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Thread: 71 gs 455 engine knock

  1. #1
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    71 gs 455 engine knock

    car has been sitting for 5 years, i thought it may be the lifters and that they would fix themselves but so far no luck, we have no oil pressure gauge, there is no knock at idle, or atleast i cant here it i used a piece of wood and put it on the valve cover toward the back of the motor it sounded like the knock was coming from there. im thinkin spun bearing or a bad piston.? can somone please help me !!
    Last edited by 71455gs; 06-21-2011 at 02:27 PM.

  2. #2
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    Get an oil pressure gauge and see what the pressure is.

    Could be a cracked flex plate. Check your torque converter bolts, and look closely at the flex plate. Maybe you are lucky and that is all it is.

    Did you change the oil? any metal shavings in the oil?

    good luck.
    86 GN, all factory options, engine build in progress
    72 Skylark Convertible, 462, TH 400
    72 GSX Clone 464, TH400, 3:42. 10.71 @126mph best

    We build GM carburetors and specialize in custom built Quadrajets

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  3. #3
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    i checked the oil. no shavings. oil was normal. i just replaced the oil and the oil filter. iv been told to check the flex plate. but im new to mechanics, where exactly is that?
    thanks.

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    Flex plate is bolted to the rear of the crankshaft, inside the transmission housing. Also called flywheel. It's what the starter engages to, to crank the engine.

    You can usually "feel" a main bearing knock with you hand on the carb. If you can feel the knock when it is knocking, it could be bad. But the flex plate crack can also give the same feeling.
    86 GN, all factory options, engine build in progress
    72 Skylark Convertible, 462, TH 400
    72 GSX Clone 464, TH400, 3:42. 10.71 @126mph best

    We build GM carburetors and specialize in custom built Quadrajets

    www.quadrajetpower.com
    mark@quadrajetpower.com

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    If you should go down the bad lifter path a little more, I have found diesel oil, 15-40 to be a good choice to help free them up.

    You could actually pull the rocker cover and put your screwdiver or hearing device right on the suspect rockers.


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  6. #6
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    thanks iv heard the 455's like to crack the flex plates, ill check that out. is the rockers cover the same as the valve cover? i feel like such a idiot!

  7. #7
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    71455GS

    By all means, get an oil pressure guage and install it where the sending unit is on the block
    Remove all the plugs and do a compression test
    remove the valve covers and, while cranking the engine over, watch each rocket arm/valve to see if they are all opening and closing. watch to see if oil is coming out of each passage. I use the air compressor to blow them out

    if the compression is equal on all eight cylinders (my 455 is between 130-150) BUT, it had been sitting on a barn floor for 5 years, and I had to free up a couple of lifters (I used penetrating oil & Mystery Oil, needed a ball peen hammer on a couple of them). I had it turning freely BEFORE I put sparkplugs in it.

    If the oil pressure is real low, check the pressure release valve in the oil pump, make sure it slides freely.

    You are going to have to change the oil again because of the penetrating oil and the gunky oil that you have forced out of the passageways.

    If you have a 71GS w/455, take your time, we wanna see that baby back on the road
    Butch
    64Wildcat 445
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    73 455 on bench
    67 Impala 327

  8. #8
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    thanks!! you guys are a big help im do gonna all this and ill update when im done!

  9. #9
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    wow, you're close to me (ish).

    you should get an auto stethoscope to help you pinpoint the noise. If the motor didn't knock before it was stored, it probably is simply something stuck.

  10. #10
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    i just used a solid piece of 2x2 wood. worked pretty well, i put it toward the back of the left valve cover and i kinda sounded like it was there.

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