Home How-to Video Jeep 232, 258 and 4.0L Inline Six and 2.5L Valve Adjustment

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

by Moses Ludel

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.

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Terry March 15, 2020 - 5:26 pm

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

Moses Ludel March 16, 2020 - 6:21 am

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

Line Dean July 17, 2020 - 3:33 pm

thanks for the video and the text description above.

I have a 1975 jeep 304 v8. The lifters are hydraulic. The rockers are each stud mounted. I finger adjust/tighten the stud nut to the point where all slack between the rocker to push-rod to lifter clearance is zero. The lifters do not have oil yet.

Now then, at this state, how much more should I turn the rocker stud to achieve the proper push-rod to lifter preload depth?
one full turn? 3/4 turn ? 1/2 turn ?
is it a torque specification ? (would be very light torque ?)

I am unable to find this information anywhere. Conversations become confused because not all 304 v8 engines have the same rocker arm configuration. Some tighten down rocker retainer all the way and the preload is determined by the length of the push-rod.

thanks much for your help,

Moses Ludel July 17, 2020 - 8:18 pm

Line Dean…The rocker arms each have a retaining nut. The procedure for a stock engine is to torque this rocker stud nut to 20-25 lb-ft. In stock form, the additional nut rotation from zero clearance (lifter plunger extended as you describe) will set the correct lifter preload. The nut must be seated/torqued, or it will otherwise loosen in service.

Since you have watched my commentary in the video, I draw attention to those factors that will make this method not work: 1) changes in block deck or cylinder deck from machining, 2) mixing pushrods or installing new pushrods of incorrect length, 3) a major change in head gasket thickness (very rare) and 4) valve seat height or valve head/face changes when installing replacement valve seats or valve grinding.

If a machinist matches the valve’s original valve seat position and the valves are new, there is generally no issue. Recall, there was a traditional way to restore AMC (6 or V-8) valve stem height when valve refacing and seat grinding: a bridge gauge determined how much to grind from the valve stem tip to adjust the valve stem’s height in relationship to the cylinder head topside.

Since you need to torque the nut to specification at a fixed position, you’re right to be concerned about the lifter clearance/preload. I can suggest three methods for setting preload. First, you can use a CompCams adjustable pushrod length measuring tool to select the right pushrod length. (This is very accurate because it allows for any head or block deck, valve seat or valve face concerns.) With the lifter resting on the heel of the camshaft lobe with the lifter plunger fully extended, take a measurement between the seated rocker arm and the extended lifter plunger. Compare the tool’s measurement with your pushrod length. The difference between these two lengths is the amount of lifter preload.

Secondly, you can use the CompCam’s tool and measure from the the rocker to the extended plunger length. Add 0.020″-0.060″ (I prefer 0.030″-0.040″ range) to the tool’s measurement. This takes into account the preload position for the lifter plunger and the additional pushrod length that you want for the right lifter preload. Compare the tool’s length to your pushrod. You can buy the Melling pushrod(s) that most closely set the correct lifter preload.

Lastly, and simplest, is the Chevrolet small-block V-8 approach that you hint about…The threads on a Chevy small-block V-8 rocker stud are 3/8″-24. Factory valve setting on the original small-block Chevy with hydraulic lifters was zero clearance (as you describe for your current measuring point) plus an additional 3/4-turn. Chevrolet simplified the entire process by calculating that by rotating the adjuster nut 3/4-turn, the lowered rocker arm would create the right amount of lifter preload. The thread size and pitch determine the amount that the rocker arm will drop.

Note: Chevrolet used full thread studs and self-locking rocker nuts to eliminate issues around tolerances, machining, valve stem heights and the rest. AMC rocker nuts are not self-locking and need to seat securely at the rocker stud’s thread stop. Chevrolet rockers were rechecked with the engine running and warmed. The rocker nut was backed off to audible clearance; lash was closely adjusted to the zero point to eliminate noise or perceptible rocker-to-valve stem gap; then the nut was turned slowly, a quarter turn tighter at a time, until 3/4-turn was reached.

