Push Rod Length

Discussion in 'Street/strip 400/430/455' started by crazyjackcsa, Mar 12, 2018.

  1. crazyjackcsa

    crazyjackcsa Big and Untame

    Looking for advice on a subject I am WAY out of my depth on.

    I'm rebuilding my 71 455 and I already ordered new pushrods. It never occurred to me to inquire about length.
    So I went stock length.

    Then (no word of a lie) as I'm lying in bed last night, I sat upright when I realized that both the heads and the block have been machined. Nothing too crazy, just a resurfacing. (I'll have to ask my engine guy how much he shaved off).

    The question is this: At what point can I no longer use the stock length rods? How much tolerance is there?
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Bottom line, if you are rebuilding an engine, you should measure for proper push rod length. Best way is to buy an adjustable push rod. Even just replacing the cam can alter what length you need.
     
  3. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    If the engine came with steel shim head gaskets from the factory and you are going to use the thicker Fel-pro blue head gaskets you might be ok?

    Usually the length needed is checked with an adjustable push rod before ordering push rods for your engine.
     
  4. crazyjackcsa

    crazyjackcsa Big and Untame

    This is all good info! So how do I measure with an adjustable?
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You get each lifter on the base circle of the cam lobe. The base circle is the back side where there is no lift at all and the valve is closed. Then you slowly extend the adjustable push rod until you get to zero lash. Start with the push rod short enough where you can move it up and down. Slowly extend it while spinning the push rod. You can feel zero lash as a subtle resistance as you spin the push rod and take up all the clearance. You can do this several times to get the feel of it. Once you get to zero lash, extend the push rod half a turn for lifter pre load. Then remove the push rod and measure it. I used a digital caliper to measure. You can do several or all of the push rod positions and measure each one. They should be very close in length. Then order the correct length. You could also just use an entire set of adjustables. I did exactly that in my first engine.

    Both valves are closed at TDC of the compression stroke for each cylinder. If you mark the harmonic balancer every 90*, you can go around the firing order and do each cylinder. That is the simplest way. The balancer diameter is 6 3/4". The circumference is 6.75 X 3.14 (pi) = 21.2" in circumference. That would be 360*. To make a mark every 90*, just measure from the factory mark on the balancer and mark it every 5 1/4". Then start with #1 cylinder making sure you are on the compression stroke. Measure the push rods of cylinder #1. Then turn the crank 90*, and do cylinder #8. Follow the rest of the firing order 4-3-6-5-7-2, turning the crank 90* each time. You'll need to turn the crank 2 complete revolutions (720*) to do all 8 cylinders (16 push rods). It 's easier with all the spark plugs removed.
     
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  6. Bluzilla

    Bluzilla a.k.a. "THE DOCTOR"

    One thing that folks often overlook and can play into the equation is correct valve tip height. If and when a valve job is performed or new seats have been installed a competent machine shop will check for, .... and correct, valve tip height when possible. That equalization of tip height helps to keep pushrod length uniform, as well as factoring into the desired geometry.

    Larry
     
    Last edited: Mar 12, 2018
    Julian likes this.
  7. Schurkey

    Schurkey Silver Level contributor

    And UNLIKE Chevrolet's ball-stud system, when a "non-adjustable" rocker-shaft system is employed, pushrod length affects hydraulic lifter plunger preload but NOT rocker geometry.

    If there's enough (but not too much) lifter plunger preload...you'll be fine. I've seen lifters with more than a quarter-inch of plunger travel. I've also seen some that were deliberately limited to a few thousandths of an inch.
     
    Mark Demko likes this.
  8. Bluzilla

    Bluzilla a.k.a. "THE DOCTOR"

    As I was saying, .... often folks don't realize that no matter what style rocker is used, the Valve Tip Height Most Definitely DOES Affect Geometry, ..... as well as alters the necessary pushrod length. With our shaft mounted rockers, keep the tip height correct and uniform to achieve the best geometry, then size the pushrods for length.

