EEEEEKKK! Airhead Valve/Piston Interference

Discussion in 'Airheads' started by wmax351, Oct 2, 2011.

  1. wmax351

    wmax351 Been here awhile

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    Just took the top end off of my rebuilt '71 R75 after 100 miles due to a ridiculously loud banging sound from the right side. Seemed almost like rod knock, but it wasn't. Turned out to be this...:eek1

    Right Side:
    [​IMG]

    [​IMG]

    [​IMG]

    Left Side:
    [​IMG]

    [​IMG]


    Any ideas what can cause this? I used the original, .5 mm base gaskets/shims rather than the low test, 1.2mm shims. Could that have done it?

    It seems as if the valves only hit when they were seated, as it would have likely grenaded due to a bent valve otherwise.

    What should I do here? Any chance of the heads or pistons being salvageable? They are a fresh rebuild with under 100 miles on them.

    What should I do to make sure it doesn't happen again?
    #1
  2. Renner

    Renner combustophile

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    who did your heads?
    have you asked them about this?

    your good luck served you well regarding this bit of bad luck
    #2
  3. Beater

    Beater The Bavarian Butcher

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    Absolutely that could have done it. Any narrowing of that gap can do it. But holy crap, I am shocked you didn't snap a valve. I would put in the thicker gasket and go from there. It's a good thing you pulled that apart when you did ...

    On a side note, I thought the space between the piston at top dead center and the valve was like 3 to 4mm .. How long have you had the bike? Do you know any history on it? Did anyone shave the cylinder head?
    #3
  4. Jon-Lars

    Jon-Lars Jon-Lars

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    What cam is in there? and is there an offset cam key?
    #4
  5. wmax351

    wmax351 Been here awhile

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    Heads were done for the PO, by an unknown source. They were professionally done though, and I have all the original parts. I assembled the rest of the bike myself.




    I have had the bike since a year and a half ago, when it was in 3 Rubbermaid boxes. Didn't appear to have a shaved cylinder head.

    Think it is fine to run these parts without replacing them?

    AFAIK, it is a stock cam, with no offset at all. Set up by Dave at Recommended Service.
    #5
  6. Renner

    Renner combustophile

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    I'd run them down to Scotts Valley to talk about them with Ted Porter at the Beemershop.
    You could call and offer to send pics first.

    My money is on the piston being ok and a valve job required.
    #6
  7. AntonLargiader

    AntonLargiader Long timer

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    Looks like an oversized exhaust valve. Can you measure its diameter?
    #7
  8. wmax351

    wmax351 Been here awhile

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    I measured them. ~38 mm (1.490") exhaust,~42 mm (1.650") intake. They appear to be standard. Could the seats be protruding too far?
    #8
  9. supershaft

    supershaft because I can

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    Your intake valve is hitting too! You are running short reach plugs in long reach holes!

    I am thinking it has to be a too thin base gasket.

    I am not so sure your other cylinders valves aren't touch too?
    #9
  10. caponerd

    caponerd Kickstart Enthusiast

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    Looks like the exhaust valve is the one that's hitting. I'd be looking at anything that can cause a delay in valve timing. Would a loose cam chain cause that?
    Maybe just a couple of degrees retard in the valve timing would allow the valve to just touch the piston without bending the valve?

    Come to think of it, with those valve pockets in the piston top, can the piston even hit a closed valve without first hitting the head itself?
    #10
  11. wmax351

    wmax351 Been here awhile

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    I'm pretty sure the other side is touching, just not as much.

    That's what it seems like. It doesn't add up.

    Its a fresh cam chain, sprockets, and tensioner, installed by Dave Gardner of Recommended Service. I'd say its highly unlikely that that is the cause.
    #11
  12. supershaft

    supershaft because I can

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    Who says the piston is hitting closed valves? It (they?) isn't (aren't?). Pistons do not hit closed valves.

    Look closer. The intake is hitting too!
    #12
  13. fishkens

    fishkens Long timer

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    It was mentioned by the OP earlier but that's possible w/out crushing the exhaust valve on the exhaust stroke.
    #13
  14. wmax351

    wmax351 Been here awhile

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    That's a good point. It does seem as if the marks are exactly where the valve would be if it were at least mostly closed. Otherwise, the marks would be closer to the center of the piston.
    #14
  15. Airhead Wrangler

    Airhead Wrangler Long timer

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    Any chance you're a couple of teeth off on your timing chain??
    #15
  16. supershaft

    supershaft because I can

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    Yes, the valve is mostly closed but it isn't closed. The piston isn't around either valve when they are mostly open. The exhaust is closing as the piston comes up.

    Since both your valves are hitting and it seems to be running alright otherwise, it has to be the base gasket? You did adjust your valves correctly with the proper lash?
    #16
  17. wmax351

    wmax351 Been here awhile

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    Valves adjusted to proper lash, on the loose end for break-in, checked at both hot and cold.


    I'm wondering about that. I farmed out the bottom end to a guru, and he took care of the timing chain. Seems an unlikely option.
    #17
  18. supershaft

    supershaft because I can

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    They either won't run or barely run thusly. I have seen people do it and nothing touched in the cases I have seen. Twice I think? Maybe they were timed early both cases?

    Plus, if it were that I would think only one valve would be getting into the piston? Favoring the exhaust valve would be late timing and I don't see that getting the intake into the piston at all.
    #18
  19. JonnyCash

    JonnyCash turd polisher

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    ^ good thinking! I'd check that out. If the timing is right, I'd try the thicker base gasket, but you should check the clearance with modelling clay or rosin core solder to see that this does in fact give you the needed clearance.
    #19
  20. wmax351

    wmax351 Been here awhile

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    It ran perfectly, so that may be lower on the likelihood scale.
    #20