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Valve Clearance for Roller Tappets

Discussion in 'Stelvio Chat & Tech' started by canuck1969, Feb 13, 2016.

  1. John Warner

    John Warner Cruisin' Guzzisti GT Contributor

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    I've been thinking since I did my Roller conversion (1000 miles ago).
    It's well known/documented that the clearances open up when hot, so is there any reason you couldn't/shouldn't set them both to 0.1mm, or even tighter?
     
  2. kiwi dave

    kiwi dave GT Reference

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    I too have wondered this, but not game to follow through.
     
  3. abbienormal

    abbienormal High Miler GT Famiglia

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    They open when hot? Everything I have been taught says they close up when hot which is the purpose of the clearance. You have a long thin valve stem that runs hotter than the cylinder head itself. It will grow more in length and thus decrease clearance. Also the push rods will grow decreasing clearance. If somebody has info proving otherwise I am listening. Apparently the rollers change the ratio of growth as you can run slightly tighter clearance but not that much.
     
  4. scottmastrocinque

    scottmastrocinque Scott Mastrocinque GT Famiglia

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    I don't know where this got started but it is pure nonsense and flies in the face of thermodynamic atomic properties of metals.

    Show me a cast metal that CONTRACTS when heated and I will show you the next Nobel Prize Winner!

     
    Last edited: May 19, 2017
  5. WayneOrwig

    WayneOrwig GT Reference

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    Maybe because aluminum grows at a much higher rate, pushing the cam and rockers apart faster then the valve stem grows.
    Maybe. Maybe not.
    No reason the fiddle with the spec. Nothing to be gained.
     
  6. canuck1969

    canuck1969 GT Reference GT Contributor

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    I believe it is more to the fact that the gap gets larger over time as compared to shim and bucket which tighten over time. But I too have heard that the gat gets bigger when it gets hot. Have never verified that claim, but others have.

    Even with rollers I have them set a 4 and 6 and have never had any issues. Run great with a stock setup and quiet.
     
  7. scottmastrocinque

    scottmastrocinque Scott Mastrocinque GT Famiglia

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    In linear thermal expansion, there is no maybe, maybe not. The engineering of thermal expansion coefficients is established fact and there are defined mathematical formulas to describe the process. It is entirely predictable and calculable. Granted, aluminum has roughly twice the TEC as ferrous metals, but there is more in play that just the metal type. It has to also do with length of the metal, mols of the metal, and there are complex calculus based integrations to take these factors into consideration.

    When an object is heated or cooled, its length changes by an amount proportional to the original length and the change in temperature. This linear expansion coefficient varies both between materials, and also will change it's coefficient rate over a given temperature range versus another.

    The valve itself is the hottest by far, and one of the longest pieces as well. Hence, the valve will always out expand the surrounding metal.

    You can test it. Set your valve train. Ride your bike to operating temperature. Stop engine. Remove valve cover. Measure valve lash gap. It will be much tighter compared to when it was cold.

    Blah blah blah..LOL

    but suffice it to say, the standard gaps are well established and I hold with the others here that there is no benefit nor reason to deviate from them. YMMV of course.
     
    Last edited: May 19, 2017
  8. GuzziMoto

    GuzziMoto GT Reference GT Contributor

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    I do not know of anyone who has tested this on a newer 8 valve motor. But it has been tested on an older 2 valve Guzzi lump. What was found was the clearances opened up as the motor got hot. That was not speculation, or discussion about theory, that was real world measurements. I know Scott did not agree with those measurements, and insisted they were wrong. But until someone else comes up with better measurements that say otherwise I will go with what was found to be.
    That said, there is a good chance that the 8 valve motor does not behave the same way. Without someone actually measuring I don't know. Some seem to think it is simple, metal gets hot, metal expands, clearances get smaller. But it isn't that simple. There are multiple metal parts, made out of different metals. Looked at individually, as some of those metal parts expand, the clearances get smaller, but as others expand the clearance would get larger. The net effect needs to look at all the aspects of thermal expansion, not just one. There is also the aspect of different metals having different rates of thermal expansion. For example, steel tends to expand less when the same amount of heat is applied as the same part in aluminum would as I recall.
    That said, I would never run a Guzzi with tighter then recommended clearances. Looser clearances, yes. But tighter clearances, no.
     
  9. canuck1969

    canuck1969 GT Reference GT Contributor

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    Scott, just do a search on gap expansion for tappet engines hot vs cold. Lots of explanations as to why it happens. Has to do with differential expansion and the rockers them selves moving up due to expansion.

    I took back what I said earlier, I did try this when I first got my bike as there was a debate on setting hot vs cold with the danger being setting them hot would cause tighter valve gaps once it cools down. The gap does get larger on an 8V motor.
     
