Jeep 232, 258 and 4.0L Inline Six-Cylinder and 2.5L Valvetrain Noise and Valve Adjustment

[Moses Ludel]

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.

Moses

[philliproos]

Good day, Moses

My son has a very interesting scenario on his XJ 2000 model, 4.0.

He had a head gasket blown, got the head resurfaces, valves seating done and fitted it back with a new head gasket.

The engine runs fine, but one of the rockers hits the tappet cover, both side of the rocker hits the cover...

We checked everything but can't determine why this is happening, any advise?

Thanks

Phillip - South Africa

[Moses Ludel]

Hello, Phillip...Good day to you as well...This is interesting.  Since there is no mention of the camshaft and lifters, I assume the camshaft is "stock" and not an aftermarket grind with high valve lift.

When the head was resurfaced, that lowered the "deck height" of the cylinder head.  The head now sets lower and that affects the pushrods and rocker arms.  Adding to this, when the valve seats were ground into the cylinder head, unless the stem tips were also ground to compensate, the static height of the valve stems increased.  Simply put, the valve stems stand higher at the rocker arm tip.

After the cylinder head work, you did not check the lifter preload or clearance.  The preload increases when the cylinder head sets lower due to head surfacing, a thinner head gasket, etc.   Raising the valve stem heights from the seat grinding raises the rocker tip height at the valve stem end of the rocker arms.  So, with the cylinder head essentially lowered, and the valve stem heights essentially raised, the rocker arms would stand higher on both the pushrod side and on the valve stem side.  The issue is twofold:  1)  The tall height on the valve stem side of the rocker is the result of valve stems that are now too long and 2)  the lowering of the cylinder head height made the original pushrod lengths too long at the lifter/camshaft side of the rocker arms.

Since the engine runs okay, apparently the lifters have enough plunger movement/range to accept this higher than normal preload.  (Regardless, the plungers are riding too low in the lifters.)  If you check the lifter preloads or clearances, they will be excessive.  You can remedy this with the use of shorter pushrods.  Below is a link to one of our thorough discussions on how to measure for the required pushrod lengths with a CompCams pushrod gauge.  You select the right length pushrod for each valve position.  The procedure described for the 4.2L six is similar to the 4.0L in your son's 2000 XJ Cherokee.  I would not correct the pushrod lengths until the valve stem heights are corrected.  If you decide not to correct the valve stem lengths (i.e. too much work to remove and rework the head or whatever reason), you can correct the pushrod lengths now and deal with the valve cover issue:

Once you determine the correct pushrod length(s) required, the rocker arms will ride lower on the camshaft/lifter side and not hit the rocker cover.  However, the rocker arms will still be high on the valve stem side, as this is governed by the height of the valve stems.  In my video (above), I talk about the "bridge" gauge tool that was once popular for AMC six cylinder head work.  The aim of the gauge was to measure the standing height of the valve stem with the valve closed...Visualize looking at the cylinder head from the side.  Consider where the valve stem tip stood before the valve seats were ground.  If the original seats were ground, the valve stem height would be higher.

After the valve seats were ground, if the original valves' faces were also ground, that further raised the height of the valves at the rocker tips.  To restore the original valve stem heights would require cutting the stem tips to shorten the valve stem lengths.   Traditional valve grinding equipment has a provision for grinding the valve stem tip ends squarely.  The aim is to restore the height of the valve stems to their original standing height.  The lifter clearance (i.e. pushrod length) would be impacted in doing so.  In any case, checking and restoring the pushrod lengths with a CompCams (or equivalent) gauge is essential for establishing the correct lifter preload following cylinder head work.

The correct pushrod lengths should stop the rocker interference on the pushrod side of the rocker arms.  You will have to determine whether the valve stem heights are still an issue on the valve stem side of the rocker arms.  To grind/shorten stems would require removal of the cylinder head and disassembly.  If the interference is slight, at the rocker/valve stem side, you might "dimple" the valve cover to provide clearance or find a taller valve cover in the aftermarket (Clifford, Offenhauser, etc.) that is designed for a high performance camshaft with more valve lift.

As a footnote, when there is this kind of stem height, the valve springs require shimming to provide adequate spring tension rates.  Visualize that the valve stem keepers now set higher than stock.  This means that the valve springs do not compress sufficiently, which reduces valve spring pressures.  This can cause valve "float", usually at higher rpm, the result of the valve spring tension being weak.  Weak spring tension will not seat the valve(s) tightly.

There are two issues involved here:  pushrod length and valve stem height.  This should clarify why your rocker arms are hitting the valve cover on the pushrod and valve stem side.

Moses

[philliproos]

Moses,

Thank you so much for your absolutely fantastic explanation.

This is amazing information and I know exactly what to do to correct this.

Thank you so much once again.

Nice to have real experts on my side.

Have a nice day.

Regards

Phillip

[Moses Ludel]

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 clearance.  The 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.

Moses

[Moses Ludel]

philliproos...Thanks for your comments about the information.  I'm glad the details provided some direction.  Your questions prompted me to tighten up the discussion at this topic and focus on the issue you encountered.  Your 4.0L engine highlights the effects of cylinder head work (valve grinding and resurfacing) and block decking.

Let us know how far you take this rocker clearance issue.  If you stop at getting a taller valve cover, check the pushrod lengths, anyway.  This can be done with the cylinder head in place.  I discuss the procedure with "Stuart_Snow" (1977 CJ 258 engine in the forum topic link provided above). 

The CompCams tool is available at Summit Racing and Amazon.  I'm not clear about its distribution at South Africa.  Here is the Amazon listing for the 8.800"-9.800" length tool that works for your 4.0L Jeep pushrod length range:

https://www.amazon.com/Competition-Cams-Hi-Tech-Checking-Pushrod/dp/B00062YER2

Typical OEM production Mopar "stock" pushrod for a 2000 4.0L engine is Mopar Replacement 33002986AB (various listing lengths show 9.640" at Summit Racing to 9.659" or longer from other sources).  This is a generic pushrod that would likely be too long for your reworked cylinder head in its present configuration. 

When you use the gauge, there are aftermarket pushrods of various lengths available from Melling and other sources.  Given the work that was performed on your son's XJ cylinder head, I would measure each pushrod length carefully and order the necessary lengths by each valve position.  I like a lifter preload range of 0.020" to 0.040" (optimal), measured with the camshaft lobe on its heel (lowest point).

Moses