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Old 04-27-2010, 09:04 PM
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Will compression test reveal bent rod ?

My 65 Elky has a Chevy 350 which has a vibration starting at 2300 rpm standing still, which rules out drive train vibration. My mechanic tells me its a slipped balancer or bent connecting rod. If I do a comp test and one cyl varies greatly from the others would that indicate a bent rod?
Has anyone done this test and proved a bent rod was the cause of the low reading ?
I know replacing the balancer would answer the question, but if a comp test would also give me an answer it would be easier for me than replacing the balancer.
Thank You for your help. Crash

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Old 04-27-2010, 09:33 PM
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I could be wrong but I don't think a compression test would reveal a bent rod unless it was bent so bad that it broke a piston skirt and was forcing the rings to leak badly, and if this was the case it should smoke something terrible and foul the plug. Bent connecting rods rarely happen, something really bad usually accompanies that happening and it will be very evident - spun bearing, smoking, knocking, or rod hanging out of the pan, etc.

Are you sure it is something wrong and not just a cam/intake/carb related harmonic that is causing a vibe at this rpm?? Does it go away above this rpm or get worse the higher it goes?? What motor and trans mounts are you using and how old are they?? How old is balancer??
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Old 04-27-2010, 10:19 PM
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comp test for bent rod

Vibration continues to get worse, drive everyday, no smoke, knock etc. Know very little about engine, motor mounts and trans mount are new, didn't help problem. No idea how old balancer is but there is some signs of cracked rubber.
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Old 04-27-2010, 10:24 PM
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On a 400, it's a balancer/damper. On a 350, it's just a damper, not a balancer. There is no imbalance possible in a 350 harmonic damper. If the inertia ring slips in relation to the hub, the worst of it is that you can't time the motor properly with a light. Look somewhere else for the problem.

I would start by taking all the drive belts off and running the motor without any accessories or water pump.
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Old 04-27-2010, 11:30 PM
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Is this a problem that just started or a problem from the beginning?
I had a problem like yours once after swapping a 360 AMC in place of a 343.
343 had a new flex-plate so I swapped it for the one on the 360. It wasn't a good idea. Had to pull tranny and re-install the original flex-plate.
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Old 04-28-2010, 05:53 AM
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A bent rod on a 350 will usually cause a "knock". As the crankshaft counterweight hits the piston just below the pin boss. Won't show on static compression test.
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Old 04-28-2010, 07:06 AM
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Quote:
Originally Posted by crash
My 65 Elky has a Chevy 350 which has a vibration starting at 2300 rpm standing still, which rules out drive train vibration.
Not quite. There's still the torque converter as well as the front pump spinning at a standstill in Park. Before ruling them out, you'd need to disconnect the TC from the flex and run it up to the speed of the vibration.

Good luck.
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Old 04-28-2010, 06:16 PM
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engine vibration

This has been since i bought it, but of course I didn't notice before I bought it. It does have an alt. and water pump only, i'll try without them, then I guess torque conv. disconnect. I think maybe they could have used an incorrect flexplate. Who knows? There is definately no knock.

Thanks for all the suggestions. Crash
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Old 04-29-2010, 12:53 AM
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compression test

If you do a leak down test you should be able to tell which cylender is low on pressure for one. Two you can tell if it is a sticking valve, but tappong can only make you wonder if it is a lifter or the push rod. If you take the rod out and use a straight edge level to check flatness for the block you should be able to see of the rod is bent as well.
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Old 04-29-2010, 05:46 AM
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Engine vibration - bent rod

There is no tapping or knock its just a vibration. I'll check a few things and then swap the engine for my spare if I have to.
Thanks, Crash
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