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post #1 of (permalink) Old 12-16-2000, 02:24 PM
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4.0head on 4.2 swap

I realy like the 4.0 head swap and I want to do it soon to my 4.2 (88 YJ). Just a Question about the heads. I have a line on a 91 Cherokee head. Is there a difference between the H.O. head and the normal 4.0 and would the 91 be an H.O.? Also, do I need to change the exaust manifold aswell or is the 4.0 header just an added bonus? My head is in rough shape and the gasket is gone so I want to do this soon. A help is greatly appriciated. Thanx.

'88 Sahara
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post #2 of (permalink) Old 12-16-2000, 07:28 PM
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Re: 4.0head on 4.2 swap


The head casting number is 7120. It is on the driver side top. You can see it even with the valve cover on. If this what you are looking at, it is the right head.

As far as exhaust goes, you will need to use an exhaust header/manifold for a 4.0 head. All of the stock ones are tube type headers and there are aftermarket ones as well. The 3 into 2 into 1 stock header is prone to cracking where all the two sides go into the final pipe.

Go to http://www.monsterslayer.com/jeep for a complete write-up on the 4.0 head swap. That page is Jaffer's who is on this board all the time.

Your valve cover should work on the 4.0 head. Earlier 258's need a new valve cover too (I got mine from a junk yard for $25).

The whole cost if you buy a head from a junkyard will be around $500. This includes the head, exhaust, machining, gasket, bolts, etc. You can get brand new heads from Hesco for around $1,100. You would still need bolts, gasket, exhaust going this route so more like $1,400.

258 4.0 Head/MPI,NV4500,D300,4.10's,HS9500
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