Do Intake Valves Get Tight And Exhaust Valves Get ‘wide?’

Discussion in 'Technical Help' started by StuarT595, Feb 27, 2021.

  1. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    #1 StuarT595, Feb 27, 2021
    I can safely say that the intake valves get tight, because 5 out of 6 of mine (Triple 955/595) have zero gap.
    However, is this also the case with the exhaust valves, or do they get progressively wider (though I guess this wouldn’t make sense), with only one of my exhaust valves being on the edge of spec by being on the wide range.
    The reason I ask is I have the head off and the opportunity to switch a few shims around to get the best out of the tolerances over the long term.
     
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  2. Tricky-Dicky

    Tricky-Dicky Crème de la Crème

    Dec 12, 2016
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    #2 Tricky-Dicky, Feb 27, 2021
    Valve to follower/cam clearances generally close up with use.
     
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  3. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    #3 StuarT595, Feb 27, 2021
    Thanks, TD, I know it seemed an obvious question, but I just wanted to check because one of the exhaust clearances was borderline out of spec, making me think it was either a wrong choice or that the gaps increase in the exhaust?
    Clearly then the best option is to have them shimmed at their widest specification if there are shims going begging?
    Many thanks again! :)
     
  4. Pegscraper

    Pegscraper Elite Member

    Jun 12, 2020
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    #4 Pegscraper, Feb 27, 2021
    As TD has said, clearances reduce with use, not due to cam/follower wear so much as the fact that the valves hammer down into the seats but it can vary massively between different engines which is why some specify long valve clearance check intervals. I always aim for a clearance some where in the middle of the specs. In theory, setting them at max reduces valve lift and duration while minimum gives the the opposite with a risk of burnt exhaust valve seats but in reality it's much more critical in highly tuned race engines. JMO
     
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  5. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    #5 StuarT595, Feb 28, 2021
    Many thanks, Pegscraper, I’d never visualised it quite like that before, re the length of time that the valve is open, etc.
    Truly appreciate you taking the time.
     
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  6. Tricky-Dicky

    Tricky-Dicky Crème de la Crème

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    #6 Tricky-Dicky, Feb 28, 2021
    The major stress/load on any combustion engine is on the valve train at idle or low rpm mainly due to the spring pressure keeping the valves closed so its no wonder the seats get hammered.
     
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