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Discussion Starter · #1 ·
I wastold that it is posible to put a 4.0 head on a 4.2. IS THIS TRUE? And if so what is needed to do so? Is there any power gain or loss? Would i just be better off to just put in a 4.0? I am new to the jeep thing and am realy trying to understand. Thanks for any help /information you can give me.
 

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iamfawkinwithyou
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use to be a write up on cjoffroad, but the site is down right now. def gives more power
 

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bitter clinger
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has to be a head off a mid 90s or newer. cant remember exact year but the newer ones provide better air and exhaust flow. check out Clifford thats where I got my info from, they are good with i6s and they have lots of performance parts.
 

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on jeepforum.com I was told it would give you up to 50 horsepower...I was considering it, I am guessing you would need the head, intake manifold, computer from a 4.0, fuel pump in the tank...and probably a few other things I am forgetting, but yes it will bolt up, you need to plug a few of the water jackets in the 4.0 head iirc.... if you go to jeepforum.com and go to the yj forums and search there is a lot of discussion about it usually and you should be able to find what you need if you didn't from the link above.
 

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I am the law.
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Would i just be better off to just put in a 4.0?
Yes. Just use the entire motor. If you have a 4.0 and a 258 at your disposal, you can assemble a stroker. Google. :thumb:

People on JF don't explain things very well and have serious misconceptions about certain "popular" CJ mods, as well. The power gain they are describing is to use the entire FI system on the 258, which is only worth it if you know that your 258 has a lot of life ahead of it.
 

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Discussion Starter · #8 ·
I have a 4.2 in my 88 YJ with less than 60 thousand on it . I'm hopeing it is gonna have some good life in it yet. I am just looking for better performance and more power , and I have access to a rebuilt head and all the fuel injection stuff off a 4.0 So I was just looking for the cheapest way to do it . THANKS AGAIN FOR ALL THE ADVISE!!
 

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iamfawkinwithyou
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I have a 4.2 in my 88 YJ with less than 60 thousand on it . I'm hopeing it is gonna have some good life in it yet. I am just looking for better performance and more power , and I have access to a rebuilt head and all the fuel injection stuff off a 4.0 So I was just looking for the cheapest way to do it . THANKS AGAIN FOR ALL THE ADVISE!!
I did HEI dist, Howel FI and a Header on my 4.3 and it performs great, might be a better route for ya
 

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You can keep it carbed,use the aluminum 2 bbl manifold& mc 2100 or weber.....you do need to use the 4.0 exhaust manifold.The 258 has a longer stroke than the 4.0,so it makes more torque than the 4.0.
 
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