Pushrod Length Tolerance

Discussion in 'Cams, Heads and Valve Trains' started by Ronno6, Feb 9, 2019.

  1. Ronno6

    Ronno6 Member

    Using a Proform pushrod length checker, I found my pushrods to be a bit short.
    Using feeler gauges I determined them to me .040" - .045" short.
    Do I conclude correctly that that is TOO short?

    Myy pushrods are 7.205" long.
    Comp Cams offers the 5/16" pushrod off the shelf in a 7.250" length.
    That may put me .005" or so over on a cylinder or 2, but I would recon that to be
    considerably better than .040" short....(?)
     
  2. Indycars

    Indycars Administrator Staff Member

    What does the witness mark on the valve tip look like? How far off center is the mark?
     
  3. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    "
    That may put me .005" or so over on a cylinder or 2, but I would recon that to be
    considerably better than .040" short....(?)"

    yes .005 over is better than .040 under.... but either should work, its better to try an adjustable push rod to verify the wear pattern.


    "[​IMG]
    [​IMG]
    yes I use BOTH the push rod checker tools to get the quick check on length, and the adjustable length push-rods and a marker too method verify the correct length with a cross check by looking at the wear/sweep and location, on the valve tip
    [​IMG]

    if your within .050 away youll generally be ok, if your within, .030 or less that is ideal,
    at some point your effectively "SPLITTING HAIRS" and yes ideal length is preferred
    if you have adjustable length push rods you can play with the length to find ,
    what gives you the best and most consistent and ideally narrow and preferably centered,valve tip wear pattern
    that way your not guessing , you see what minor changes do to the wear pattern
    get as close as you can, and it does not mater much if its a bit over or under length as long as its reasonable close

    read the links

    http://garage.grumpysperformance.co...sh-rod-length-checker-tools.13735/#post-70650

    http://garage.grumpysperformance.co...rect-custom-length-pushrods.14241/#post-72366

    http://garage.grumpysperformance.co...train-clearances-and-problems.528/#post-79273

    http://garage.grumpysperformance.co...on-these-push-rod-length-checker-tools.13735/

    http://garage.grumpysperformance.com/index.php?threads/rocker-push-rod-wear-issues.9815/#post-56576
     
    Last edited: Feb 9, 2019
  4. Ronno6

    Ronno6 Member

    The "witness mark" appeared to be pretty good IMO.
    There were a couple that were high, maybe by half the width of the mark.
    However, those were taken using studs which were very inconsistent as to concentricity.
    I have changed the rocker arm studs to ARP's which are MUCH more concentric than
    the previous PBM studs.
    I have also changed rocker arms as the wide-butt Scorpions I was using were scraping the valve covers.
    As I wish to be as good as I can get, I will try the longer rods.
    I'll see how the mark looks then.
     
  5. Indycars

    Indycars Administrator Staff Member

    When possible, take photos of your work. Then when there is a question, it's much easier for
    others to voice a accurate opinion. It's much harder when all you have is words.

    Just a suggestion.
     
  6. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    If you have a choice, it is better to err on the slightly short side. Why?
    Because the valve spring pressure is only exerting it's seat pressure while the rocker arm is contacting the valve tip towards the intake port side.
    Meaning that the side thrust to the valve guide is much less in this situation, as compared to if the pushrod was too long.
    Then the side thrust toward the exhaust side would be much greater because the valve spring is fully compressed at max valve lift.
    Think about it, which scenario is going to wear your valve guides sooner - pushrod too short or too long?
     
  7. Ronno6

    Ronno6 Member

    I a ma horrible photog....I can never seem to really capture the subject to my satisfaction.
    Here's the best I could do:

    shortrod1.JPG

    This gap is typical of the other pushrods.
     
