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Piston Replacement on a Mercury 150 V6, Some Advice Needed

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  • Piston Replacement on a Mercury 150 V6, Some Advice Needed

    Took the boat out this weekend for the first time with the new to me '86 Mercury 150 XR2. It seemed to be running great although there was some hesitation and every once in a while i would hear a strange clanking/knocking sound that I though was a misfire. The 150 on my 18' bowrider absolutely flew, scary fast. All of a sudden, horrible castostrophic banging/clanking sound at 3/4 throttle, kill the motor. Long story short, get it started back up and limp home with basically no power and it would die if given any throttle.

    I start it up the next day and it seemed to run perfect, no sounds. Run compression check, 115-120 psi on 1 through 5, but then 0 on #6. Pull the head and found destroyed piston on #6, see below. So there is very very light scoring on the cylinder walls, can barely feel it with my finger and cant see it with the naked eye.

    Here are my questions:

    1) Since the scoring is so light, is there any way I can just hone it and throw in a new piston? It was scary fast with 5 cylinders, as long as it wouldnt cause it to fail again, does it absolutely have to be honed? I am out of budget at this point and cant really afford that shop bill...

    2) What other things should I look for damage wise? I have most of the motor off and I will be pulling the power head tonight to crack it in half.

    3) I am trying to figure out what caused this damage, it was not running hot and it definitely had oil. I did replace the stator and a switch box prior to taking it out. Could the stator or switchbox have caused detonation resulting in this?

    4) Any general tips or suggestions on the piston replacement? I never replaced a piston before, but I have stumbled through a good deal of engine work. I do have the Seloc manual and I will be using that to assist.

    Click image for larger version

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  • #2
    Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

    Don't like being tha one ta bring "bad" news but tha motor has ta come completely apart. That kinda damage put debris through out tha motor. Could have possibly got tha crank and some of tha bearings also. It also spreads to tha front half also. Inta tha reeds and reed blocks. A "hone" will not fix it............jmo

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    • #3
      Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

      There was a bunch of debri in the bottom of the reed block at the #6 cylinder and behind the #6 piston, but I didnt see any debri in any of the other cylinders. I guess I will know more once i crack it in half and can get a good look at the crank. Maybe I should just part this thing out instead if it is really that bad.

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      • #4
        Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

        On tha "bright" side being a steel 2.0 motor and other holes are still good and crank not hurt (?) it can be fixed rather reasonably if you can do all but tha machine work..........Ya just need ta do some research on "cause" and what it will take in parts and machine work.......jmo
        Last edited by Dukedog; September 9th, 2013, 10:13 AM.

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        • #5
          Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

          As to the cause, I did some searching and one thing I can across it that Mercury recommended premium gas for these XR2 motors and that running regular unleaded could cause major detonation issues??? I definitely put regular unleaded in as I had no idea of this, had no clue... I see where some people say they run these with regular unleaded with no problems though. Is it possible running it with regular unleaded one time could cause detonation that would have resulted in the edges breaking off the piston???

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          • #6
            Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

            To my knowledge Mercury didnt require anything more than 87 octane on those 150 HP 2.0 liter fishing motors.
            Get something black on the back.

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            • #7
              Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

              Originally posted by snowbrd84 View Post
              As to the cause, I did some searching and one thing I can across it that Mercury recommended premium gas for these XR2 motors and that running regular unleaded could cause major detonation issues??? I definitely put regular unleaded in as I had no idea of this, had no clue... I see where some people say they run these with regular unleaded with no problems though. Is it possible running it with regular unleaded one time could cause detonation that would have resulted in the edges breaking off the piston???
              NO.........

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              • #8
                Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                I wouldnt think so. So I got the powerhead on the workbench and the crankcase cover removed. The lower end bearing ball bearings were reduced to small deformed metal chunks. There is a little damage to the crank case cover down at cylinder 6 from metal getting pressed into the crank case cover by the connecting rod im guessing. Is that going to matter? basically some gouges in the channel. Also, a couple of the crank shaft seal rings were broken in half and fell out when I cracked the case open.

