Science Behind Tight Valve Clearances?

Discussion in 'Technical Help' started by StuarT595, Dec 4, 2020.

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  1. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Can anyone explain to me why all the valve clearances on my T595 are tight?
    None of the inlet valves will even take a 0.05 gauge, and the exhaust are also under...what could be the cause? I can’t get my head around it at all? I always thought things wore away?
    If it’s relevant I’m stripping the engine down for a head gasket change.
     
  2. dilligaf

    dilligaf Guest

    The valves wear into the valve seats making the clearances tighter :confused:
     
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  3. Callumity

    Callumity Elite Member

    Feb 25, 2017
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    #3 Callumity, Dec 4, 2020
    Last edited: Dec 4, 2020
    I would wonder who was last in there and how long ago. You would expect valve seat recession to be worst on exhaust valves through hot gas erosion.
    If they are all tight I would question the sums or intentions of the last person to shim them. You are on the edge of optimal breathing through being too tight.
     
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  4. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Isn’t it strange, that never crossed my mind...I was only thinking of the surfaces at the top of the valve stems/shims etc.
     
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  5. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    So this engine has been running rough, hard to start, and blowing the water out once it gets going. I’m just about to crack the head because many wiser than me think it to be a head gasket.
    The oil is perfect and there’s no oil in the water. So, here’s a thing...what are the chances my “HG issue” is nothing but valve clearances?
    Would the engine massively overheat with 5 out of 6 inlet valves not taking a 0.05 and 5 out of the 6 exhaust valves taking a maximum 0.15?
     
  6. ChasChas

    ChasChas Marxist Scum

    Aug 9, 2020
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    My understanding (YMMV) is that with rocker and some sort of screw locknut valve adjustment they open up and get 'tappity' due to wear in the adjustment chain. Magnified if you have pushrods.
    With cam direct to bucket and shim there's no wear to make it looser, the only wear is to the seat. Which makes them tighter.
    And you can never discount a previous visitor to the engine getting it wrong or thinking 'that'll do'
     
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  7. Callumity

    Callumity Elite Member

    Feb 25, 2017
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    As the engine warms the clearances disappear so you could have leaky valves which would be obvious on the inlet side with blowback. Frankly, you need to get the head off because you could just have a leak into the water jacket where the offending piston(s) pressurises the coolant system and it blows out. You would not necessarily impact the oil supply.....a good thing! Look for tracks on the block or head as alloy erodes and if in doubt get them skimmed.
     
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  8. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Thanks for the input, chaps...business as usual, then...back to undoing the head. I’m only the head bolts and head to frame bolts away, now...so I may as well check it all out properly I guess
     
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  9. Pegscraper

    Pegscraper Elite Member

    Jun 12, 2020
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    I'd second more or less what's been said. Tight valve clearances means the valves will stay open longer and run hotter (and quieter) with less time on the seat to lose heat to the head. Too tight and the clearance will shrink to zero when hot, potentially holding valves permanently open causing poor running and burnt valves and seats. Can't see how it would be the direct cause of the coolant loss problem.
    I had a R1 years ago that was weeping oil from, what looked like, a leaking head gasket but when it went in to be fixed under warranty it turned out to be a flaw in the head casting. Fair play to the dealer who replaced the cylinder head rather than trying a quicker/cheaper fix with sealer, even though the bike was of the road for 2 weeks in high Summer.
     
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  10. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Cheers, pegscraper,
    As always it’s really reassuring to hear the forum feedback and experience. I’ll continue with the strip down and take a good look at the valves. Likely going to need a grinding at best.
     
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  11. Yorkshireman

    Yorkshireman Crème de la Crème

    Dec 12, 2015
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    I’ve only ever done the valves on one bike, an 8 valve Kawasaki Z650 so be prepared for quite a bit of work. Don’t go too hard with the coarse grinding paste, if at all. It doesn’t feel like much is happening until you put it back together and find the valve sits higher in the head and closed the clearance right down. (I know you are already lumbered with tight clearances, the point I’m making is what seems like a gentle spin of the valve with the coarse paste takes quite a bit of material away). I actually finished off with solvol metal polish after the fine paste and got a real smooth finish. You’ll likely need a large selection of shims and the patience of a saint to set the clearances afterwards but it’ll pass those long winter evenings, that’s for sure.
     
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  12. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Thanks for sharing this, Yorkshireman...I will be sure to go steady. I’m under a little bit of pressure, however, because I have to work on the bike outdoors, storing it under a cover between sessions. Clearly I need to get the engine zipped back up ASAP...the rest can of course wait!
    I will of course be sourcing a nice warm shelter for the old girl to live in once she’s purring pretty again.
     
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  13. Tricky-Dicky

    Tricky-Dicky Crème de la Crème

    Dec 12, 2016
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    As an interim diagnosis method i would run a leak-down test on it that would tell you if the valves are burnt/leaking and or if there was any kind of leak from the head gasket, modern materials mean lower expansion rates so clearances are often quite small however if too close then your definitely at risk of burning valve seats and would explain some of your symptoms.
     
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  14. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    I’m a little too stripped down now I think...8 or so bolts and I’ll be in with a visual.
    I’d really appreciate your thoughts when I get some pictures up though
     
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  15. Pegscraper

    Pegscraper Elite Member

    Jun 12, 2020
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    When you get the head off, if the combustion chambers are in the head and not the pistons, turn it upside down and fill them with petrol to see if and how quick it drains away. If the valve seats leak petrol there's no way they'll be gas tight.
     
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  16. MyEvilTwin

    MyEvilTwin Well-Known Member

    Mar 14, 2020
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    I found this video to be the best simple illustration regarding this topic. However, his model is shim-under-bucket type. My 865 is shim-over-bucket, but same concept, but your design may vary depending on engine.


     
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  17. StuarT595

    StuarT595 Active Member

    Nov 19, 2020
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    Many thanks for that...getting ready for work now, but will definitely watch it overnight
     
  18. learningtofly

    learningtofly He’s not the Messiah, he’s a very naughty boy!
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    Sep 25, 2018
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    What a great video - I've never really understood the whole issue of valve clearances before, and now (I think) I do :)

    So, when people refer to tappet noise, is that because the clearance is too great/shim too small?
     
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  19. MyEvilTwin

    MyEvilTwin Well-Known Member

    Mar 14, 2020
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    Slapping noise when COLD engine tolerances EXCEED spec (loose bucket; worn shim). Therefore upon heat up when things close in, there is no flush contact to the shim bucket, and the noise is the slapping of parts.
     
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  20. Ducatitotriumph

    Ducatitotriumph Crème de la Crème

    Apr 25, 2019
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    Might be a good idea to get a new(er) head?
    Might be saving yourself alot of heartache and pain? Or, adding to it of course...
    We have @dilligaf 's twat for that if we need/want it!! ;)
     
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