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In speaking with leading connecting rod manufacturers, you often hear that a high percentage of rod failures don't occur during the high pressure of combustion. Rather, it's during the exhaust stroke that a rod gets "yanked" away from TDC. This sudden movement of the piston causes abnormally high tensile loads in the rod's beam and leads to a fracture in this area, typically somewhere just below the piston pin end.
Also, failures can occur during either valve float or conditions of over-revving the engine. What happens is that the open valves (and lost combustion pressure) don't provide any sort of a cushion for pistons heading toward TDC. So when they pass through TDC, there's nothing to stop them from being "pitched" at the cylinder heads, often leading to another cause of tensile fracture in the beam section. In fact, the "effective" or dynamic weight of a piston passing through TDC under these conditions can be far in excess of its actual static weight. Multiple times, in fact.
Yet another common location for rod failure is a portion sometimes called the "hinge point," which is generally where a connecting rod's beam section changes in cross-section area (wide to narrow). Connecting rod designers frequently work in this area to determine the best compromises between rod strength and material selection. Of course, you should always include proper rod side-clearance, making certain not to provide excessive dimension that allows oil to create over-oiling of cylinder walls. Insufficient side-clearance can lead to over-heated and failed rod bearings, as well.
hmm, thinking about it at work our small engines if the connecting rod breaks, it breaks more toward the piston pin where the profile does get smaller and changes alitle.
Hard to tell where in the stroke it breaks other then the piston being left towards the head of the motor, but when it lets go it totally fubars the bore and even had them punch a hole the size of my fist out of the block.
Mods: Steeda CAI, FRPP hotrod's, Black GT500 staggered 18's, GT/CS front end, Airlift Suspension, and more
Near future..... Ford racing intake and hotrod cams. Not really feeling the nearly $1000 a piece price tag on new ford racing heads. So I'm going to see if I can get a stock set machined, ported and a valve job and use them. Then we'll see where we need to go from there.
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