Re: [AMC-list] 4.0 to 4.2 question

[Frank Swygert <farna@xxxxxxx>]

Mark got you on the casting number. But what will you be doing with this engine? For better low speed power (normal driving) the later model head should be a better choice. The only thing is you need a 2001 or later model. The 99-00 head has a thin spot in the center that's prone to cracking with age. I think I have that problem solved as I'm running a 2000 head on my J-10 (258 with carb) and have had no problems... yet. Been running it about a year now. 

The later head has smaller exhaust ports but has a bit better flow at low speeds due to higher velocities. It will fall off in power quicker than the 7120 head though -- peaks out at 3500 or so rpm instead of 4500 or so (with stock or near-stock cam). But below 3500 it does a bit better. So for a street vehicle the later model is better, but for total performance the 7120 is the better head. 

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Date: Thu, 18 Mar 2010 19:04:06 -0400
From: Richard Estermyer <javelinman74@xxxxxxxxxxx>

I had done reading on the 4.0 head swap and suggesting the use of the #7120 head casting of the 4.0 head circa 1990.  Does anyone know where the casting number would be located on the head?  I'm looking at a 1991 and want to make sure I am on track.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
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