    Larry
     
    Last edited: Mar 13, 2018
  9. Schurkey

    Schurkey Silver Level contributor

    You are correct, and I'm not contradicting you. In the Buick system, valve tip height could affect pushrod length--although there's a LOT of wiggle-room due to lifter plunger travel. Pushrod length with the Buick shaft-mount rockers is not critical like the Chevy ball-stud system--as long as there's some lifter preload, but the plunger isn't bottomed-out. Within the range of "some but not too much" lifter preload, pushrod length is immaterial. By contrast, the Chevy system is sensitive to a 0.050 (or less) change in pushrod length assuming the preload is adjusted the same for each length of pushrod. A shorter or longer pushrod changes the contact pattern (position of contact, and width of the contact "sweep") of the rocker arm at the valve tip. Not so with Buick.

    And that was my point. In a stock Buick, valve tip height can affect pushrod length, but pushrod length does not affect valve tip height, or any other valve/rocker/lifter geometry other than hydraulic lifter preload. (If the pushrod is too long, the lifter plunger could bottom-out, and that might hold the valve open--but that's not the way to measure valve tip height.)
     
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  10. Bluzilla

    Bluzilla a.k.a. "THE DOCTOR"

    A lot of good useful information here guys! .............. I'm a real stickler for proper valve train geometry so I use this snazzy fixture to easily and accurately measure valve tip/stem height. I may start a new "Valve Train Geometry" thread one day as to not deviate too far from the OP.
    101_4782.JPG 101_4784.JPG
    Larry
     
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  11. Bens99gtp

    Bens99gtp Well-Known Member

    Too much lifter preload occurs way before the plunger bottoms out. Anything over .050 can run the risk of lifters pumping up and hold the valves open. If running upper rpms often shorten that to .03 or .025 can be a wise
     
  12. Schurkey

    Schurkey Silver Level contributor

    It's all "philosophy".

    Some guys run minimal preload so the plunger can't "pump up". Typically, it's more a matter of the valves floated, and the plunger simply takes-up clearance. This can be more common with weak valve springs.

    Some guys run the preload at factory spec.

    Some guys run the preload almost all the way to the bottom of plunger travel, so that the lifter can't "leak down" and cause loss of lift with heavy valve springs.

    Of course, you can buy lifters with severely restricted plunger travel, so you can have the plunger at the top of it's travel (minimal pump-up) yet also be near the bottom of it's travel (minimal bleed-down.)

    So plunger at the top, plunger at spec, or plunger way down...it's all a matter of what you fear.
     
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  13. crazyjackcsa

    crazyjackcsa Big and Untame

    All good information! As is my way, I called the engine builder, he said he pulled 0.010" off the head and stock lifters will be fine. So that's what I did.
     
  14. Brett Slater

    Brett Slater Super Moderator Staff Member

    Larry,

    Is proper measurement achieved in your directions prior to the mention of both valves being closed at TDC? And the math?
     
  15. Schurkey

    Schurkey Silver Level contributor

    With shaft-mounted rockers, if you have adequate lifter preload, but not too much preload, you're fine.

    There is no "math". Get your adjustable pushrod set to the needed length as Larry described. Measure your adjustable pushrod, order a set of pushrods to suit. The only difficulty is that there's AT LEAST three different ways to measure a pushrod, so you need to use the same method as the company you're buying your pushrods from. That way you're "speaking the same language". It's not super-critical, shelf-stock pushrods come in .050 increments, so if you're +/- .025 or closer, and still have acceptable lifter preload, you're likely OK.

    https://www.compcams.com/valve-train-geometry

    Comp Cams used to have a nice illustration of the three methods of measuring pushrod length. I can't find that illustration any more.
     
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  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Not sure I understand Brett. At TDC, both valves are closed, there is no lift. That is where you adjust the pushrod, then 1/2-3/4 turn to get preload.
     
  17. Brett Slater

    Brett Slater Super Moderator Staff Member

    Your first paragraph ends with "then order the correct length". The novice/layperson may think what you laid out in that first part would be all one needed to do to get proper measurement.

    That's what I meant.
     
  18. Brett Slater

    Brett Slater Super Moderator Staff Member

    I'll call TA and get input.

    Thanks!
     
  19. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    After 0 lash, you go 1/2-3/4 turn to lengthen them to preload the lifter. Then measure them. I have a caliper that I used. I just measured them end to end. It worked for me.
     
    Brett Slater likes this.
  20. Redmanf1

    Redmanf1 Gold Level Contributor

    When I have my engines done they are measure for proper push rod length as Larry said.
     
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