  10. Quattroa4m

    Quattroa4m Tuned and Synch'ed

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  11. scottmastrocinque

    scottmastrocinque Scott Mastrocinque GT Famiglia

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    I understand thermal expansion guys and differing coefficients of expansion.

    Also, I have measured exactly like I described on an 850 Eldorado, a SP III, a California II, a California 1100i, a Caifornia EV 1100, a Centauro, and my '12 Stelvio NTX. I did not find a single one that has ever become looser in its lash gap.

    Do what you will, but as the engineer that taught me once said and I never forgot, nobody is willing to back up thier claim of valve lash expansion by setting the cold lash to zero and run the engine under load because they know they will fry the valves and destroy the engine. YMMV and you may disagree. Okie dokie. To each his own.

    Show faith in your conviction and set your lash to zero and trust in your belief that the gap will open up. Obviously I and my teacher are dead wrong and you have nothing to fear.
     
  12. canuck1969

    canuck1969 GT Reference GT Contributor

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    Really Scott.....

    Setting the lash to zero is not a valid test. Starting up a cold bike with no lash would be one step below stupid. That is not what we are saying. And we are not talking 1/2" difference.

    I have seen it with my own eyes, as have others in both motorcycles and car engines (yes, not the exact same setup, but same principles). As the bike cools down, the valve lash gets smaller due to different expansion rates of different metals. Not sure what to tell you but I think the debate is over. Believe what you will or won't, the choice is up to you.

    By the way....nobel prize forth coming.........negative thermal expansion of metals is not new. Been going on for decades. You will just never find them in an engine........just search the internet and you will find several of them.
     
  13. kiwi dave

    kiwi dave GT Reference

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    There was one guy (here or on the other forum) who set his clearances close to zero, and benefited from quieter tappets. I did not note his name or whether he still frequents these forums. To say that an 8 valve motor is quiet is really saying that it's not tremendously loud, but there is still a considerable amount of noise from those tappets.

    If only the hydro motor had survived. Now that was a quiet motor.
     
  14. scottmastrocinque

    scottmastrocinque Scott Mastrocinque GT Famiglia

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    Canuk1969,

    If you are going to try and slam me for something I write, I suggest you read carefully what I have written.

    "Show me a cast metal that CONTRACTS when heated and I will show you the next Nobel Prize Winner!"

    I don't have to search the internet, I need only reach behind me at my desk, to one of my university textbooks in Inorganic chemistry. I know about negative thermal expansion compounds. I've even worked with Cubic Zirconium Tungstate.

    Non of these compounds are CAST METALS.

    In the labs that I taught at the university, (last semester even), PURE WATER will exhibit negative thermal expansion at just above freezing, but then reverse and expand extensively when frozen, thereby becoming less dense, and hence it floats.

    I already stated that you can believe what you want, see what you have seen, whatever, I have the same experiences but only different. In the grand scheme of things, who gives a flipping hoot? Do what you want. I really don't care.

    The bottom line is I don't see anybody here recommending to set the lash tighter because you don't need to worry because the valve lash is guaranteed to loosen up as it gets hotter.
     
  15. john zibell

    john zibell Moderator Staff Member GT di Razza Pura

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    OK guys, this is getting a bit much. Bottom line, set valves to factory specifications, period. Further back and forth on setting tighter than factory specification and expansion rates of different metals will result in this thread being locked.
     
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  16. scottmastrocinque

    scottmastrocinque Scott Mastrocinque GT Famiglia

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    Arguing with a scientist is a lot like wrestling with a pig in the mud. After awhile, you realize the pig likes it!

    Sorry for being a pig in the mud. My bad.
     
  17. canuck1969

    canuck1969 GT Reference GT Contributor

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    You know it was a nice quiet place here for the last while. Guess time to move on. Too bad, I was enjoying the company
     
  18. John Warner

    John Warner Cruisin' Guzzisti GT Contributor

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    S**T, what have I done?

    It was only really a 'thinking-out-loud' sort of question, apologies that it provoked such a 'heated' debate . . . :sweating:

    Probably my fault for being someone that tends to question everything they read, no matter who says it, or how 'set-in-stone' it's supposed to be.


    Better not ask that Oil-related question now . . . :lipssealed:
     
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  19. Quattroa4m

    Quattroa4m Tuned and Synch'ed

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    What I see is a bunch of really good guys who are passionate about what they ride. We are very fortunate to have such a good forum.

    I'd be honored to ride with any and all of you. I think that speaks volumes about the caliber of the folks here.
     
    Adam Davis and scottmastrocinque like this.
  20. KraZ440

    KraZ440 Just got it firing!

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