4.2 to 40 head conversion - JeepForum.com
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Unread 06-27-2003, 05:38 PM   #1
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4.2 to 40 head conversion

I just read about using a 4.0 head on a 4.2 engine (258) does it work or is it myth? it sounds like a cheep and easy way to find some horsepower

1984 CJ-7 offroad special loves to get dirty and play rough. unlike the girls I meet
258 rebuilt with torquer cam/ weber 32/36 carb, HEI ignition ,headers 4 in lift AMC 20 detroit rear locked and trussed / tru trax front 4:56 Gears ,solid axles on 35's

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Unread 06-27-2003, 10:19 PM   #2
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The 4.0 head has a better port design than the 258. That's the main difference. I'm not sure if it changes the compression or not. You will probably have to plug some of the water passages and you have to use the 4.0 exhaust manifold.
'86 CJ7, 258 w/Mopar MPI, 4.56 gears, rear Detroit/front Truetrac, full cage, OBA, 8274, 4" lift via Alcans/Rancho, 35's on beadlocks, other stuff. Built for rocks, mud is a four letter word.
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Unread 06-28-2003, 06:14 AM   #3
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I have done the swap to my 90 wrangler with the 258 and I am pleased with the performance gains. You can really tell a difference in the power. Plus, my MPG went from 12 to almost 16. It isnt as cheap as you think. I have almost $600 into my project. Also, there is some other fab work you must do to make eveything come together. All in all, its a good project to do to your 258.
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Unread 07-17-2003, 07:52 AM   #4
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I'm getting ready to do this swap. What all is involved with the exhaust? I know that I have to use the manifold for a 4.0 which I was gonna do by buying a header that would go on a 4.0 in a Wrangler. Is there a direct bolt on exhaust system? I guess what I mean is, can I use the 4.0 header then use an exhaust system designed for a Wrangler and have it "cooperate" with my frame and the stuff the exhaust pipes attach to? I hope I didn't confuse anyone. Simply put, what kind of things make the exhaust easy to install?

1979 CJ5 project
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