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11.5:1 pistons - broken crank?

Old 08-20-2006, 10:24 AM
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11.5:1 pistons - broken crank?

Well, after just getting my SH back together last November (spun rod bearing) and after only 2k miles she left me stranded again a couple weeks ago. I finally got the motor out and pulled the rods/ pistons last night. Not what I expected - broken crank. Luckily, it stayed together wiping out only the outside of the bearings on the front rod. The front rod is fine and the rear rod and bearings are 100% unaffected. I haven't split the cases, but I'm hoping they're okay, too.

Anyway, the Q is, is/has anyone else with JE or other high compression pistons had any crank issues or is this a freak occurence with a 17k mile crank? It was not detonating, the rotating assembly was properly balanced prior to the build, and the AFR was on the fat/safe side when I had it dyno'd with the exception of a strange spike to 14 at 5k RPM; however, the front plug was closed, but I figure that was due to it dieseling after the crank broke. Just trying to figure out which way to go with the rebuild - thanks.
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Old 08-20-2006, 01:06 PM
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Originally Posted by Motohead
Well, after just getting my SH back together last November (spun rod bearing) and after only 2k miles she left me stranded again a couple weeks ago. I finally got the motor out and pulled the rods/ pistons last night. Not what I expected - broken crank. Luckily, it stayed together wiping out only the outside of the bearings on the front rod. The front rod is fine and the rear rod and bearings are 100% unaffected. I haven't split the cases, but I'm hoping they're okay, too.

Anyway, the Q is, is/has anyone else with JE or other high compression pistons had any crank issues or is this a freak occurence with a 17k mile crank? It was not detonating, the rotating assembly was properly balanced prior to the build, and the AFR was on the fat/safe side when I had it dyno'd with the exception of a strange spike to 14 at 5k RPM; however, the front plug was closed, but I figure that was due to it dieseling after the crank broke. Just trying to figure out which way to go with the rebuild - thanks.
I've seen plug gaps close due to high compression. The plug could have closed and then the cyl. could have loaded up with unburned fuel. This effectively raisies the compression even more causing the piston or crank failure.
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