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GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

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  • GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

    Okay so I bought a nice 84 Doral 15' boat for $500. It needed a motor.

    Took the motor apart and found it had a broken piston (number 3) and a bent rod and scuffed up cylinder wall.

    Took it a rebuilder, claims he only rebuilds MARINE blocks. He bored all 4 cylinders to 30,000 over and put new pistons, rods, lifters, machined the cam, new oil pump, all new bearings and seals etc. etc.. Supposed to be like new.

    Put the motor in, went out for 10 minutes, water temp read 180 degrees, got some blow back, and then KNOCK KNOCK KNOCK.

    Pulled the motor out, took it back to the guy. He said it had a bad piston. (number 4)

    I put it back in. Took it out for a day, it ran at 180 again. Next day KNOCK KNOCK KNOCK.

    This time I opened the motor, saw another piston seizing up (number 3). I changed the piston, honed the wall. This time I changed the water pump, the impeller, and thermostat (I know I should have done that the first time).

    The exhuast manifold was all cleaned out, the carb rebuilt, I went over everything with a fine tooth comb.

    I thought NOTHING could go wrong.

    Sure enough the 3rd time, 30 minutes in, we got the knock, ran at 180 (which I don't know how).
    Got into shore, took the thermostat housing off, steam was coming from the engine. We checked for lots of water flow from the impeller, no problem. The thermostat was opening up as it should at 160. The water pump is brand new.

    I took the thermostat right now and it was running at 110 or 120 if that helps.

    So here we are again. I will be pulling the motor again today. I cannot afford to continue to put new pistons in.

    Is this a sign of overhearing? or another problem altogether? I didn't think 180 would be enough to cook a piston, I thought it would blow a head gasket before this.

    Was there a mistake when the engine was rebuilt the first time?

    Any help would be appreciated.

    Thanks in advance


  • #2
    Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

    I should add some more detail.

    Each time the engine had problems, what actually was seizing up was the WRIST PIN on the piston, First number 4, then 3 and now it sounds like 2 (haven't opened the motor yet) which leads to me believe this was an oil flow problem or something along those lines?

    Hope this helps.

    Comment



    • #3
      Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

      You checked the thermostat, with housing off-fine. Just because you have water coming in doesn't mean it can get out-meaning FLOW.

      I would never put old manifolds/elbows on a new rebuild.

      Comment



      • #4
        Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

        never seen a chevy seize a wrist pin, they are oiled from oil slung from the crank, make sure the pistons are on the rods right and that the rods are on the crank right if not it will put the pin in a bind

        Comment



        • #5
          Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

          Originally posted by xtraham
          never seen a chevy seize a wrist pin, they are oiled from oil slung from the crank, make sure the pistons are on the rods right and that the rods are on the crank right if not it will put the pin in a bind
          Yeah my thoughts are that when they did the rebuild, the tolerenances were not calculated right. The wrist pin is pushed in the rod and floats. I think it's not getting enough oil once they get warm and it's binding up and then the piston starts to cuff the wall.

          Comment



          • #6
            Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

            if the pins have been floated were the rods drilled so the pins could get oil? and did you change to pistons with snap rings to keep the pin centered?

            Comment



            • #7
              Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

              Originally posted by xtraham
              if the pins have been floated were the rods drilled so the pins could get oil? and did you change to pistons with snap rings to keep the pin centered?
              The pin only floats on the piston. It's fixed on the rod. There's no snap ring.

              Comment



              • #8
                Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                If you want to blame DEEP engine problems-go ahead. Knock yourself out, over and over and over.

                Your solution is allot simpler than you are thinking. See above.

                Comment



                • #9
                  Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                  Originally posted by DWJ
                  If you want to blame DEEP engine problems-go ahead. Knock yourself out, over and over and over.

                  Your solution is allot simpler than you are thinking. See above.
                  The manafold has been disassembled and cleaned thoroughly. Lots of water coming out and moving through, in and out. I don't see how that's what it could be. Please explain.

                  Comment



                  • #10
                    Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                    take it back to the rebuilder and make him fix it...thats what you paid him for, is'nt it?

                    Comment



                    • #11
                      Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                      The bent rod and broken piston sounds like a case of water ingestion. I would bet you have a hole rusted through in the manifold allowing water to enter either the exhaust or intake side of the manifold and into the cylinders. If it is the original manifold I would replace it. I would pressure test the manifold and riser for a water leak into the intake or exhaust. Any water is going to go to the back cylinders because of the angle of the engine.
                      Also because marine engines run at higher loads and generate more heat in the piston and rings, ring end gaps and pistion to cylinder wall clearences neeed to be greater than used in automotive engines

                      Comment



                      • #12
                        Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                        if 3 & 4 I`d be worried about the manifold or riser dumpin water in the motor.when you pulled the head were any of the pistons devoid of carbon?A sure sign of water in a cylinder.If you beat a piston down on the pin from water ingestion it will appear to be a seize issue but caused by water.
                        NO PERSONAL QUESTIONS, THIS IS WHAT THE FORUM IS FOR.

                        Comment



                        • #13
                          Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                          Update:

                          I took the advice to test the manifold for leaks. There are none.

                          There is also no water mixing with the oil.

                          We pulled the motor and took it apart with the rebuilder.

                          Turns out, Pistons 1,2 & 3 are all made my Hastings and are all burnt up. Number 4 (furthest from the water pump and most prone to overheating) was made by Federal Mogal and is perfect.

                          So we have a piston problem. This one brand of pistons once they get hot and expand, are seizing, breaking up, and cuffing the wall.

                          So needless to say I won't be getting the same brand pistons installed this time.

                          I am going to reuse the manifold because I see nothing wrong with it and cannot justify another $400 after all these expenses.

                          So hopfully we will get this thing going for one real boat ride before winter.

                          Will keep you posted.

                          Comment



                          • #14
                            Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                            Incorrect ignition timing can cause high temperatures and piston failure. I would not be too quick to blame it on the brand of piston. Each piston manufacture provides specifications for piston to cylinder wall clearences depending on the type of metal used and application,auto, marine, racing, etc. Forged aluminum pistons require much more clearence than cast or hyperutetic pistons. Also generally piston ring end gaps need to be larger than normal in marine engines. I would look more at the engine builder not following manufactures specification especially if he is not familar with the special needs of marine engines.

                            Comment



                            • #15
                              Sign up today
                              Re: GM 2.5 L OMC engine rebuild 3 X - NIGHTMARE

                              jeez- this summer I ebayed an awesome running 2.5. I got a whopping $200 for it... It had 115 psi on all 4 cyls. I even posted a video of it running on the auction.
                              Are you sure it's smart to rebuild these.....
                              I have a line on more....
                              Get a 3.0.

                              Comment


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