4.2 vs 4.0

[Gsmithl10]

I'm in the process of rebuilding my AMC 258 i6 / 4.2l . Block and head are at machine shop now. I can get the AMC 258 i6 / 4.2l build similiry to the 4.0, meaning springs valves etc as buying the H.O. conversion from odessa. Is the 4.0 that much better? Has anyone built a AMC 258 i6 / 4.2l ? I plan on keeping the carb. for now and have heard the 4.0 is more for FI.

 

[coldair]

I built a AMC 258 i6 / 4.2l i have the thread on here abut it with all my info. BTW I love my little AMC 258 i6 / 4.2l ... with that being said i beleave you can build a AMC 258 i6 / 4.2l to be as powerful as a mildly built 4.0

 

[Gsmithl10]

I built a AMC 258 i6 / 4.2l i have the thread on here abut it with all my info. BTW I love my little AMC 258 i6 / 4.2l ... with that being said i beleave you can build a AMC 258 i6 / 4.2l to be as powerful as a mildly built 4.0

Yea I've been keeping g up with it. My concept for building mine has been very similar to yours. I want to run the comp 239 cam or 235. with a carb. Originally I wanted the 4.0 head but now I'm kinda waffling. And haven't found jack on here about building a AMC 258 i6 / 4.2l .....Guess that should be a sign.... just figured I'd see what input is out there?

 

[Gsmithl10]

Just to clarify.... I'm talking about the heads. I'm keeping the AMC 258 i6 / 4.2l block just not sure what head to run.

 

[thiswontbetooexpensive]

I went with just rebuilding the stock head. Can't say for how it runs yet, but time will tell on that. Her eis the link to my thread on this topic of rebuilding the AMC 258 i6 / 4.2l I6.

http://www.jeep-cj.com/forums/f2/what-do-you-think-16973/

Hope it works well for all of us!

 

[coldair]

Just to clarify.... I'm talking about the heads. I'm keeping the AMC 258 i6 / 4.2l block just not sure what head to run.

Here are the casting numbers for the 4.0 heads. If you go this route use a 7120 or the 0630 head. Stay away from the early non HO head. Also stay away from the 0331 head they had smaller exhaust valves and are very prone to cracking.
Year............Casting No
1987-90......2686
1991-95......7120
1996-98......0630
1999-04......0331

 

[Jimbos76cj]

Running a HO head will up your compression. Add a bigger cam and a Mopar thin head gasket and you will be in the 10-10.5-1 compression ratio. Im sitting on a AMC 258 i6 / 4.2l that is bored .060 over, has a big cam, 7120 head and thin gasket. The PO of the motor says hes had it dyno'd at over 300hp.. Not sayingthats gospel but Ive put that info into quite a few calculators and they show the same results.. Im waiting to find something to put it in.

 

[Gsmithl10]

So you don't have it in a vehicle??? Have you run a set up like this before???

 

[Jimbos76cj]

No I have not run it. It was actually a buddies and he ran it for 3 yrs. when the machinist put it together he put the AMC 258 i6 / 4.2l valve seals on the 4.0 head and it burnt some oil because of it. He got a great deal on a 4.7 stroker so he swapped the motor's. with the HEI ignition, offy 4bl intake and Holley carb it was a monster.. I was going to put it in the jeep I'm building now but didn't want to spend the cash on a new intake for it. It will go I to my next jeep.

 

[1986cj7]

I'm in the process of building my AMC 258 i6 / 4.2l . I have one question. I have a 4.0L head that I bought but will be putting Chevy valve in it. My question is since both the 4.0l head and AMC 258 i6 / 4.2l head would have to be machined to install the Chevy valves would it make sense to just use my AMC 258 i6 / 4.2l head and have them port and polish the runners to get the flow of a 4.0l? And possibly smooth out the combustion camber?

 

[Jimbos76cj]

What else are you don g to your motor? I'm by no means a high up jeep motor guy but seems that unless your putting in some big cam that the valves and port and polish would be a waste of money for the return. I e talked to a few builders about porting and polishing heads in a lot of other applications and hey always seem go with spend the money on some better internal parts..

I've never heard of putting Chevy valves in a jeep head.. I'd assume they are bigger that's why they would be swapped..

 

[1986cj7]

What else are you don g to your motor? I'm by no means a high up jeep motor guy but seems that unless your putting in some big cam that the valves and port and polish would be a waste of money for the return. I e talked to a few builders about porting and polishing heads in a lot of other applications and hey always seem go with spend the money on some better internal parts..

I've never heard of putting Chevy valves in a jeep head.. I'd assume they are bigger that's why they would be swapped..

I'm putting a clifford cam 97-2021k Chevy 1.94 intake and 1.50 exhaust valves. Boring .030 over. and a weber 38 carb.

 

[Gsmithl10]

with the HEI ignition, offy 4bl intake and Holley carb it was a monster.. I was going to put it in the jeep I'm building now but didn't want to spend the cash on a new intake for it. It will go I to my next jeep.[/QUOTE]

You put an offy on a 4.0 head? Didn't think you could do that. How do u make a 4.7 stoker?

 

[Gsmithl10]

Has no one built a AMC 258 i6 / 4.2l head???????????

 

[coldair]

Call Clifford performance they claim you can make a AMC 258 i6 / 4.2l head work better then a 4.0 head.

 

[coldair]

Another thing I thought I would let you guys know about Clifford performance they take for ever to do anything threw. I ordered a cam from them waited 3 months finally canceled my order because I got tired of waiting.

 

[Gsmithl10]

Last I talked to them they where out of cam blanks.

 

[coldair]

Last I talked to them they where out of cam blanks.

Ha ha ha yep same thing they told me!!! I don't like talking bad about a company but when you pay for something and then told month after month we are out of this or can't get this in makes you wonder. Pretty funny they are still out of cam blanks after a year or so isn't it?

 

[Gsmithl10]

Its been 6 months since I talked to them.

 

[Jimbos76cj]

with the HEI ignition, offy 4bl intake and Holley carb it was a monster.. I was going to put it in the jeep I'm building now but didn't want to spend the cash on a new intake for it. It will go I to my next jeep.

You put an offy on a 4.0 head? Didn't think you could do that. How do u make a 4.7 stoker?[/QUOTE]

I believe the po had to do some work to get it to fit. I'm going to run the stock AMC 258 i6 / 4.2l intake.

I believe boring a 4.0 block .060 and using a AMC 258 i6 / 4.2l crank and rods you get the 4.7 stroker.