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Should I bore only one cylinder?

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  • Should I bore only one cylinder?

    I have a two cylinder with one burnt piston and cylinder wall. I am being told to replace one piston with an oversize after Boring the one cylinder wall to .020 over (Hopefully that is all that will be required). I am being told since the other cylinder wall and piston look fine to leave it alone and only replace the rings and maybe hone lightly the other cylinder wall. Does this sound right? Certainly not what I am accutomed to with cars motors. I still do not know why this one burnt. Carbon on piston indicating good fuel and oil. Burnt on exhaust side the top and side of piston. This was the lower piston on my two cylinder.


  • #2
    Re: Should I bore only one cylinder?

    get a second opinion on it, whoever is telling you not to bore a second one may be right, but from what i'vebeen taught to do to "small engines"... i have been taught to bore all the cylinders for simply the life of them..

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    • #3
      Re: Should I bore only one cylinder?

      OK. The cylinder LOOKS fine but how does it measure? Yes, it's OK to bore one but I would bet that for the minimal cost of boring and one piston you want to go that way. You need new rings for the other cylinder after you hone it anyway. Have you figured out what caused the failure?Good luck!

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      • #4
        Re: Should I bore only one cylinder?

        We can not find any obvious cause for the failure. I was not on the boat when it occurred. It was "loaned" out. I don't know if it was overreved, it does not appear to have been lean, any thoughts?

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        • #5
          Re: Should I bore only one cylinder?

          Sandman... You don't say what engine you have. If your engine has more than one carburetor, more than likely your engine failure was due to the lower carburetor becoming fouled, resulting in an improper mixture (the carbon build up on the piston does not indicate a proper mixture). Always rebuild all carbs when rebuilding an engine, and make sure the ignition is perfect.Now, if you have damage on only one piston/cyl, and the other(s) are fine, do not go to the expense of boring a perfectly good cylinder and the added expense of another oversized piston. If your engine was a V/6 with damage to one (1) cyl/piston, would you bore the other five cyls and go to the expense of purchasing five (5) more oversize pistons? (not likely... or needed)! The difference in weight is about zilch and will cause no off balance problems. The bigger bore on one cyl is of no consequence as the bigger piston will in effect have the compression equal/even out with the standard piston. Good Luck.....
          Our Questions Require Answers... If You Refuse To Answer Our Questions... How Can We Answer Yours?

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          • #6
            Re: Should I bore only one cylinder?

            Yes, because you are 90% there. I would do them both. It will also increase HP (more bore/stroke=more HP),even if only a small amount.And you will keep the original ballance of the crankshaft intact.
            "Did you hear about the guy whose whole left side was cut off? He's all
            right now."

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            • #7
              Re: Should I bore only one cylinder?

              Find the cause. The damaged piston is a result -- not the cause. If you don't you are setting yourself up for a repeat failure. Old fuel, wrong plugs, dirty carb, double firing pack -- something caused the piston to "burn". Good luck!

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              • #8
                Re: Should I bore only one cylinder?

                I agree, something caused this but we have been unable to determine why. Are there suggestions or steps to follow that may help us to rule out things as we go along? The manual helps us in repairs but I do not see much help in trouble shooting. Can you suggest steps in determining what went wrong?

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                • #9
                  Re: Should I bore only one cylinder?

                  Howdy, Sandman.I agree with those above. Whatever caused your problem is probably still there. Your carburetion system is a strong suspect. Be sure to fix the cause before running the engine again.Overboring one jug is a sure way to unbalance your engine. BAD news. Don't even consider it.Red sky at night. . .JB
                  sigpic

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                  • #10
                    Re: Should I bore only one cylinder?

                    Hey Sandman here comes my two cents worth. I agree with Joe Reeves, I've overhauled a lot of outboards; it don't make any differance. As far as hopping it up, I don't think that you will notice any differance. One of the reasons that you can get away with boreing just 1,2,or ever how many is that each cyl. is a little engine in itself, and fires each time that the piston approches top dead center. A 4 stroke fires ever other time the piston aspproches TDC and all cyl.s interact with each other through a common crankcase. I have just completed a 90 HP that 2 cyl.s have been bored to .064" one at .020" and one at .044". I don't think that I would do my own quite so radical, but this is a rental engine and the owner just wants to get it back in service. I know that some people might dissagree, but that's what I've always been taught, I have never had any trouble with it.

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                    • #11
                      Re: Should I bore only one cylinder?

                      Only 2 cyl.

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                      • #12
                        Re: Should I bore only one cylinder?

                        Yes, it is a Johnson 1990 40HP 2 cylinder.

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                        • #13
                          Re: Should I bore only one cylinder?

                          Hi, Joe and ARA.I don't know where you guys learned your physics.A two, three, or any number of cylinder engine is ONE engine, not a bunch of little independent engines that don't affect one another or contribute complimentary and opposing forces to the whole. The same is true of the reciprocating masses in that engine. A +.030 piston weighs more than a stock piston. If you have one of each you have imbalance.Your crank, seals, etc. may tolerate an imbalance. . . for a while. That doesn't make it a correct thing to do, but if it's your engine, take whatever chances you wish. If it's someone else's engine, that is a different matter.Red sky at night. . .JB
                          sigpic

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                          • #14
                            Re: Should I bore only one cylinder?

                            Just a thought -- Wiseco has made sure their oversize piston assemblys are within weight tolerence of standard pistons. That's why you can run different oversize pistons in a two cycle outboard motor. Go by cylinder measurements rather than weight. I still think you will end up boring both anyway. By the way -- did we figure out what caused the failure?

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                            • #15
                              Re: Should I bore only one cylinder?

                              I completely agree with joe and almost retired, it won't hurt anything to just bore one cylinder, I have done it hundreds of times and never had a problem. Balance in a 2 stroke is not nearly as important as it is in a 4 stroke, unless your racing or turning the engine 9000 RPMs. In fact when johnson built the first 3 cylinder everybody told them it would never last because it wasn't balanced but look at them now, they are a very popular engine for reliablilty.

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