  8. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    the pictures just fine!
    [​IMG]

    and as previously stated, why not use an adjustable length push rod.
    to find the ideal slightly longer in this case push rod length then use your 12 caliper
    to measure the test push rod and order the length you find that available thats less than .050 in length different.
    BTW the push rod should not touch the push rod guide slot ,
    I generally stick a plastic zip tie in the guide slot to center the push rod location in the guide slot, before measuring the ideal length ,
    if its leaning in toward the rocker stud the measurement tends to read shorter than it should be.
    [​IMG]
    [​IMG]
    yes I use BOTH the push rod checker tools to get the quick check on length
     
  9. Ronno6

    Ronno6 Member

    OK. This is better. I realized that I wasn not rorating the crank correctly to get the contact patch....1 cycle CW and 1 cycle CCW gives bogus readings.
    these show things better with 2 CW roratin cycles:

    contactpatch4.JPG contactpatch5.JPG

    I apologize for the blurry pic, but I turned the flash off in order to get a more distinct image. I didn't hold too still....
    Anyway, contact patch too far up......pushrods too short.
    Correct??
     
  10. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    it may be a bit short but its rather well centered, (yes it could be better)
    which is good, I think if your push rods are a bit longer
    (maybe .050, that may be fine,)
    but don,t guess check and accurately measure,
    it ideally will look like this, and yes you need to properly adjust the valve train and rotate the engine,
    a full 720 degrees remember the crank rotates twice for each cam rotation.
    its why the cam gear is twice as large as the crank gear on the timing chain
    http://garage.grumpysperformance.co...ectly-and-get-it-to-last-cam-install-info.90/
    its the attention to details and checking clearance etc. and time and care taken,
    that makes the difference between a barely or average functional ,
    and a rather exceptional engine build,
    yes the urge to get it running is strong,
    but getting it done correctly is the goal, when checking the valve train,
    its advisable to always spin the engine in the normal clockwise ,
    looking from the front of the crank damper,
    direction of rotation, checking valve train clearances in the other reverse rotational direction,
    changes the clearances and valve train geometry very slightly.

    [​IMG]
    [​IMG]

    http://garage.grumpysperformance.co...train-clearances-and-problems.528/#post-79273

    http://garage.grumpysperformance.com/index.php?threads/two-loose-valve-locks.9687/#post-36006
     
    Last edited: Feb 17, 2019 at 7:21 PM
  11. Ronno6

    Ronno6 Member

    The ordered pushrods are .045" longer...should be 'bout perfect....
    I fully understand the cam/crank relationship....
    I had been turning the crank CW thru the lifter rise and fall, then reversing the direction to CCW for the second cycle.
    That yielded a much wider contact patch and gave false center reading.....

    contactpatch2.JPG
     
  12. Ronno6

    Ronno6 Member

    You should see my lengthy dissertations on mixing brake systems and, especially, manipulating deck height and head gasket thickness to obtain optimum
    quench and compression ratio which I posted on another forum...........
    Talk about attention to detail........
     
  13. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

    yes admittedly at some point in any engine build,
    striving for excessive precision,is counter productive,and
    rapidly has the potential for or tends too devolve into absurdity,
    but slapping parts together to get the engine up and running,
    or grabbing easily available and mis-matched parts, seems far more common.
     
  14. Ronno6

    Ronno6 Member

    And, of course, rotating the crank 720° will rotate the camshaft exactly 1 revolution.
    That will actuate all valves no more or less than 1x.........
    I believe I have read that the lifter should be cycled 2x..that would require 4 turns of the crank.
     
  15. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    [​IMG]

    What is guiding the pushrod in this picture? Am I missing something?
     
  16. Ronno6

    Ronno6 Member


    Not a thing! You are not missing anything.....except the self aligning roller rocker not in the picture !!
     
  17. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    That's what I was hoping you would say.
     
  18. Ronno6

    Ronno6 Member

    The pushrod guides would not permit enough separation to properly align the original rocker arms over the valves,
    so I bought self aligning rocker arms and cut off the guide fingers, using the plates as spacers under the studs.
    I could not return them and thought it foolish to purchase washers. So...............
     
  19. T-Test

    T-Test Well-Known Member

    The joys of Hot Rodding will drive yo crazy or broke--pick your poison.:):):)
     
  20. Grumpy

    Grumpy The Grumpy Grease Monkey mechanical engineer. Staff Member

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