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Im looking at building a motor for my 83 Cj7

I have options, I have a 96 4.0 parts motor with 0630 head plus a spare 7180 head. I have two 258s one is a 1990 (006CL25 / EF3235444) the other an 80 (009C09 / 3235444).

The 1990 258 i could just re-ring and drop a 4.0 head on, i wouldn't even have to plug the water jacket holes in the head as the 90 block is squared of on that side like a 4.0 block, re cam with a RV unit and i'm hauling like never before. Pretty strait forward.

Or; Take the crank and rods from the 1980 block and put them in the 4.0 block. With this one i would keep the 4.0 pistons, install new rings and because the 1980 crank is the last of the 8 counter weight heavy cranks i would assemble it and assume im pretty close on balance and see what it runs like.

Im thinking the second one is a bit of a gamble on the balancing but i've put lighter pistons in small block chevs before and never had to rebalance, so ive been there done that.

Anybody out there done a heavy crank to 4.0 swap with all stock stuff? think it could work?

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GoatBoy4570...I'd like to see others' comments here...My opinion leans toward the 4.6L since you have the parts.  The stock 4.0L pistons with a 258 crank and rods may create a piston deck height issue, depending upon pin location and crown-to-pin center line spacing.  What's your expectation here?  I'm intrigued about the 1990 block being tolerant of a 4.0L "bolt-on" head approach.  Was 1990 the only year this was the case? 

If the pistons are "in the hole" a minor amount, this can be resolved by decking the block then making sure the lifter clearances (plunger heights) are correct, using the right length pushrods or correcting valve stem heights.  Not complicated, either way, and you do wind up with more displacement at a minimal expense.  I would target 8.5:1 to 8.7:1 compression for use of any unleaded fuel grade.  4.6L at this compression ratio with an RV cam would be a real stump puller.

Of course, you do have the Holy Grail crankshaft, and that sweetens the whole idea of a 4.6L or a very durable 4.2L.  Are you planning on EFI?  If so, either displacement with MPI would be a strong performer.  A 258 with a 4.0L head, aftermarket intake manifold and carburetion is also okay, just not as versatile, and certainly not the performance expected of MPI.  Even TBI is not the same as MPI, as the long plenum runners to the outer cylinders of an inline six create fuel delivery imbalance.  In ranking order, the best induction is MPI, next is TBI and then carburetion.  Each will work, depending upon parts availability and budget.

 As for balancing, I would talk to the machine shop.  If the pistons and rods are weight matched, and especially if you use the 258 rods that fit with that 258 crank, I doubt you would have any  issue.  (Presumably, you're also using the 258 flywheel and damper.)  Reciprocating parts that are more critical to balance would be the crank, damper and flywheel, toss in the clutch assembly for a fully "balanced" package.  

Note:  Keep in perspective that an inline six is "inherently balanced" at most harmonics, superior to other engine designs.  (This is why I prefer the Cummins ISB diesel to a Duramax, EcoDiesel, Powerstroke and other V-8 and V-6 designs.  May not be race bred, but the ISB is capable of pulling a loaded race trailer for 500K miles or more!)  Given the Jeep inline six's rpm ceiling with an RV cam (4800-5000 rpm would be the typical limit, 4,500 rpm would be a practical "redline"), OE/factory balancing of matched parts is often enough.

There's a distinction between weight matching and balancing reciprocal parts.  Piston weight differences between 258 and 4.0L types should be a negligible factor.  The machine shop can confirm this.

Others with additional comments? Jump into the discussion!


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My understanding (what ive read around the net) is 88-90 blocks are cast similar to 4.0, my block for people who what to keep an eye out for one can easily be spotted as it has 5 freeze plugs like a 4.0 not 3 as with pre 88 4.2s.

My understanding, which as i keep investigating is probably wrong was that the 4.0 and 4.2 have the same compression height, the difference in stroke is made up by the rod. I was hoping the 242/258 were similar to the Chev 350/400 in that the engines used the same compression height but just shortened the rod to accommodate stroke change. What ive read lately about the 258/242 is they changed deck height in different years so that complicates things more. The Chevy is easy compared to this motor as there is a ton of info where the AMC is bits and pieces here and there not always correct.

I think im gonna do the 1990 258 head swap and the 4.6 stroker as a more long term thing, swapping the motors out later could be done over a weekend.

For the intake im using the 2000-up efi intake and im going to weld or epoxy a 2 barrel carb adaptor to it.

iLL know more about the 242/258 parts interchange when i pull the motors apart and start measuring parts.

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GoatBoy4570...Keep us posted on your intake manifold approach.  Should be interesting!  

Have you considered a Clifford or Offy manifold with a 2-barrel adapter/reducer for the 4-barrel (square flange) mount?  If you're considering a Holley 2-barrel carburetor (2300 series), the OEM carburetor from a 266 I-H Scout/Pickup V-8 works well.

Shy away from the "universal" 2300 performance carburetors.  Consider an OEM carburetor from an engine with displacement close to what you are building.



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