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Old 10-26-2006, 10:02 AM   #1 (permalink)
 
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4.0 vs 4.2

I recently ended up hurting my motor in my 89yj 4.2L Now I can't find any 4.2's locally or at any of the junk yards. I know I can use the 4.0 head on the 4.2 block with some modifications but what do the actual blocks of the 4.0 and the 4.2 have in common? Could I swap out my 4.2 with a 4.0 longblock and just put my carb setup on it? Are the mounts in the same position? Any help would be greatly appreciated.
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Old 10-26-2006, 03:44 PM   #2 (permalink)
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I want to say yes, but you are going to need a 4.0 exhaust manifold...The block will physicaly bolt right up where the 4.2 was, but the intake is higher on the 4.0 head than the 4.2 head. Why not just do a 4.0 swap? MPI and all?

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Old 10-26-2006, 03:46 PM   #3 (permalink)
 
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Usually cheaper and easier to just swap it for a complete 4.0 The mounts should be the same. You can usually find a used 4.0 for a few hundred dollars.
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Old 10-26-2006, 08:15 PM   #4 (permalink)
 
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Theres a local that has a 4.2 complete for $300 it just seems theres no parts for these things anywhere anymore unless it's brand new. I'm thinking I'll get a 4.0 from the junk yard for $125 and throw it all in and when I get around to it pull the 4.2 apart and see what I could use for a stroker down the road.

So the 4.0 will mount up in all the same places but I can't use the 4.2 manifold/carb? I don't want to switch this thing over to efi what so ever if possible. Thanks for the replies guys.
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Old 10-27-2006, 09:44 AM   #5 (permalink)
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I don't want to switch this thing over to efi what so ever if possible.
Why on god's green earth not? The 4.0 will be no better than the 4.2 without the injection.

There are tons of writeup's on doing the swap, injection and all, it's your best bet.
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Old 10-27-2006, 12:11 PM   #6 (permalink)
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Make sure you get the HO 4.0 from 1991 to 1995. You can use the 4.2 intake with just a small modification to it. The locator dowls on the head will hit the bottom of the intake instead of the holes for the 4.2 head. You just need to grind them enough so that the intake rests on the dowls and the ports line up between the intake and the head.

Get all the FI stuff with the 4.0. If you don't inject it now you might want to down the road. Look at cjoffroad.com, they have a good section just on this subject.
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Old 10-27-2006, 09:30 PM   #7 (permalink)
 
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bullshit the 4.2 it better then the 4.0! why would you even say that? maby cuz its bigger? well not true. well now that i got that out of the way the 4.0ho head will bold right up the the 4.2 block but you will need the intake and the exaust form a 4.0 thats the only probluml. but what the hell if you got a deal on a 4.2 for 300 what are you waiting for stroke that bad boy make it a 5.0 and then slap a blower on it. add a mpi kit and you will be done. email me if you need help ive dont this may times. good luck and i hope to hear from you soon.
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Old 10-27-2006, 10:08 PM   #8 (permalink)
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Quit dancing around and get the 4.0 HO. Either build a stroker or run the 4.0 but run it FI. Get into this century. You will never look back and say " I wish I still had my BBD and all that emmissions stuff."
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Old 10-27-2006, 10:44 PM   #9 (permalink)
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I didn't say the 4.2 is better.

Exercise your critical thinking skills...I said that the 4.0 is no better without the fuel injection. It could be a 2.0 or a 7.0, but if it doesn't breathe right, it won't matter.

Do the 4.0 (because the head breathes better and it has the injection) or a 4.2 with a 4.0 head WITH the fuel injection (same reason). It isn't difficult and you'll be very pleased with the results.
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Old 10-29-2006, 03:53 PM   #10 (permalink)
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I wouldn't be afraid of the earlier 4.0. That's what I used. The only difference really is smog related. You can use an open air filter for the HO and be smog legal. You need the box for the earlier one. I use an open filter anyway. I don't have to smog in my county.

One positive thing the early ones have is the anti-knock sensor. No pinging problems here.
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