                I removed the piston and the connecting rod looks good. Feeling the inside of the cylinder wall, the scoring is so light that i cant feel it with my fingernail, but i can feel it with my finger. I think what I am looking at is the following:

                1) Remove all pistons and crankshaft to clean out the block of any debri.
                2) Hone #6 cylinder (should i hone all the cylinders?)
                3) New piston and rings for #6
                4) All new crank shaft seal rings
                5) New lower end bearing
                6) New upper end bearing

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                • #9
                  Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                  Originally posted by snowbrd84 View Post
                  1) Remove all pistons and crankshaft to clean out the block of any debri.
                  2) Hone #6 cylinder (should i hone all the cylinders?)
                  Really needs ta be checked for tolerances. Never had much luck with just a hone and replace. If ya do hone tha others do so very, very lightly. A hand held hone can destroy a cylinder in tha wrong and/or unskilled hands real quick. A "ball" hone is good but tha ports tear one up pretty quick.

                  3) New piston and rings for #6
                  Rod/rod cap needs ta "torqued" (not on tha crank) and checked for true before ya put it on tha new piston.

                  4) All new crank shaft seal rings
                  Just replace tha broken ones. Leave tha originals in place. Broken ones were probably done when ya pulled tha front half off. That's usually tha case. When replacin' tha two take sand paper and lightly take some of tha "sharp" edge off both sides of tha ring.

                  5) New lower end bearing
                  Double check crank runnin' surface.

                  6) New upper end bearing
                  Did this one show damage/metal particles? Here again double check crank runnin' surface. Especially this one.
                  Double check tha valve/bibs on top and bottom of front half. Tha ones that had tha small tube connecting them. Tha bottom is a one-way bib. tha top is both way.

                  On tha front half with tha damaged runner. Make sure there are absolutely no penetrations completely through it. If not make sure to debur any and all "marks" in tha runner where tha debris went thru tha motor.

                  Now ya gotta figure out which went first. Piston or bearing. I'd lean toward piston. Reason, 'cause tha top bearin' didn't go to. Top get its fuel/oil mix from tha bottom. Could be tha other way 'round just as easy though...........

                  I'm sure I missed something but its a start. GL....................
                  Last edited by Dukedog; September 11th, 2013, 08:30 AM.

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                  • #10
                    Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                    Thanks Duke, appreciate all the advice. Upper end bearing looked fine, but all the rollers fell out when i pulled it off.

                    I think I may just bite the bullet and take the block to a machine shop to have the cylinder honed or bored. Im not sure if the bearing went first or the piston. Considering I was able to start it up the day after taking it out and it sounded fine, reved up with no bad sounds like the day before on the water, I am leaning towards this is the reason the guy sold the motor and just flat out lied to me. I think all that debri in the bottom of the motor was just sitting down there, and the motor would run until some of it started making contact, then all hell broke loose. It started up and ran so good after buying it, i didnt check compression, lesson learned....

                    Im planning on rebuilding the carbs and other various things.

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                    • #11
                      Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                      Originally posted by snowbrd84 View Post
                      Thanks Duke, appreciate all the advice. Upper end bearing looked fine, but all the rollers fell out when i pulled it off.

                      All early 2.0 and 2.4's do that. Don't lose 'em. Clean 'em all up. A little grease and patience it goes back together..........

                      I think I may just bite the bullet and take the block to a machine shop to have the cylinder honed or bored. Im not sure if the bearing went first or the piston. Considering I was able to start it up the day after taking it out and it sounded fine, reved up with no bad sounds like the day before on the water, I am leaning towards this is the reason the guy sold the motor and just flat out lied to me. I think all that debri in the bottom of the motor was just sitting down there, and the motor would run until some of it started making contact, then all hell broke loose. It started up and ran so good after buying it, i didnt check compression, lesson learned....

                      Im planning on rebuilding the carbs and other various things.
                      Very good idea. No sense doin' this again later on...................

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                      • #12
                        Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                        Well I pulled the crank and found this, ball bearing got slammed through...Click image for larger version

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                        • #13
                          Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                          So is the block toast or can a shop fix that hole?

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                          • #14
                            Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                            Take to a marine shop as it can be welded but it will have to be machined so seal rings will have correct radius and surface.Get a estimate first as it maybe cheaper to get a block.
                            When in doubt,Trim it out!!!
                            Business Owner & Certifed Technician
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                            • #15
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                              Re: Piston Replacement on a Mercury 150 V6, Some Advice Needed

                              the seal ring surface is unmarked, what you see in the pic is just some oily grit. All the damage is down in that race where the end of the crankshaft rotates. I cant afford a new block, every block i have found online costs more than just buying another used motor....

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