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How to Assure a Properly Machined and Rebuilt Jeep 4.2L Engine


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I am having my 1985 4.2 completely rebuilt.  It is difficult to know which builder will do a great job due to my inexperience.  What are the most important steps in the process of a first class rebuild?  Any pertinent questions I should ask? How important is use of a torque plate in the honing process?  The build is not a performance rebuild (Weber 38, stock intake and exhaust), but for a rock solid daily driver. 

 

Thanks,

 

Reid

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This is a very good question, Reid!  I have been exposed to the engine rebuilding and "reman" industry for a half-century.  Here's what I expect from a rebuilder at the local machine shop level.  Think "remanufacturing":

 

1)  Disassembly of the engine with removal of freeze plugs, oil galley plugs and camshaft bearings.  Disassembly of the cylinder head and valve mechanisms.

 

2)  Hot tanking of all castings and reusable parts.  Bore brushing of oil arteries and galleys.

 

3)  Block cylinder boring, crankshaft main bearing saddle line boring, connecting rod aligning, fitting pistons and new pins, machine honing the bores to precisely match piston clearances.

 

4) Completely rebuilding the cylinder head, including 3-angle (typically) valve seat cutting, new stainless steel exhaust valves, refaced or replaced intake valves, rocker arm tip machining if advised, new valve guide silicon-bronze inserts and guide fitting, new guide seals, new valve springs if advised, new valve locks.

 

5) Head should be surfaced, block decked, too, minimal amounts to flatten and restore surfaces.  Torque plating is essential for accurate block boring.  Confirm or restore valve lifter "clearances" (plunger depths) after surfacing the head and block deck.

 

6) New camshaft bearings and freeze plugs (brass or steel work).  Typically new oil galley plugs get installed with thread sealant.  New high volume (Melling, typically) oil pump with new screen.

 

7)  I like moly rings with United Engine Silv-O-Lite hypereutectic pistons.  Forged pistons are unnecessary for your plans.

 

8)  New camshaft and lifters, my favorite grind for overall street/trail use is the CompCams 252 grind and a 4500-5000 rpm ceiling.  (I never spin these engines past 4,000 rpm.  That's enough.)  Make sure pushrods are in good condition or replace.  Pushrod length determines hydraulic lifter clearance on mid-'70s up AMC/Jeep inline sixes.

 

8)  King or Clevite crankshaft bearings.  Crank should be cleaned, machined and polished, then balanced with rods and other reciprocating parts.  Balancing does pay off.

 

See the magazine's video series where Tony Hewes and I discuss a 4.6L build.  (Simply type "Tony Hewes" without quotes into the search bar at www.4WDmechanix.com.)  I'm not saying you must do this build, but now is the time to consider.  You would need the 4.0L block and head cores.  You have the rest.  (There's the rod option of 4.2L or 4.0L.)  The result would be satisfying, near V-8 level power with great bottom end torque.

 

Moses

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Moses,

 

I so appreciate such a detailed response.  It is exactly what I needed.  I have found a shop with a reputation of meticulous rebuilds of Jeep and AMC engines and they use a torque plate when boring cylinders.  I found another shop with excellent credentials and they don't have a Jeep 258 torque plate and as expected downplay it as an important part of the boring process.  The remainder of your list will allow me to know if I am getting a top notch rebuild.  The shop with AMC Jeep experiance and the torque plate, are seriously more expensive, but I think it will be money well spent.

 

Thank you for the tip on the video, I will give it a look.

 

Thank you again for taking the time to answer my question so fully. I'll keep you posted on the outcome.

 

Reid

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You're welcome, Reid...Blocks were bored with reasonable success in the pre-torque plate era.  However, an inline six has a long deck and really should be torque plated.  This eliminates any risk of cylinder/piston/rod alignment, and like balancing, a torque plate bore job provides insurance that your engine will last a very long time.

 

Keep us posted!

 

Moses

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  • 2 weeks later...

Hi Moses,

 

A slight twist on the issue of honing plates for my Jeep 4.2L rebuild has come up.  I was told by the engine builder that honing plates are indeed part of

the machine work. He volunteered the machine shop uses a 4.0L plate.  I guess they don't want to buy two.  My presumption is that since 4.0L heads can be installed on 4.2L blocks this may be OK as head bolt stresses would be similar if not the same.  

 

Your opinion would be greatly appreciated. 

 

Reid

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This should work, Reid.  The bore centers of a 4.2L and 4.0L are the same.  The 4.0L has a larger bore than the 4.2L, so there's plenty of space for the boring bar.  As you note, the bolt patterns are the same for head/block bolts. 

 

I would trust the shop's approach here.  The whole idea of using a torque plate is to prevent cylinder distortion during boring, to hold bores true when the head is torqued in place, all this while maintaining rigid bore centers.  The boring bar will cut perpendicular to the crankshaft centerline.  (Typically, the block/main caps are line-bored to assure a true crankshaft centerline.)   The plate will simulate the torque applied with the head and head bolts torqued in place...

 

Moses

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Thank you Moses!  I learn more than I ask from each of your replies.  Its time I got the engine out of the Jeep and over to the builder ASAP.  The hood, fenders, and grille are off. I have no excuses now.

 

Thank you again,

 

Reid

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