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1972 Broken Piston Engine Rebuild

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  • 1972 Broken Piston Engine Rebuild

    Some of you maybe saw my thread from last week about re-sleeving my block due to scoring inside my number six cylinder wall. Weak compression was shown on a gauge and the car now burns significant amounts of oil. Yesterday, Todd and Dave helped me yank the engine out and strip it down.

    There were some good surprises when removing parts, such as a two row timing gear set and chain, the internals of the engine looked clean and fresh for a car with 100k miles on it. The best news is that the end of the cam was stamped with "GP2." SCORE!

    The PO sold me the car as a cosmetic refresh, he turned a sitting car into a nice looking driver. When he got the car, it started right up on an auxilliary tank and other than rebuilding the carbs, running new fuel lines etc. no other engine work was done by him. No claims of a rebuild were made by him.

    A receipt dated in the late '80s from a PO prior to my PO lists a new cylinder head being installed (the car has a post '72 head), cylinders honed, the crankshaft being cleaned up and a new camshaft being installed. I have no idea if this is when the GP2 cam was done. But according to the receipt, this was approximately 25k miles ago. Maybe the car over heated and cracked the head?????

    Since I was unaware that I had a GP2 cam, the valve lash is set to 0.010" instead of the recommended .017/.015 lash by Goodparts.

    My question to you all is, what caused my piston to let go? Pics follow;

    Todman turns wrenches



    Dave Helps



    Deep gouge and another 180 degrees opposite



    Side 1



    Side 2



    Piston markings





    1972 Sapphire TR6 #CC84,something

    1959 Red TR3 (Wife's)

  • #2
    The top compression ring is not broken, the middle one is.
    1972 Sapphire TR6 #CC84,something

    1959 Red TR3 (Wife's)

    Comment


    • #3
      Could have been because the ring wasn't gapped adequately...when heat expanded it, it broke..
      You got to check them gaps.
      Driving a 1973 TR6
      Doing ZS carb repairs
      email kencorsaw"at"aol.com

      Comment


      • #4
        You think after 25k miles? Me thinks it was perhaps that truarc ring broke and thrust upwards compressing the ring lands breaking the first solid ring. Temperature could have been a factor, did it overheat?
        72 Pimento w/overdrive

        Comment


        • #5
          Could have been because the ring wasn't gapped adequately...when heat expanded it, it broke..
          You got to check them gaps.

          I'll second Ken's opinion. Bad ring gap + too much heat = a bad day

          Nice find on the cam, Aaron. Now get er all set up properly and you'll have a reliable engine.

          Cheers,

          Kevin
          ps keep an eye on Toad though, when your not looking, he'll unbolt a part he needs :woohoo:
          76 TR6 CF58170UO (The Lady)
          72 TR6 CC80068UO (The Slut)
          68 TR250 CD4893L (retirement project)

          Comment


          • #6
            Where is the other end of the circlip in the 4th picture? If that broke off it would cause all sorts of issues.
            1976 TR6 originally white now Porsche Voodoo Blue
            Frame off resto started May 2015
            Tshirts and TA boxes replaced
            Diff braced and reinforced
            Engine and head rebuilt
            Header and Intake ported
            SS exhaust
            Floors repaired, new metal at rear bumper mounting points
            New Rockers
            AAW wiring - GM alternator
            Complete suspension and brake rebuild, poly bushings

            Comment


            • #7
              I heard of some increasing the cylinder size for the piston nearest the firewall because this area generates the most heat. The increase is suppose to help with expansion. Anyone think this is a good rule of thumb to follow?
              1972

              Comment


              • #8
                I heard of some increasing the cylinder size for the piston nearest the firewall because this area generates the most heat. The increase is suppose to help with expansion. Anyone think this is a good rule of thumb to follow?
                The circlip was broken and on the side(probably sticking w cold oil) of the piston, but hard to say when & where it was. What you don?t see is both sides of that piston broke so the perfectly round hole for the wrist pin and circlip ain?t so round any more.

                Comment


                • #9
                  heat. Now what caused the heat? without a metallurgy analysis of the piston??? When going back together I would make sure piston/cylinder wall clearance is towards the wide side, allowing more room for expansion. Make sure the rings are gapped and gaps positioned properly. See one of the oil control ring gaps over the wrist pin, which is a no no, but still better than being on a thrust surface. Make sure the honing is done properly with extensive soap and water clean up after honing and lots of lube on the cylinder wall, piston and rings going together.

                  Comment


                  • #10
                    And the engine doesn't have to overheat to crack a ring without sufficient gap..
                    But, the ring could also have gotten broken when 'whoever' installed those AE pistons.
                    I can't remember which time it was but unexpectedly I discovered a couple broken rings during an engine teardown for other reasons..It's not always a catastrophic event. Under some circumstances, broken rings can remain in place around the piston for who knows how long.
                    Compression in that cylinder might be a little (or a lot) lower than others or oil consumption might be higher than desirable for the mileage...the effects can vary.
                    Driving a 1973 TR6
                    Doing ZS carb repairs
                    email kencorsaw"at"aol.com

                    Comment


                    • #11
                      If you look at Skootch's 4th picture (side 2) I believe you can see the other half of the truarc ring thrust up into the bottom piston ring on top of the remaining in (sort of) place truarc ring. That's why I was wondering if that went first and caused the grooves to deform and break the ring.
                      72 Pimento w/overdrive

                      Comment


                      • #12
                        If you look at Skootch's 4th picture (side 2) I believe you can see the other half of the truarc ring thrust up into the bottom piston ring on top of the remaining in (sort of) place truarc ring. That's why I was wondering if that went first and caused the grooves to deform and break the ring.
                        I think you're right on Lou.
                        When my engine broke a ring it peeled up the top edge of the piston, but did no damage to bore.



                        CF1634U+O Pimento/Chestnut
                        2nd owner, since 1975
                        Soon to be in Fair Oaks, CA

                        Comment


                        • #13
                          It is at the scene of the crime.
                          Driving a 1973 TR6
                          Doing ZS carb repairs
                          email kencorsaw"at"aol.com

                          Comment


                          • #14
                            Thanks guys for the help.

                            Is a broken ring the cause of all the missing metal around the circlips for the wrist pin? Baws' pictures shows a perfect circle around the pin as do my other five pistons. But mine is U-shaped on one side of the piston and is missing the top bit of metal on the other.
                            1972 Sapphire TR6 #CC84,something

                            1959 Red TR3 (Wife's)

                            Comment


                            • #15
                              It's pure speculation, but I don't think a piece of a ring could do all that...more than likely now that I think about it and look more at your pictures, whatever damaged the piston around the wrist pin and circlip set off a chain reaction of destruction including the piston ring...
                              Driving a 1973 TR6
                              Doing ZS carb repairs
                              email kencorsaw"at"aol.com

                              Comment

                              1972 Broken Piston Engine Rebuild

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