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How to Measuring for Correct Push Rod Length.

This is a discussion on How to Measuring for Correct Push Rod Length. within the Internal Engine forums, part of the LSx Technical Help Section category; Every once in a while something comes along that you wish you had more info on. This is one of ...

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    Spaz is My Mentor SMWS6TA's Avatar
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    How to Measuring for Correct Push Rod Length.

    Every once in a while something comes along that you wish you had more info on. This is one of those times. Will (98TransAmWs-6) and I spent a lot of time going over this for a few weeks. It seems there was very little info in regards to this subject.

    GM has ended the LS1 lifter and replaced it with the LS7 lifter. A lot of discussion on several forums about if you need to recheck the push rods after installing LS7 lifters with stock heads or can you just run with stock 7.4" rods and be good. Some have had no issues with OEM length while others have. Too long and you can get coil bind or PTV issues, too short and the valves don't open far enough. Both = bad.

    So we felt it was necessary to do a write up.




    Measuring for Correct Push Rod Length.

    Whenever you install new cam, mill the cylinder heads, new rockers, or even change lifters it is always a good idea to check to see if the push rod length has changed with the repairs/mods being done.


    These directions are for just checking push rod length and installing the rods. Not for head removal, cam install or lifters.


    Tools require:


    Ratchet
    8mm socket
    24mm socket (for rotating the crank for a final check)
    15mm socket (IIRC the size) for removing the spark plugs.
    Comp Cams Push Rod checker, Adjustable from 6.8" to 7.8" (cost $20)




    Step 1: Pick a cylinder; you’ll do this for each head. First turn the engine over manually until the cylinder is TDC. This can be done by placing a paper towel in the spark plug hole of the cylinder and when it blows out you are TDC or placing a wire in the hole and when the piston its it you are TDC.

    Step 2:
    Loosen both the intake & exhaust rockers. Pull push rod out & set aside.

    Step 3: Take your Adjustable pushrod checker and open it up to 10 full turns. Next install it into the intake pushrod cavity.

    Step 4: Checking for Zero Lash. There are two ways to check this next step.

    1. Now reinstall the rocker arm and tighten it down but don't torque it. Now feel for play (tap the rocker) at the tip of the rocker arm, if you hear a tapping sound the pushrod is too short and should be readjusted by rotating the checker one full turn at a time and rechecking it until there is very little play and the tapping sound goes away. Rotating one full rotation = 0.050”

    If the pushrod is too long there will be no play at all, meaning not even to the side, a proper length pushrod will have no play up and down but will have some side to side at all and the pushrod will need to be adjusted 0.050” smaller at a time until the proper side is reached.

    This may take several attempts to achieve “Zero Lash”

    2. Instead of tightening the bolt down, remove the bolt from the rocker and press it on the pedestal. Hold it firm with one hand while tapping the rocker if you hear a tapping sound the pushrod is too short and should be readjusted by rotating the checker one full turn at a time and rechecking it until there is very little play and the tapping sound goes away. Rotating one full rotation = 0.050”

    If the pushrod is too long there will be no play at all and the pushrod will need to be adjusted 0.050” smaller at a time until the proper side is reached.

    Doing this method eliminates the possibility of torquing the bolt down effecting the measurement of the checker. I’ve done both ways and each came to the same measurement.

    Step 5: Once you find the correct length remove the pushrod. Next count the turns it takes to fully close the pushrod. Write down your count. We’ll get to the math formula later.

    Step 6: Repeat steps 4 & 5 for the exhaust, remember to write it down.

    Step 7: Repeat steps 1thru 6 on the other cylinder head.

    Step 8: Calculating Push Rod length:

    Take the # of turns and multiply that number by 0.050” Add 6.80”. (This is the length of the checker) Now add lifter preload* This is your pushrod length

    Looks like this: 9 (turns) x 0.05” = 0.45” + 6.8” = 7.25” +0.075”= 7.325”

    Note *= Lifter preload may vary depending on the lifter. I used LS7 lifters because this is the direct replacement for all LS1/2/6 motors. If you use aftermarket lifters, check with manufacturer for preload.

    Step 9: Order new push rods

    The following steps are for re-using same rods or after new push rods come in.

    Step 10: Install new push rods and torque them down to 22 ft-lbs. This needs to be done when the cylinder is TDC.

    Step 11: After all have been installed rotate the crank a couple full cycles. This will ensure all rockers have been cycled thru the compression cycles for both intake & exhaust.

    Step 12: Recheck the torque on the rockers to make sure they are still torqued down to 22 ft-lbs. By doing this repeat step 10.



    When referring to the "Rockers" we are referring to the stock OEM rockers. If you are using aftermarket rockers use their torque specs and sequence.


    Hope this helps.
    Last edited by SMWS6TA; 02-12-2013 at 02:45 PM.

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    Good job Scott. This definitely needs to be stickied.
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    Sticky

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    Good info guys


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