1. 451Mopar

    451Mopar Well-Known Member

    Messages:
    2,444
    Likes Received:
    1752
    Joined:
    Oct 25, 2011
    Location:
    Aurora, CO
    Local Time:
    3:25 PM
    Was watching bad videos on youtube about adjusting valve lash. The find zero lash and tighten 1/2 turn stuff without explaining where that comes from, and why it is usually wrong for our Mopars. Most of that 1/4 to 1/2 turn adjustment is from adjusting stud mounted rockers, not shaft mounted rockers. When adjusting a stud mounted rocker arm like that, it is like a 3rd class lever. The lower the rocker ratio, the greater the adjustment has to the pushrod end. It also does not account for thread pitch of the adjuster which might be 3/8x24 tpi or 7/16 x 20 tpi (Stud mount rocker arms.) For example, a 1.5:1 rocker arm on a 20tpi stud adjuster, 1/2 turn is 0.0417" preload. With a 1.6 rocker and 24 tpi adjuster, 1/2 turn is 0.0339" preload. Not a big deal except for when suggesting using 1/4 turn. With thermal expansion the lifter could end up with zero preload.
    On our Mopar shaft mounted rockers, the adjuster is at the pushrod, so figuring preload is easier. 24 tpi is 0.0417" per turn of the adjuster, and 20 tpi is 0.050" per turn.
    On the Stud mounted adjuster, it only takes about 1/2 turn to get the same preload the shaft mounted rockers would require 3/4 to 1 turn of preload.
     
  2. Cranky

    Cranky Banned Staff Member FBBO Gold Member

    Messages:
    22,952
    Likes Received:
    20973
    Joined:
    Dec 4, 2009
    Location:
    Pasa-getdown-dena Texas
    Local Time:
    4:25 PM
    Wouldn't the push rod expand some once it was heated up which would increase lifter preload? I usually shoot for .040 preload max cold with the juicers.
     
  3. IQ52

    IQ52 Well-Known Member

    Messages:
    1,653
    Likes Received:
    2208
    Joined:
    Aug 2, 2011
    Location:
    Idaho
    Local Time:
    3:25 PM
    Yes the pushrod will expand some. But so does the cylinder head, aluminum or iron (though aluminum expands more), which lifts the rocker shaft away from the camshaft and the valve tip. Thereby decreasing the preload.
     
    • Like Like x 1
    • HEMI-ITIS

      HEMI-ITIS STREETER on LI FBBO Gold Member

      Messages:
      12,226
      Likes Received:
      14858
      Joined:
      Dec 14, 2008
      Location:
      Long Island,New York
      Local Time:
      5:25 PM
      Iron heads & PRs might grow a thou or 2 that's it. When I had the iron heads the would grow 3 thou,with the aluminum they grow almost 6 thou!
       
    • 451Mopar

      451Mopar Well-Known Member

      Messages:
      2,444
      Likes Received:
      1752
      Joined:
      Oct 25, 2011
      Location:
      Aurora, CO
      Local Time:
      3:25 PM
      I really didn't explain how I calculated the adjustment difference. Normally the rocker arm is looked at as a first class lever with the pivot between each end, so a 1.5:1 rocker arm would have some distance from pushrod to pivot, and the distance from pivot to valve would be 1.5 times that distance. For example pushrod to pivot = 1, and pivot to valve = 1.5. Total length is 2.5.
      When you adjust preload by adjusting the pivot point, the ratio is the total length / the length from pivot to the adjuster, but now the pivot point is at the valve stem (third class lever like an engine hoist.) Example total length = 2.5 / pivot to adjustment = 1.5, so ratio is 2.5/1.5 = 1.667:1, so if the stud adjuster is turned in (down) 0.020", the pushrod would be pushed down 0.020" x 1.667 = 0.03334".
      As the rocker ratio gets higher, the ratio decreses as the adjustment gets closer to the pushrod. Example of a 1.7:1 rocker, 1.7+1 = 2.7/1.7= 1.588:1. So the same 0.020" at the adjuster is now just 0.020x1.588= 0.0317" at the pushrod.
       
