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Many Jeep owners need how-to information on checking valve clearances and adjusting the hydraulic valve lifters on the inline 232, 258 and 4.0L six-cylinder engines and the 2.5L Jeep pushrod engine.  Between the model years 1971 and 2006, Jeep used these AMC-design 232, 258 and 4.0L sixes and the 2.5L straight four-cylinder engine (1983.5-2002).  The hydraulic lifter and valvetrain design has particular needs, especially the valve clearances.

When these engines develop valvetrain noise, owners often think a valve adjustment will cure the problem.  In each of these AMC/Jeep engines, valve clearances are set during assembly of the engine, and adjustment is not necessary between engine rebuilds.   If your engine has developed valvetrain noise, or if you are in the process of rebuilding the engine and need to know more about setting valve clearances, my HD video from 4WD Mechanix Video Network at Vimeo will assist.

This video was originally a Q&A Vlog at the magazine, now available for a broader viewing audience through Vimeo.


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When performing valve seat grinding, cylinder head resurfacing or block decking work, the lifter clearances will change.  Preload on a hydraulic lifter is critical.  If there is too much preload, the lifter plunger can bottom and prevent the valve from seating.  Too little clearance leads to lifter noise or, at worst, the plunger dislodging the retainer clip and coming apart.  Lifter preload is determined by the pushrod length on AMC inline sixes (232/258/4.0L) and 2.5L AMC four cylinder engines.  The installed valve rockers are at a fixed position that cannot be altered.  

Visualize the lifter plunger at its normal, fully extended position and the camshaft on the heel or lowest point on the lobe.  Now envision the pushrod taking up the clearance between the rocker arm pushrod seat and lifter's pushrod cup.  This would be zero clearanceThe additional depression or "preload" of the hydraulic lifter plunger is determined by the pushrod length.  Since these AMC design engines use hydraulic lifters, they require lifter plunger preload.  Preload is the amount of lifter plunger depression from a fully extended position downward into the lifter.

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 a specific plunger preload height, which keeps the valve stem tip-to-rocker arm clearance at a zero-gap position.

This is a great generic illustration courtesy of a G.M. Performance website.  Note the pushrod and "socket" position in the preloaded mode (right).  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: 


This is why I talk about the use of a pushrod length gauge to measure the zero gap dimension.  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.

For an arbitrary pushrod/lifter 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 hydraulic lifters with oil, let them fill when the valvetrain is fully installed and the engine is 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.

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 position within the lifter—a height or position 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.

A CompCams or equivalent pushrod gauge tool is inexpensive insurance when cylinder head, block deck or valve service work is performed.  Check the pushrod length requirements for the new rocker arm-to-lifter relationships.  Do this for each valve.  Install the correct length pushrod(s) to establish the right lifter plunger preload.


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