4.2 to 40 head conversion - JeepForum.com
LinkBack Thread Tools
post #1 of 4 Old 06-27-2003, 06:38 PM Thread Starter
Registered User
gumby's Avatar
Join Date: Nov 2002
Location: I am at your girlfriends house
Posts: 463
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

"Two roads diverged in a wood and I - I took the one less traveled by, and that has made all the difference."-
Robert Frost
gumby is offline  
Sponsored Links
post #2 of 4 Old 06-27-2003, 11:19 PM
Registered User
SamFromCO's Avatar
Join Date: Mar 2002
Location: Aurora, CO
Posts: 404
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.
SamFromCO is offline  
post #3 of 4 Old 06-28-2003, 07:14 AM
Registered User
Join Date: Jan 2003
Location: Sunbury, PA
Posts: 13
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.
Scramblin_Man is offline  
post #4 of 4 Old 07-17-2003, 08:52 AM
Registered User
MudLover's Avatar
Join Date: Nov 2002
Location: Loveland, Ohio
Posts: 395
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
"Life is an issue of mind over matter. Ya see, I don't mind, and well...you don't matter."
Red Jeep Club Member #2
MudLover is offline  

Quick Reply

Register Now

In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

Similar Threads
Thread Thread Starter Jeep Forums Replies Last Post
WTF 4.0 head conversion prob!!! whit450 Jeep CJ Forum 23 01-31-2016 08:49 AM
258 Cylinder head conversion? RipLip Jeep CJ Forum 3 06-12-2009 06:29 PM
Ebay 4.0 conversion head jeepmor Jeep CJ Forum 7 08-28-2008 05:06 AM
258 4.0 head conversion 5artist5 Jeep CJ Forum 3 11-03-2006 02:30 PM
258 rebuild w/ 4.0 head conversion cduemig Jeep CJ Forum 8 08-13-2006 08:04 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome