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Depending upon the year and engine source, 4.2L AMC/Jeep®/I-H version timing covers can be different than a 4.0L cover.  The TDC mark may be many degrees away from the 4.0L damper mark, which is typically the damper used in a Mopar EFI conversion.  Determine the true TDC for #1 piston when referencing TDC;  this is determined by measuring the piston's position in #1 cylinder with the spark plug removed.  The piston must stand at its peak when determining TDC. 

Note:  Inexpensive TDC tools are available at Summit, Amazon and elsewhere:  https://www.summitracing.com/parts/sum-900189 or TDC tool at Amazon.  I prefer the Amazon listed tool for its yielding contact with the piston and gauge. 

If your damper mark does not match up with #1 piston TDC, place the #1 piston at true TDC and scribe a timing mark on the damper to match the timing cover's TDC mark.  You can add MSD or similar degree tape.  For accuracy, make sure the tape matches your damper's diameter!

This is one builder’s 4.0L/4.6L experience.  He has a 4.0L long block from a ’98 TJ Wrangler and installed an OE 4.2L/258 crankshaft.  The 4.2L crankshaft keyway did not match his OE 4.0L crankshaft keyway position:

“I had stashed away parts from the 4.2 that I stole the crankshaft from to build my Stroker. Last night I dug out the timing cover and put it next to the one from my 4.0. Now I see why the notch in the harmonic balancer doesn’t line up with the tab on the 4.0 cover! The woodruff key slot on the 4.2 is cut in a different location than the key slot on the 4.0 course crank… This might be one for the forums- probably most people never even look since the timing marks won’t be used to set distributor timing but some people like me will be very confused if they do.”*

*Footnote:  The 258/4.2L timing cover and crankshaft source was an early seventies I-H 4.2L (AMC) inline six.  The I-H/AMC 4.2L crankshaft with a unique key-way offset is the holy grail 12-counterweight type.  When you're sourcing parts, know the origins of the crankshaft and the original timing cover's TDC and advance/retard tab location on the cover.

4.0L Damper with 4.2L Crankshaft.jpg

4.2L Crankshaft with 4.0L damper and timing cover.

4.2L and 4.0L Timing Covers.jpg

4.2L timing cover (top) and 4.0L timing cover (bottom).

Know which parts you have and always index the timing cover TDC mark with precise TDC of #1 piston.

Moses

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