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The timing cover on a 4.2L is different than the 4.0L.  TDC mark is many degrees away from the 4.0L damper mark, which is typically the damper used in a Mopar EFI conversion.  Use true TDC for #1 piston when referencing TDC. 

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 TDC.  This is one builder’s 4.0L/4.6L experience.  He has a 4.0L in 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.”

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.


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