Mikuni, Keihin and Walbro. So yes depending on what you have it could very well be metric.
                            Don't sound surprised to hear that metric threads are used on an old American product. Ever find a spark plug with imperial threads?
                            Home, down in the valley behind the Red Angus
                            Bad Decisions Make Good Stories​

                            Location: British Columbia

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                            • #15
                              Originally posted by Willy View Post
                              What Randy said.
                              Don't sound surprised to hear that metric threads are used on an old American product. Ever find a spark plug with imperial threads?
                              Next your going to tell me the OD is not even imperial and my spark plug sockets all lie.
                              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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