4.0 Head Conversion on Older Model 258 Block

Howdy Folks,
New to the forum so I don't know if this topics been covered- perhaps someone can point me in the right directtion. I'm wondering if the 4.0 head to 4.2 block is possible on older model 258s ('75 thru '79). The casting no. on my block is 3224098. I'm pretty well versed on the Tim Weaver's write up on the topic but it doesn't specifically address the question at hand. Same with the AMC 4.2L rebuild w/ 4.0L conversion head article from the bustedjeep website. Anyhow, any input is appreciated.

Thanks,

Tadilllac

 

They don't address it because it's not relevant. Just curious - why do you think that's important? In looking at Tim Weaver's article, he does not exclude any years of 258s. The only year-related consideration is the head bolt size, which Weaver also covers. Quote: Generally 1971-1980 and 1988-90 258/4.2L Engines have 1/2 inch Head Bolts. No changes required. 1981 through 1987 engines have 7/16" Head Bolts. Do not reuse your old head bolts. I would recommend new head bolts and have a machinist tap the block to ½ inch. Clifford Performance does sell a kit with spacers which will center 7/16" bolts in the 1/2" holes in the head if you do not want to tap your block. Hesco too IIRC.

The 258 was first introduced in 1971, and those engines were essentially unchanged through 1980. The engine was lightened in 1981, which changed some things like the valve cover and manifolds, but otherwise left the engine unchanged.

Welcome from Boston! Post up in the Intro section if you like.

 

I'm interested in this topic. Do you have a link to that write-up?

 

Clint, the link has been dead for a while. Hopefully it will come back. Send me a PM.

 

I am in the process of doing this mod. (Well kind of, I have the head sitting on my bench. I am not sure when I will get time to actually do it). I bought the head, gaskets, etc. from www.hesco.us which they modified by blocking the appropriate holes and I wil need to use head bolt spacers as I have the 7/16" head bolts.

 

Thanks for the warm welcome Timgr.
I'm just curious because out of the write-ups I've read, I haven't seen anything covering '75 thru '79 block castings (or anything earlier for that matter). Just want to be sure before I get in to deep if you know what I mean.
Though the original site's no longer available, I have found a link to weaver's write up that's current (http://www.bigredheep.com/article20.html).
Anyhow, thanks for follow ups!
Cheers,
Tad

 

I put a 91 4.O H.0. head on my 72 258.

Everything bolted up fine, and i used my 258 offenhouser 4 barrell intake. I'm sure that intake didn't flow as well as it could of, but it worked fine.

If you go for the 4.0 head swap, I'd use ARP head bolts, and don't forget to plug those coolant holes.