      Last edited: Jul 30, 2018
    • PurpleBeeper

      PurpleBeeper Well-Known Member

      Messages:
      4,694
      Likes Received:
      3426
      Joined:
      Mar 20, 2011
      Location:
      Chicago
      Local Time:
      4:25 PM
      Wow, I'm a "math guy" and my head hurts. Could you dummy this up for me so I know how many turns of pre-load I should put onto my hydraulic lifters with the old Isky ductile iron rockers (I think 1.5 ratio, but not sure) that I bought in the 80's? I'm about to adjust them again. I went with 3/4 turn in last time.

      I get the "general idea" of what you're talking about with lever arm length and thread pitch, but not the finer details. Is there a relatively easy way to adjust the rockers that is better/more precise vs. the desired thousandths of an inch pre-load? I've always just heard "1/2 turn in"
       
    • 451Mopar

      451Mopar Well-Known Member

      Messages:
      2,444
      Likes Received:
      1752
      Joined:
      Oct 25, 2011
      Location:
      Aurora, CO
      Local Time:
      3:25 PM
      Trying to state the differences between adjusting the stud mount rockers vs shaft mount rockers and how the "turn the adjuster" a specific fraction of a turn can result in different preloads depending on the setup.
      The amount of actual preload to use has always been debated. Some racers turning high RPM like near zero lash with the engine hot so the hydraulic lifter will not pump up and hold the valve open. Many OEMs like > 0.080+ so the lifters are really quiet and have plenty of adjustment tolerance. Just about every performance cam manufacturer recommends somewhere between 0.020" minimum and 0.060" maximum. This is the preload with a warmed up engine, not setting the preload cold.
      With iron block and aluminum heads, I use 0.010" of expansion from cold to hot. Figuring about 0.008 to 0.012 depending on actual head expansion for aluminum, and a but less for iron heads.
      Anyhow, for the shaft mounted rockers with adjusters over the pushrod, I just use 1 full turn cold for most setups.
      If the adjuster is 24 tpi, this is 0.0417" preload cold, and hot it should end up around 0.030" to 0.036" preload
      If the adjuster is 20 tpi, then 0.050" cold, and 0.038" to 0.044" hot.
       
      • Agree Agree x 1
      • 451Mopar

        451Mopar Well-Known Member

        Messages:
        2,444
        Likes Received:
        1752
        Joined:
        Oct 25, 2011
        Location:
        Aurora, CO
        Local Time:
        3:25 PM
        Last edited: Jul 30, 2018
        • Like Like x 1
        • Thanks! Thanks! x 1
        • MoparLeo

          MoparLeo NRA BENEFACTOR LEVEL LIFE MEMBER FBBO Gold Member

          Messages:
          1,303
          Likes Received:
          1095
          Joined:
          Nov 24, 2015
          Location:
          Moreno Valley, So California
          Local Time:
          2:25 PM
          And yet they still run. Just goes to show that you don't need a degree to make your car run. You make life too complicated. How many out there have broken any parts whether you went 1/4 turn or 1/2 turn. What does the FSM say? There are enough variables in an engine to give anybody a head ache.
           
          • Agree Agree x 1
          • dvw

            dvw Well-Known Member

            Messages:
            5,622
            Likes Received:
            8106
            Joined:
            Feb 16, 2010
            Location:
            waterford mi
            Local Time:
            5:25 PM
            Preload doesn't mean squat on a hydraulic cam as long as you have the folowing
            A: the valve spring controls the valve train in the engines usable range
            B: the plunger isn't bottomed
            C: there is enough preload to have a oil cushion under the plunger at all times
            My 2 cents
            Doug
             
            • Agree Agree x 4
            1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
              By continuing to use this site, you are consenting to our use of cookies.