Correct Push Rod Length & Lifter Pre-load?

Discussion in 'Fox 5.0 Mustang Tech' started by ks65stang, Sep 12, 2012.


  1. ks65stang

    ks65stang Member

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    I just rebuilt my 1965, 289 and installed 1993 Cobra GT40 heads w/ Scorpion pedestal 1.6 roller rockers. I’m running a new Hydraulic Flat Tappet Camshaft (see cam card attached). My stock 289 pushrods are too short. So I bought an adjustable push rod to get the correct size...

    My question is, how do I know I have the correct lifter pre-load? I have installed the adjustable push rod and rockers on my assembled heads and adjusted to zero lash with out depressing the hydraulic lifter. I have also taken a sharpy pen to see the wear pattern. I can get the wear line it in the middle of the valve stem, but how do I know I have the correct pre-load on the lifter?

    I can lengthen the pushrod to were the lifter is partially depressed at zero lash and still get an acceptable pattern on the valve stem?

    Anyone running this combo or have a suggestion?
     

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  2. ks65stang

    ks65stang Member

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    I was making this way too hard. Below is the response from Scorpion:

    The pushrod length on a non-adjustable rocker is for lifter preload only and you're shooting for approx. .030 to .080 preload with a standard type lifter. The best way to check with an adjustable pushrod is to make the pushrod shorter than needed. Put the lifter on the low part of the cam lobe (valve closed), put the pushrod in, install the rocker with all hardware and tighten the attaching screw. Lengthen the pushrod (lightly) until you've just taken the up and down movement out (zero lash without depressing the lifter). If the intake manifold is off you can see if you start to depress the lifter. Remove the pushrod measure it and then add the amount of desired preload to the the pushrod length and you're done.

    I would suggest checking more than one cylinder as machining variances with heads, cam, block etc. may vary this. If this is not an all out, high rpm race engine you don't have to get carried away as the engine basicily will not perform any different if the preloads vary. As long as you keep it out of valve float the engine only knows it's running with no lash. No you can't use a solid lifter as the pushrod seat height may not be the same as your preloaded hydraulic lifter.

    If the valves are close to stock length then you would not have to worry about the rocker to valve contact as it is fixed with the pedestal height.

    I would suggest using at least a .080 wall pushrod for best performance with any engine.
     
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  3. Rick 91GT

    Rick 91GT SN Certified Technician Site Sponsor

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    Yes that is the correct method.. Id suggest a Trick Flow pushrod, they are .080" wall chrome moly 1 pc pushrods, very cost effective.
     
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  4. ks65stang

    ks65stang Member

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    My push rod length at zero lash and no pre-load on the lifters is about 6.87" and per my camshaft instructions acceptable preload is .030-.060" on the lifters. The Scorpion tech guy said .030-.080" of lifter preload is acceptable.

    I ordered the push rods below.

    http://www.jegs.com/i/Comp-Cams/249/7828-16/10002/-1

    Push rods specs:
    Comp Cams #249-7828-16
    High Energy Pushrod Set
    • Ford 255-302ci 1968-85
    • 5/16'' Diameter
    • 6.936'' Intake; 6.936'' Exhaust (+.060'' Long)
    • End Type: H-H
    • 16/pkg.

    This puts my push rod length now at 6.93 and preload on the lifters at 0.060" (the max). I may shim the pedestals up .020 if I feel needed.
     
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