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Really dumb question - what is a burnt valve?

Discussion in 'The Garage' started by mattman, Sep 5, 2002.

  1. mattman

    mattman 1/2 ton status

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    As it says, what exactly is a "burnt" valve and how does it affect your enginge? Also, how do you get a burnt valve?

    I have always heard the term but never got a good explanation what it was. Hoping to learn this once and for all.

    Thanks.
     
  2. BorregoK5

    BorregoK5 1/2 ton status

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    When the exhaust valve seat becomes overheated, molten particles of the seat are removed by the valve head and the rotating action of the valve produces abrasive wear on the seat and effectively lowers the valve head into the seat. This action rapidly reduces valve clearance. The results of this reduction in valve clearances will cause the exhaust valve opening time to be advanced exposing the exhaust valve head to the high temperature heat flash of the still expanding combustion stroke. It will also decrease the time the exhaust valve head is in contact with the seat which is the main area of heat dissipation for the valve to the cooling system. Both the early opening of the exhaust valve and the decreased time that the valve remains on the seat dramatically increase the temperature of the exhaust valve which in turn accelerates valve recession problem. This cycle continues until there is no valve clearance and the valve head can no longer discharge its heat to the cooling system. This results in a burnt valve and a total loss of compression.
     
  3. MontanaLifted

    MontanaLifted 1/2 ton status

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    Now THAT is what I call a great reply!
    (is he still breathing?)

    I love this site
     
  4. BorregoK5

    BorregoK5 1/2 ton status

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    I can't take all the credit on that one. It's a rather lengthy explaination and I knew I'd stumbled across a good exerpt on it recently so I "cut and pasted"... that way I could save my breathe and still get the point accross well. /forums/images/icons/grin.gif
     
  5. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    I have a nice pair of burnt valves in my center console from my supra. Had an injector go lean on it. Nice divot on the one valve and you could see on the second valve for that cylinder was starting to burn. The hardend valve seat faired fine. Hand lapped all the valves (that was over a year ago and I still have calluses from spending better part of the day lapping that 24 valve head) and the #2 cylinder had a little better than average compression compaired against the other 5.
     
  6. BorregoK5

    BorregoK5 1/2 ton status

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    Lapping the valves by hand is a PITA weather its a 4 valve head or a pair of 2 valve heads. Talk about getting carpal tunnel. Not a bad stop gap measure. I had a Honda CRX that took well to a good lapping and pulled another 100,000 miles on the stock head, until I sold the car. It has gone another 50,000 by now with the current owner so I figure it was worth it. sounds like yours worked out good too.
     

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