Thread: Building A looper Beast

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    Quote Originally Posted by R Austin View Post
    Yes, the answer is in the pictures. Remember all cylinders have like clearences. The guestion you have to ask is how can the piston hit at both top of stroke and at bottom of stroke.
    Just a few thoughts: 1) The closest I've been able to run piston to head clearance without damage for OMC triples (with lighter/smaller reciprocating assemblies) spinning 8000-8500 is .028" (factory is appx .045"). At .025", engraved markings are embedded in the combustion chamber. 2) Connecting rods are designed for compressive loading. From the looks of it, the rod failed in tension, likely induced from contact with the bearing housings as shown. 3) Free revving such a wicked beast might easily have achieved 10,000+ RPM. Memory from prior Merc 1500 post mortems suggests the rods were only good for 7500-8000. 4) It there's evidence of contact on other pistons/rods, you should crack check all parts at a minimum, and seriously consider a new set of rods.

    Tim

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    Quote Originally Posted by Powerabout View Post
    crank pin offset?
    Crankpin offset is "STROKE". Stroke is a fixed number machined into the crank and can not change. However, effective stroke can change. ????

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    Quote Originally Posted by Tim Kurcz View Post
    Just a few thoughts: 1) The closest I've been able to run piston to head clearance without damage for OMC triples (with lighter/smaller reciprocating assemblies) spinning 8000-8500 is .028" (factory is appx .045"). At .025", engraved markings are embedded in the combustion chamber. 2) Connecting rods are designed for compressive loading. From the looks of it, the rod failed in tension, likely induced from contact with the bearing housings as shown. 3) Free revving such a wicked beast might easily have achieved 10,000+ RPM. Memory from prior Merc 1500 post mortems suggests the rods were only good for 7500-8000. 4) It there's evidence of contact on other pistons/rods, you should crack check all parts at a minimum, and seriously consider a new set of rods.

    Tim
    Again all cylinders have the same clearances. There are no indications of any contact of parts of any kind in any other cylinder. Piston to head clearance was set at 20 thou. The failure was unique to this cylinder and nothing related to clearances, fit-up or assembly.

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    Quote Originally Posted by R Austin View Post
    Yes, the answer is in the pictures. Remember all cylinders have like clearences. The guestion you have to ask is how can the piston hit at both top of stroke and at bottom of stroke.
    The answer is bad rod bolts and/or rod threads, or under-torqued rod bolts, that allowed rod over-travel.

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    Quote Originally Posted by Tim Kurcz View Post
    The answer is bad rod bolts and/or rod threads, or under-torqued rod bolts, that allowed rod over-travel.
    Or, possibly an oversized rod bore, undersized crankpin, or any combination that allowed the overtravel. Could even have been something as simple as a defective rod.

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    Quote Originally Posted by Tim Kurcz View Post
    Or, possibly an oversized rod bore, undersized crankpin, or any combination that allowed the overtravel. Could even have been something as simple as a defective rod.
    Thinking in the right area. Rod bolts with bad threads or bad material will not torque properly, they will either strip out or just soft stretch and break before torque value is reached.
    Think about the micro pieces between the piston and head. Both rod bolts are broken and look alike. I think they both broke simultaneously by another cause that can change effective stroke.

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    Quote Originally Posted by R Austin View Post
    Thinking in the right area. Rod bolts with bad threads or bad material will not torque properly, they will either strip out or just soft stretch and break before torque value is reached.
    Think about the micro pieces between the piston and head. Both rod bolts are broken and look alike. I think they both broke simultaneously by another cause that can change effective stroke.
    It's possible the rod may have been brittle due to improper heat treat; the bolts would have torqued perfectly, then threads or other features fractured with a rap on the throttle. Suggest stereoscope examination of the failures and Brinnell testing if the debris will support it.

    Another possibility is the rod bearing diameter may have been over-size and/or the crankpin may have been undersize. Such a mis-match would not be detected rotating the assembly by hand, and cause a hammering sufficient to break bolts or the rod at extreme RPM.

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    My guess and it is purely a guess, I would say that this particular rod was a just a bit too long compared to the others. The way it is bent led me to this conclusion and would explain why the others were not affected.
    Gardner Miller
    Lone Star Outboard Racing Association

    "Water is for racing. Asphalt is for the parking lot."
    Remember....Freedom isn't...."Free".......

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    Quote Originally Posted by Tim Kurcz View Post
    It's possible the rod may have been brittle due to improper heat treat; the bolts would have torqued perfectly, then threads or other features fractured with a rap on the throttle. Suggest stereoscope examination of the failures and Brinnell testing if the debris will support it.

    Another possibility is the rod bearing diameter may have been over-size and/or the crankpin may have been undersize. Such a mis-match would not be detected rotating the assembly by hand, and cause a hammering sufficient to break bolts or the rod at extreme RPM.
    The crankshaft and the rods were all seasoned parts from a service engine. There is no bluing on any parts in the motor including the damaged rod or its rollers from skidding.

    I still believe that the most important clues are the micro metal particles in the piston and the head, and the fact that not a single piece made a trip between the piston and the cylinder wall or sheared between the piston and port. This would indicate that the piston stopped after its last compression stroke when the exhaust port opened and there was no energy left and no rod cap to move to complete BDC. That means that the rod bolt failure was both simultaneously and off the rod in one partial revolution. Had one bolt broken, it would have made a couple of cycles and released rollers into the ports and/or cylinder.

    My read on the sequence of events.

    The part that failed, broke and started releasing micro particles because that would be the only size that would filter out into the engine. The failure of this part would start to allow over stroke of the piston in both directions. As the cascading failure of this part continued the expansion of the stroke extension would increase, as evidenced by the piston striking numerous times in both directions. In one revolution the failing part and rollers wedged and jammed between the crank journal and rod causing a instantaneous release of the rod cap, both bolts breaking, during its last compression stroke. The piston fired with the journal still in the upper half of the rod. The piston stopped, as previously noted near BDC, with the rod hanging and with the next revolution of the crank, the journal caught the rod in the beam section just above the large end of the rod.

    The failed part (???) which you know by now, failed in a cascading fashion. The resulting catastrophic failure of the rod in just over one revolution, and less than two.

    PS. Just to be clear, I broke the motor. Maybe over reved, no it over reved. It was a totally different animal than the first start.
    Butttt, it was an OEM purchase part that failed.

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    Dick, Sorry for your loss. I know all to well the sinking feeling when one fails. I think you are correct on the rod failure part as the pictures show that one bolt either sheared or broke at the parting line and then the cap broke into three or more pieces. The failure just at the I beam was most likely the second break before total big end failure. The I beam was as you said from the crank first the bend and then the break. The fine particles are maybe from the bolt powdering out on breaking, just my thoughts. Hope all goes well next time and maybe have all rods shot peened or cryo treated after maybe a full Xray. Good luck and hope to see you at race in Constantine, Steve

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