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1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

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  • 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

    Hey there I'm was givin a 16ft fiberform boat with a 115 hp merc on it... I was told that it's a 1971 but can't confirm thy yet. The engine seems to run fine... Rough idle but revs up fine. I pulled the top plug on this inline 6 cyl. And there is a small plate of metal covering the inside of the spark plug hole. It is not the top of the piston the cylinder has compression but very little because of the metal piece... The othe five cyl below dint not have this. I'm stumped and the only I an think of is that it is there to create crankcase pressure... Any help would be appreciated thanks. I'll post a picture later on

  • #2
    Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

    That doesn't sound normal. The serial number will tell you the year of the motor. Look it up at the maxrules.com web site.

    If you remove the top transfer cover (starboard side of motor) you will be able to see into the combustion chamber and see what is up.

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    • #3
      Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

      Thats most likely a reed valve that has broken and gotten into the combustion chamber, a broken reed would also a poor idle.

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      • #4
        Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

        It would have done some damage fighting through the transfer port.

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        • #5
          Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

          Click image for larger version

Name:	115 hp cylinder.bmp
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ID:	7001924Click image for larger version

Name:	115hp plug.bmp
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ID:	7001923
          i did some research. the reed getting sucked into the cylinder is the only resonable thing that could of happened i believe. here is a couple pictures of the plug which in normal... and also the plug hole.

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          • #6
            Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

            Those pictures are too small to see. I would pull the transfer cover and see what is in there. It might be something wierd like Jimmy Hoffa's body...... I had a piece of a clam shell in my old Johnny Fat-fifty's crankcase. Didn't seem to bother it any...

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            • #7
              Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

              I have seen that a number of times, I can assure you its not Jimmy Hoffa, a reed valve or anything else that got sucked in. It is vaporized aluminum from the crown of the piston in that cyl. When you take it apart you will see a very eroded crown and likely a destroyed piston with stuck rings and melted aluminum on the cyl walls. It happens during detonation when the piston top is so hot it starts to melt and vaporize. At some point the molten mess splashes into the hole and cools. You can see the imprint of the spark plug in the metal "plug".
              Mercury inlines....nothing else stacks up
              The answer to "do i need to change my water pump impeller" is YES

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              • #8
                Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                you merc guys have a much better sense of humour than the omc guys, and im an omc guy! if you wanna get there first.....buy a merc! if you wanna get back home....buy an omc! lol (outboards are outboards.....love or hate em, they all have thier quirks!)
                many men fish for years without ever knowing its not about catching one.

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                • #9
                  Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                  thanks for the replys. i did a little more research on the motor it is actually 1975. i found an online manual that i downloaded and seems to me that the head and the block are all one piece. is this correct? whatever it is that has happened in that cyl i know it needs to come apart, so ill let you know what the problem actually is.

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                  • #10
                    Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                    Yeah I think daveswaves has it right. When you take it apart post some pics of what the inside looks like. So I can have a idea what mine looks like..lol..My 115 did that. I poked that "plug" out with a screw driver and used a magnet to get the pieces out of the cylinder. I figured that's what happend so I wheeled it to the back of my shed for later project.

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                    • #11
                      Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                      Gee, if you remove the upper transfer cover, you can see the sides and top of the piston, the rings and the cylinder walls. You may even see the spark plug hole.

                      The head is non removeable, and tearing her apart before looking is dumb.

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                      • #12
                        Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                        i pulled the motor off of the leg yesterday. split the crankcase and sure enough the top piston had melted. daveswaves says this is caused by detination... any other possible causes? im ordering a new piston and rings for it this week after i get the cylinder wall all cleaned up just to make sure it is salvagable.

                        where do you guys find is the best place to buy your parts? im live in british columbia. going to call around for some prices.

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                        • #13
                          Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                          Make sure you re-ring the other cylinders and replace the trigger as well...
                          When in doubt,Trim it out!!!
                          Business Owner & Certifed Technician
                          Boats I own & use
                          20ft Javlin w/250 Junkrude
                          17ft Bullet w/200 Merc (SOLD!!)
                          13ft Allison w/150 I-6
                          17ft Hydra-Sport 175GT
                          15ft Allison w/75 Stinger(project)
                          14 x 48 Drifter with 1100SS Merc

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                          • #14
                            Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                            Also rebuild the water pump as the TOP cyl is the first to overheat as pump output is reduced and fails to keep the block completely filled.

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                            • #15
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                              Re: 1970s 1150 mercury 115 hp top piston has metal plate over plug hole???

                              Yes I am ordering new rings and a new piston... The trigger? Which parts is this?
                              Yes the leg will get pulled aswell rebuild the water pump and check out the gears.

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