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Valvetrain noise and rocker arm interference after cylinder head work can be issues on the AMC design engines.  Causes can include sluggish oil flow through the hydraulic lifters, the lifters bleeding down, or possibly excess valve/lifter clearance from valvetrain wear.  At 200K miles, that's a very real possibility.  Another possibility, since you're certain it's not a lower end bearing noise, is a carbon buildup knock (not likely with an MPI engine) or a piston-to-wall clearance issue.  Piston skirt wear is likely, too.

It works like this: Pistons naturally expand from heat.  There is the normal piston-to-wall clearance to accommodate expansion when new.  Over time, the cylinder bore wear creates cylinder "taper" toward the top of the cylinder.  The piston skirts also wear.  Over time, the cylinders and pistons wear. 

Cold, the engine was noisy, the pistons expanded, and for a while, anyway, the engine quieted down when warm.  Now, the lifters are clacking, the cylinders are worn, the pistons have worn, and you get the sound effects!

Another noise can be piston pin wear, which causes a double-knock rap...You would notice this clearly as a dominant sound when the engine is unloaded and you tip the throttle in and out.  Use of an automotive stethoscope, a copper tube or a piece of PVC tubing can help isolate engine noise.  Be aware that these noises will be transmitted widely and very exaggerated while using these sounding probes!

At 200K miles, these engines have done a heroic job tugging a Jeep around.  2.5L models with 4.10:1 axle gears, which make the piston travel extreme over this many miles, wear an engine out sooner.  An engine with this kind of mileage needs a suitable burial or rebuild it completely, restoring the short block and cylinder head to OEM specifications with pushrod lengths checked (changed if necessary) for proper valve clearance/lifter preload.

Some want to swap an inline 4.0L six in place of the AMC 2.5L four, and that's not easy.   I'd consider a smaller V-8 swap (GM LS 5.3L makes sense) as a practical alternative.  The Jeep YJ and TJ Wrangler frames are, for unfathomable reasons, designed specifically for either an inline four or a six-cylinder inline engine.  (You can see photos of the motor mounts I fabricated during a 4.0L swap into a 2.5L YJ Wrangler, and frankly, it would have been just as easy to install a hybrid V-8.)  Granted, the 4.0L radiator, shroud and transmission locations were straightforward, although a four-cylinder YJ/TJ model uses an AX5 transmission, which would be replaced by a 4.0L's AX15 transmission.  Aside from fabricating motor mounts, there would be wiring, cooling, exhaust, the AX15 transmission, 4.0L PCM setup and other changes.

Moses

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  • Moses Ludel changed the title to Jeep 4.2L, 4.0L and 2.5L AMC Inline Engine Noise and Valve Adjustment
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Here are some quick resources for valve clearance issues.  The link below is a topic that addresses the issues that come up after having head work done then experiencing valve clearance or rocker arm issues:

This video is a vlog that I did years ago to explain the valve clearance issues that arise from cylinder head and block deck work:

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

Moses

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