Jeep 232, 258 and 4.0L Inline Six and 2.5L Valve Adjustment

AMC/Jeep® Inline Six Hydraulic Lifters and Valve Adjustment

     The subject of valve clearance on AMC/Jeep 2.5L fours and 232/258 and 4.0L inline six-cylinder engines is topical at the magazine’s forumsand across the internet!  In this vlog, Moses Ludel talks in depth about checking and setting valve clearance on 1964-2006 AMC and Jeep inline sixes.  Want to demystify the subject of valve clearance settings on an AMC/Jeep® engine?  Watch this vlog!

     Note:  1964 to early ’70s engines have rocker shafts and non-adjustable rocker arms.  Later sixes have pedestal rocker arms that are also non-adjustable.  Moses Ludel talks about various methods for setting the valve clearance and how to properly compensate for wear, cylinder head surfacing, valve grinding and changed valve seat depth or valve stem heights.

Footnotes on Valve Clearance and Lifter Preload Settings

For hydraulic valve lifters, the valve clearance is typically “zero” plus an additional depression or “preload” of the hydraulic lifter plunger.  Imagine the lifter plunger in its normal, fully extended position.  Now envision the pushrod taking up the clearance between the rocker arm seat and lifter’s pushrod cup.  This would be zero clearance.  Since this is a hydraulic lifter, it requires a lifter plunger preload.  Preload is the depression of lifter’s plunger from its fully extended position downward into the lifter.

Jeep inline four- and six-cylinder engines with hydraulic lifters need this preload to 1) prevent the plunger from pressing against the retainer clip and popping the lifter plunger loose and 2) to allow for minor valve face and seat wear.  Face and seat wear means that the valve rides higher and tips the rocker further downward, which presses the pushrod further downward.  In actual engine running/operational mode, the lifter oil and check valve hold its plunger height and keeps the valve clearance at a zero-gap position.

Here is a useful, generic illustration courtesy of a G.M. Performance website.  Note the pushrod and “socket” position in the preloaded mode.  The lifter’s oil level maintains the zero-clearance between the pushrod and rocker arm.  “Zero” just removes the valve stem-to-rocker arm gap without unseating the valve.  This consistent zero clearance is maintained by the lifter’s plunger height, which is controlled by the check valve and oil volume in the lifter’s cavity.  Valve spring pressure seats the valve and helps establish the zero gap point:

In the video/vlog, I talk about the use of a pushrod length gauge to measure the zero gap dimension.  Again, you want to bring the piston to TDC on its compression stroke (both valves closed).  At this point, you want to know the measurement between the rocker arm seat and the lifter socket seat with the lifter plunger fully extended to the circlip.  Once you know this length, add the desired additional preload length to the pushrod.  This additional length establishes the correct lifter preload.

For non-adjustable rocker arm engines, Jeep does not provide a preload specification or pushrod length measurement anywhere in its service literature.  When valves and seats are ground, the valve stem height is crucial.  (Milling or surfacing the head will also alter the valve stem height.  So does installation of a thicker or thinner head gasket.)  Valve stem height affects the lifter preload.  The only way to “adjust” lifter preload is with the correct length pushrod or by grinding off the valve stem tip to establish the correct valve stem height and plunger position in the lifter.

If you want an arbitrary pushrod preload length, most use a figure of 0.020″-0.040″ depending upon the lifter manufacturer’s recommendation.  This means that the pushrod(s) will press the lifter socket/cup and plunger down this far with the valve closed and camshaft lobe on its heel (lowest point).  When the pushrods fill with oil (do not fill the lifters with oil, let them fill with the engine running), the oil and lifter check valves will hold the lifter plungers at this height in the lifter with the engine running.  The check valve keeps the lifter from bleeding down on the loaded upstroke of the lifter.

The term “clearance” is misleading in the case of hydraulic lifters.  These lifters create zero (theoretically 0.000″) valve stem tip to rocker tip clearance while the lifter plunger rides at a prescribed height within the lifter—a height that we call hydraulic lifter “preload”.  Valve “clearance” is the term originally applied to mechanical lifters.  Mechanical lifters require a running clearance or gap between the valve stem tip and rocker arm tip when the valve is closed and the camshaft lobe is on its heel.  This actual gap is set between the rocker arm tip and the valve stem with the piston at TDC on the compression stroke.



  1. Terry says:

    Thank you for putting this video out there. It has saved my a##

    • Moses Ludel says:

      You’re welcome, Terry! There is confusion around Jeep inline six pushrod lengths and valve clearance. Glad to clear the air. I went deeper with this topic at the forums. See this link and the December 25, 2018 onward comments about the CompCams measuring tool and how to pick pushrods from available lengths: Jeep inline six pushrod lengths. I list Melling OEM pushrod part numbers for specific length pushrods. You’ll find the coverage helpful. It applies to 232/258 inline sixes. Melling has pushrods listed for the Jeep 4.0L inline six as well (see a Melling catalog). The CompCams pushrod measuring tool is available at Summit Racing and elsewhere…Moses

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