valve train geometry problems

Discussion in 'Troubleshooting & Diagnosis' started by Lee Beason, Nov 9, 2019.

  1. Lee Beason

    Lee Beason New Member

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    72 sbc newly rebuilt trying to find pushrod length with vortec heads. If I set the rockers to 90 degrees at half lift I have a contact patch that is way too far outboard on the valve stem. To get a good contact point i have to almost bottom out the rocker and the 90 degree aspect is shot. Nowhere near. That is with the guide plates off! Even worse with the guide plates. This is checking with aluminum roller rocker and with Comp steel roller rockers. I can do one or the other but can't get both even close. Using checker springs to see what's going on without having to turn the motor. Lee.
     
  2. Vettenick

    Vettenick Vettenick

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    If that block was decked, it's possible that your current push rods are too long. You might need an adjustable push rod checker to find out what length you need.
     
  3. G72Zed

    G72Zed Veteran Member

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    How close is the pattern to the edge of the valve tip?

    In addition to what Vettenick said, it also sounds like they may have used a longer valve(s) in the rebuild.

    A possibility or quick fix would be to look at some lash caps, it would bring the valve tip closer (by a very small amount) to the rocker stud because of the added thickness, but would give you more real estate for the roller tip if everything else lines up.

    If that's not an option for you, look at some roller rockers that have a built in "back set" in your stud size and ratio your using.
     
  4. grzewnicki

    grzewnicki Veteran Member Lifetime Gold Member

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  5. twestrup

    twestrup Veteran Member Lifetime Gold Member

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    Not an expert on this but this is what I did. Bought an adjustable, measuring pushrod to help determine correct pushrod length. I did watch several YouTube videos and this is what I believe they wanted the pattern to look like: The guide plate was off a little which is why it does not look square to the valve stem. Different lengths for intake and exhaust.

    I have not run the engine yet, so I really hope it is all correct.

    IMG_3545.JPG
     
  6. Lee Beason

    Lee Beason New Member

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    Thank you all for your replies. The picture is the best possible contact patch I can get without bottoming out the rocker. I get 90 degree geometry at about .370 lift. Total lift is .525. Compromise on all fronts. Does the contact patch look wide? Or am I ballpark good? Been banging my head for a month thinking it had to be perfect. Found pushrod length wants to be 7.6. Yes I have pushrod checker as well as checker springs. Thanks again, Lee. 20191111_160519.jpg
     
  7. Lee Beason

    Lee Beason New Member

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    Maybe I am not supposed to be here with a chevelle. Sorry. This is my fathers car and we are pulling our hair out. If it helps at all he also has a 67 RS/SS Camaro. Lee.
     
  8. twestrup

    twestrup Veteran Member Lifetime Gold Member

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    I can't remember, but the picture either indicates to long or to short of pushrod. Check youtube for the
     
  9. G72Zed

    G72Zed Veteran Member

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    Well, to hit the "mid lift" geometry that would mean hitting 90* at .263 at that would indicate using a longer push rod to get your trunion center line up on the stud, that would move it out even further. What does the witness mark show when you adjust for "mid lift"

    A 7.600 is a short PR, -.200 from stock. I would be concerned with oil hole timing and added movement on the PR side. Is your block heavily decked, or the heads? regular length Valves?

    If I took AWAG, the pattern looks to be around .070, and your on the outside 3rd, but your not at mid lift. Your right, it's all a compromise on a stud setup.

    The aluminum rockers your using, are they "Millers", I have seen the "patterns" change with different manufactures with the same ratio.

    I would also do the same test with the actual springs and see your "net" lift at the retainer in order to set the true "mid lift", this would also tell you the rocker arms actual ratio based on lobe lift and loss lift due to deflection.
     
  10. G72Zed

    G72Zed Veteran Member

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    The way I see it, he is using a shorter (-.200) from the factory 7.800 PR and the pic looks like it's to long of a pushrod.

    Rocker arm design differences, and actual stud placement come into play, not to mention valve length as the stud and valve centre lines do not run in parallel.
     

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