sbc crank end play

Discussion in 'Engine Topic' started by 1981z28owner, Jan 15, 2007.

  1. 1981z28owner

    1981z28owner Veteran Member

    2,562
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    Nov 28, 2005
    midwest
    what causes a crank to have too much end play? the crank i got has .014.

    thanks
     
  2. rustbucket79

    rustbucket79 Veteran Member

    Worn crank thrust surface or worn/incorrect bearings. (#5 specifically) Is this a new assembly or an older running engine?
     
  3. Damon

    Damon Veteran Member

    12,865
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    Nov 16, 2000
    Philly area
    The crank is "captured" on both sides of the rear main bearing. That's the thrust bearing.

    In front of the rear main bearing it's a machined surface on the side/face of the rear-most counterweight. Behind the rear main bearing it's a special "lip" built into the crank, just in front of where the rear main seal souches and seals off the crank.

    How to fix... sorry, you need someone smarter than me to tell you. I recall max spec is about .008 or so.
     
  4. 1981z28owner

    1981z28owner Veteran Member

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    Nov 28, 2005
    midwest
    it is brand new clevite H bearings. the crank was turned .020 on the mains. the power manual says .005-.007
     
  5. Damon

    Damon Veteran Member

    12,865
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    Nov 16, 2000
    Philly area
    OK, Rust, you're up. I'm wondering how turning mains could prossibly get into the "walls" on either side of the rear main journal. I THINK that's not likely to happen, but you're into areas of machining that I guess I don't understand as well as I might think I did.

    I'm thinking the crank is just worn out OR the rear main bearings are just out of spec in the width dimension for some reason.

    <grabs bucket of popcorn and a beer>
     
    Last edited: Jan 15, 2007
  6. 1981z28owner

    1981z28owner Veteran Member

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    Nov 28, 2005
    midwest
    we have never had this problem before. i am going to measure my old bearings tomorrow and see if there is a difference.
     
  7. 73RS

    73RS Veteran Member Gold Member

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    Mar 7, 2006
    Gonzales LA
    I had an old GMC Sprint that had a 350 in it, it must of had about .025 to .035 end to end play. When I pulled the motor apart I realized the thrust surface on the crank was shot, it had a deep groove in it. How it got so worn I don't know, maybe some type of tranny problem or something, so I have seen it before.
     
  8. K5JMP

    K5JMP Veteran Member Lifetime Gold Member

    I have seen it before as well.. if it is a freshly machined crank, then it needs to go back where it came from. If not... time to talk to your machinist and decide if it is worth repairing....
     
  9. rustbucket79

    rustbucket79 Veteran Member

    My theory here is assuming it was a used crank that needed grinding (on the mains) that it had previous damage in the thrust area caused by the clutch or torque converter. When you grind a crank you dress the thrust surfaces with the sides of the stone, and it will add .001 to .002" end play but that isn't generally an issue. Assuming the thrust surface wasn't already oversized the only other possibility is the previous damage that the operator ground until it cleaned up without measuring the final size. (we use a normal thrust bearing and whatever feeler gauge fits in is roughly your final installed thrust clearance. You can buy main bearing sets that have a .010" or .020" overlength thrust flange, and the crank is ground accordingly, again using the overlength bearing and a feeler to determin how much has to be ground away. 1981 has 2 options here, he can get the overlength bearing set and dress off the front halves of the thrust bearings until his thrust clearance is within spec, or he can take the crank back to the machinist and have him clean up the thrust on the crank to achieve the correct clearance. You can weld up the thrust and regrind it back to standard but we use that as a last resort to save a valuable crank.
    Before the engine goes back into service, correct the problem that damaged the thrust surface in the first place, or the same damage will occur again.
     
  10. 1981z28owner

    1981z28owner Veteran Member

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    Nov 28, 2005
    midwest
    where do u get the overs sized thrust bearing? this is a crank we bought off bay several years ago. i will try to get ahold of them. aka, i wont. if i can get a .010 bigger thrust theni should be ok.

    thanks
    mike
     

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