In your case, you must seat the nut (20-25 ft-lbs) to secure it; that is how the rocker stud and nut mate up. Your stud threads should be 3/8″-24 and lifter preload should be similar to a small-block Chevy V-8. The rocker arm ratio is 1.6:1 on your AMC engine, the Chevy small-block was 1.52:1. This will not impact a static (stationary engine) preload adjustment on the heel of the cam lobe. Seat the rocker nut to proper torque while simultaneously keeping track of how much the nut turns past the lifter zero clearance point. 1/2- to 3/4-turn range would be acceptable lifter preload.

Again, use a torque wrench to rotate the nut and carefully measure the degree of rotation. The 20-25 lb-ft torque must be met at the same point the nut has rotated 1/2 to 3/4 turn beyond zero lifter clearance. If the degree of rotation is approximately 1/2- to 3/4-turn, and the nut torque is met there, you should be good. There is latitude here; the plungers have some range of travel.

You can get pushrods in various lengths from Melling or a custom pushrod source. Summit Racing carries the CompCams gauge. Here is more information at the forums that applies to pushrod length adjustments on the AMC inline sixes, the principles are similar: 4.2L valve adjustment tools. Also try the search word “compcams” at the forums.

You have some latitude on pushrod and lifter adjustment. As you note, this should all be done before the lifters get charged with oil. I use a pressure tank or oil pump drive tool to prime the engine after all lifters are adjusted and the pushrods with rocker arms have been installed.


John Keegan March 1, 2021 - 11:49 am

Fantastic Information!!! I have a 1987 Wrangler with the 4.2L that I installed a 4.0L head on and the Mopar MPI kit. I installed the stock 9.700″ pushrods and rocker arms. At TDC the valve springs were pushed in about 3/16″ from what I could measure so I purchased the pushrod measuring tool from CompCam. At TDC (the closest I can visually set using the case cover and damper marker anyway!) the pushrod checker length when taking all slack out of the rocker arm was 9.5085 which seems really far off. I checked both the exhaust and intake pushrod length using the CompCams tool and they were within .002″. Do I just add .02-06″ and get the closest one available in that range? I’ve seen on the Melling site that the Ball Ends are offered in different diameters .312 and .313, is that .001″ enough to effect seating in the lifter or rocker arm?


Moses Ludel March 1, 2021 - 2:10 pm

Hi, John…I’m a strong advocate of the CompCams tool, and we have walked through the pushrod check and selection process several times at the magazine’s forums. Here is the link to the search returns for the CompCams pushrod length tool. You can see where others with the 4.2L and 4.0L AMC/Jeep inline sixes have gone with their valve checks:

4WD Mechanix Magazine Forums search for CompCams pushrod tool

Review how we have dealt with the discrepancies. You take the measurement with the piston at precise TDC with both valves closed at the top of the compression stroke. Make sure you are not at the top of the exhaust stroke. The tool measurement should be with the lifter plunger fully extended. You measure the length/distance between the rocker arm socket and lifter plunger. The rocker should just seat on the valve stem tip, all play removed; the lifter plunger remains fully extended.

Once you have this length measurement, you are correct, just add the desired lifter preload to the tool’s length. Ideally, I like 0.040″, the middle range. You may not have this luxury, as you note. Make sure you pick a new pushrod length that lands within the 0.020″-0.060″ range. In rare instances, an adjustable or custom pushrod is required to meet this preload range.

Again, the key concerns are: 1) having the piston at precise TDC on the compression stroke and 2) making sure the rocker slack has been removed without compressing the lifter plunger. This will be the pushrod measurement baseline. Add the 0.020″-0.060″ for preload, depending upon the available pushrod lengths. Order the right pushrods.

Regarding the 0.001″ ball end size difference, this is immaterial. The reference likely has to do with tolerances in manufacture. The radius difference would be easily compensated and should not affect the lifter clearance or rocker performance enough to matter.

Visit us at the forums, and review our coverage under the “CompCams” tool…




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