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Thread: Rebuilt 455 knocks slightly

  1. #1
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    Rebuilt 455 knocks slightly

    I just rebuilt my 75 455, and it knocks slightly. Its not as consitant as a classic blown engine, sound but it is strikingly similar. This engine was my first rebuild, and i replaced all bearings, cam shaft, lifters, and piston rings. The only thing i did to the heads was had them checked for worp, and did the valve seals. The engine was recently rebuilt before i bought the motor, and i rebuilt it again, due to the cam shaft gear stripping out. The block was already bored 40 over, and i reused the pistons, connecting rods, and crank shaft, due to great condition. Any way, when i put in the new bearings i just simply lined them up and put them in with some assembly lube, was i supposed to check anything? Also after looking into some possibilities, i read something about the timing could be to far advanced. I have yet to time it at a good idle, because the only timing i did was with the starter motor, engine not running, before first start up. But i have no clue what a to far advanced knock would sound like, could someone explain that to me? And How many degrees should i set my timing for, and at what idle speed?

  2. #2

    455 knocking ?

    Did you check your clearances when you rebuilt the motor, rods and mains ? with plasti gauge ? what was the condition of the main bearings and rod bearings when you tore it down ? Myself I will always spend a few bucks at the machine shop and have the rods checked and the crank checked even on a buget rebuild , the $100 to $ 200 bucks is well worth it, buick stuff is not cheap and I only want to be in the motor one time

  3. #3
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    You said that the motor was rebuilt before. Was the crank turned ? And you just put stock bearings in it. What does the oil pressure hold at. I had a geardrive in a motor before and it chiped a tooth, that had a nice knock, sounded like a rod.

  4. #4

    knocking engine....

    First determine what the oil pressure is.... then with the engine idleing, get a listening device and determine just where the noise is coming from.... you said the noise is slight,,, does it change any at all as the engine warms up??.... a piston with too much clearance has a distinct ''click'' that changes when the engine warms up and the piston expands....
    a rod has a medium hollow knock sound, and a main has a deep solid sounding clunk....
    mains knock at start up the most,,, a rod knock is a double knock,,, and a lifter is a higher pitched kind of popping click....
    But you can probe and find what area of the engine the noise is coming from....

  5. #5
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    Well, i pulled off the oil pan to find that the pan that is just underneath the crank shaft was missing one of the 3 bolts, shock it and it did seem to have plenty of play that may have been creating the noise. Also i had my machinist, take all the messurements, and order me the parts, such as bearings. And no the crank is all original, no work other than polishing.

  6. #6
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    Just got in their and grabbed the connecting rod caps and with alittle effort i can get them to move a tiny bit, forward backword and side to side. It doesnt seem excessive to me, but again this is my first build. Is this normal?

  7. #7
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    Ask your machinist for the actual MEASURED bearing clearances. You didn't check them yourself, and if your machinist doesn't know what they are, then this is possibly your problem, and there is no easy fix, but to pull it all apart and do the detail work.

    As asked before, what is your oil pressure?

    Rods can have a slight movement, but that is not how you check your tolerances.

    Good luck.
    86 GN, all factory options, engine build in progress
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  8. #8

    rod bearings.....

    on rod bearings , you are not supposed to move them from side to side at all,,, but you are supposed to be able to move them easily along a line front to back....
    if you can move them any discernable amount from side to side,, they are too loose... if you cannot move them front to back they are too tight.... that said,, both movements cannot be excessive ... oil clearance on bottom end bearings is generally in the 1.5to 2.0 thousandths range.... stick with stock buick clearances,,, do not try to build a buick engine loose like a chebby engine....
    Last edited by DocModisett; 05-01-2011 at 05:27 PM.

  9. #9
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    Well putting the bolt in the pan didnt seem to do much long term. It sounded great, right after i put it all back together, seemed like the issue was gone, reved up the motor a few times in the last couple of days, after the repair, and all sounded well. But today, i started it up and reved it up a bit, again all sounded well, but then i noticed my oil pressure at idle seemed to drop from around 20-25 to, 17-18ish psi almost/sitting on, redline on the gauge, seemingly after warming up. I gave it a quick rev to see if the oil pressure would climb up with the rpm, and it did, but then the sound came back. Again only while reving up, can't really hear a thing at idle. It just sounds like a diesal pick up truck while reving up. Any thoughts?

  10. #10
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    Also i torqued the connecting rods down to 45 foot pounds, as suggested by my machinist, is this correct?

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