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Unread 05-08-2013, 02:00 PM   #1
pbedens
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258/ 4.0 Head Conversion

I currently have an 85' CJ7 w/ a 258 and I also have a 258 long block in the garage from I believe was a 74'. I was wanting to do the 4.0 head conversion and was wondering which block would best suit the head. I've read quite a few threads that say the head bolts on the 258 changed somewhere between my 2 block years. The 74' is a remanned motor and would be easier labor wise, just not sure if the block has the right size head bolt holes for the 4.0 head. Would it be best to go with the 85' or the 74' block for the conversion?

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Unread 05-08-2013, 02:09 PM   #2
Colt44
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I've done the conversion on my '83 and had no significant problems. One thing is that the 4.0 valve cover needs that extra stud, which I think is at the back, but my Jeep had been upgraded to an aluminum Clifford cover so I didn't have to do that.
The other thing was sealing up the coolant passages in the 4.0 head, which I did with a couple of cornstarch packing peanuts and some JB Weld. I've since heard that's optional, but I did it anyway.

You don't say, but I assume you're going to convert over to fuel injection? If so, one other thing is to make sure you get a cylinder head with the right casting number.
Some had much better intake angle, which helps airflow quite a bit. I think the magic number is 2170, but others will have the correct number(s).

There are several very good how-to threads around here, one in particular has a great deal of very detailed technical info.
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Unread 05-08-2013, 02:37 PM   #3
Dngrs1
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Casting number should be #7120
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Unread 05-08-2013, 02:40 PM   #4
bkeese
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I don't think it really matters which you use. Use the best one. Older 258 used 7/16" head bolts (I believe) and newer ones used 1/2". You can tap out the holes or buy collars to make up the difference. Your biggest concerns are the intake and exhaust manifolds. You must replace the exhaust manifold and you'll want to replace the intake, or why would you want a 4.0 head. I used the clifford and Offenhauser manifolds with the Holley 470 Truck Advenger. I would recommend an RV cam while you are at it.
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Unread 05-08-2013, 03:14 PM   #5
Colt44
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Quote:
Originally Posted by Dngrs1 View Post
Casting number should be #7120
I had the right numbers, just in the wrong order!
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Unread 05-08-2013, 06:14 PM   #6
pbedens
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Thanks, guys. I'm trying to get all of the info together to do this swap when I get home. Yes, I am going to do the 4.0 fuel injection as well. I wanted to put FI on it to begin with and figured why not get a little more power out of it while I'm at it and do the 4.0 head swap and fuel injection instead of just the Howell FI setup.

Colt 44, if you don't mind me asking, how much did the conversion set you back overall? And did you go with a new head or a redone junkyard head? I was thinking of doing a junkyard setup and getting the head redone to keep cost down. Was wiring the thing up pretty straightforward?
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Unread 05-08-2013, 06:48 PM   #7
Eddie800
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Quote:
Originally Posted by Colt44 View Post
The other thing was sealing up the coolant passages in the 4.0 head, which I did with a couple of cornstarch packing peanuts and some JB Weld. I've since heard that's optional, but I did it anyway.
Interesting, where did you see that? I guess the gasket could hold the coolant in.
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Unread 05-08-2013, 07:47 PM   #8
pbedens
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Another thing, would anyone have a parts list for this conversion? I've found a lot of info on it, but it seems like people are pulling intakes from one year, valve covers from another, and heads from a different year. If I find a good candidate with a 7120 head, could I just pull all the necessary parts from one vehicle?
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Unread 05-08-2013, 07:51 PM   #9
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Quote:
Originally Posted by Eddie800

Interesting, where did you see that? I guess the gasket could hold the coolant in.
Some blocks leave the ports open to the outside. Mine didn't but I chose the jb weld trick just for extra insurance. I cheated and used the hesco mopar fi kit, and made the swap sooo much easier.
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Unread 05-09-2013, 08:09 AM   #10
gosupes
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The blocks from 81 to 85 had the 7/16" head bolts. the rest should have 1/2". Some of the older blocks have the monster 12 counterweight crank, not sure of what years tho. The 7120 and 0630 heads are identical with the exception that the 7120 has the threaded port for the temp sender in the back. You don't want to take a chance and not seal the water jackets on the head. Sealing them off is cheap insurance. The last thing you need is to be in the middle of nowhere and have it leak.
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