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  1. #1

    1996 SL780 possible compression problem

    My Sl780 has cold compression readings of 130 - 0 - 130. It runs and drives, but lugs pretty badly. I looked at the piston through the spark plug opening and I cannot see any hole. Is it possible I just need new rings on the center piston? If so, how hard is it to change them?

    Thanks,
    Bob


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Welcome to the Hulk.

    A cylinder with zero compression is a problem. It will not fix itself so you will need to remove the cylinder head and have a look in there.

  3. #3
    What are the failure modes that would lead to zero compression besides a burt piston or bad rings?

  4. #4
    BlueFishCrisis's Avatar
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    Holed piston, broken rings, heavily scored/damaged cylinder, siezed piston, broken rod....

  5. #5
    I found the problem. The piston has a chunk missing from one side of the dome. I scored the walls up badly. How many thousandths can I hone it before I need new piston and ring sizes?

  6. #6
    woodsrider40's Avatar
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    You need to measure the cylinders to see what size you'll need.

  7. #7
    woodsrider40's Avatar
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    You need to measure the cylinders with a dial bore gauge to see what size you'll need.

  8. #8

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    You can not HONE out a scratches or scores.......It will need bored or replaced.

  9. #9
    I am thinking of having it bored 0.050 over. If so, can I keep the original pistons in the other two cylinders? Any special ideas for removing the wrist pin without removing the other pistons?

  10. #10
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    Most likely .040 or 1mm over is the largest piston you will find. If you have more damage than will clean up with that overbore, a resleeve or a whole new cylinder is needed. You may be able to find a reasonably piston/cylinder combo on ebay to install and run.

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