Manifold swap 258 I6

[bamacjer]

I recently purchased a 1982 CJ5 not realizing it had an earlier model AMC 258 i6 / 4.2l engine. This wasn't discovered ontil I was installing the headers I purchased for a 1982 model as they won't bolt up to the intake manifold without fabricating some spacers to adapt to the earlier model intake.. Tha carburator is trash and I want to purchase a 1982 intake manifold along with a two barrel carburator. My question is will the 1982 intake bolt up to the earlier model AMC 258 i6 / 4.2l and if so what is my best carburator choice. thanks in advance for any advice.

 

[elwood blues]

Short answer, No.

In 1980 AMC changed the design of the AMC 258 i6 / 4.2l intake and exhaust manifold. The pre-1980 intake bolted directly to the exhaust manifold, one won't work without the other, it looks like this;




This one is a 2bbl version, the earlier ones are 1bbl versions.


The post 1980 intake and exhaust manifolds are 2 separate pieces and look like this;

I had read somewhere a while ago that you can use the new version on the old block, but there was an alignment pin that had to be ground down off of the block. I can not speak to this, I don't know.

The biggest problem you're going to run in to is that Clifford Performance and Offenhauser are the only 2 places that I know of that make aftermarket AMC 258 i6 / 4.2l intakes.

You're looking at getting one from a bone yard or eBay. And then you have to look at getting the matching exhaust manifold.

Carburetors... Well, that's going to get you as many different answers as there are CJ owners on this site.

 

[bamacjer]

Actually I managed to install the old one barrel intake by fabricating some short spacers to push up against the header flange. I'm hoping the two barrel intake I found on E Bay will mount up some what the same, may have to grind down the pin you mentioned. I'm not sure how all this will work out but I will give it a shot and post the outcome. Thanks for your advice the pictures were very helpful. I attempeted to attach two pictures, not sure how that will work out either Haha

 

[Lumpy Grits]

Nicely done Bamacjer.
Check the manifold bolts/nuts OFTEN.
LG

 

[elwood blues]

View attachment 23165

View attachment 23166

Actually I managed to install the old one barrel intake by fabricating some short spacers to push up against the header flange. I'm hoping the two barrel intake I found on E Bay will mount up some what the same, may have to grind down the pin you mentioned. I'm not sure how all this will work out but I will give it a shot and post the outcome. Thanks for your advice the pictures were very helpful. I attempeted to attach two pictures, not sure how that will work out either Haha

The pics worked! That's some good fab work to make that work. I swapped out my original 1bbl for a '79 2bbl intake quite a few years ago. They were identical other than the carb mounting. You can stick with the 1bbl if you plan on going with a Weber carb, they make adapters for their carbs to the 1bbl intake, just food for thought.

 

[bamacjer]

I guess I got a bit ahead of myself and already ordered the 2 barrel intake off e bay. I guess I'll make the call when it arrives and I see what kind of shape its in. I wonder if there is any advantage to having the bigger opening where the carb. mounts with the 2 bbl vs the 1bbl intake. Thanks!

 

[PistolDave]

That's a mighty shiny engine! Some good looking work there.

Dave

 

[elwood blues]

I guess I got a bit ahead of myself and already ordered the 2 barrel intake off e bay. I guess I'll make the call when it arrives and I see what kind of shape its in. I wonder if there is any advantage to having the bigger opening where the carb. mounts with the 2 bbl vs the 1bbl intake. Thanks!

A few years, probably more, I remember writing up a post about that on here and it went way further down the engineering worm hole than anyone really wanted to explore.

My question was basically the same, because the diameter of the 1 hole in a 1bbl intake is larger than each individual hole in a 2bbl intake, but what is the total of the 2 and the same for a 4bbl... anyway, it was a lot of talk and no one really had a good answer.

Then someone, CheapJeep85, had a thread about milling out the 2bbl intake to make it one large oval...

https://www.jeep-cj.com/forums/f97/intake-modification-7730/


Anyway, all of this and I don't really have a good answer for you. If you want to really muddy up your thought process you can look in to dual plane intakes vs single plane Intake Manifolds: Single Plane or Dual Plane?

https://www.summitracing.com/parts/ofy-5999dp

And then there's this crazy set up from Clifford Performance that's running dual Weber 38's 08-7001J258

 

[bamacjer]

I'm afraid the cost of gas with those two Weber'swould get into my beer money

 

[Manic Mechanic]

i run a MC2100 carb Venturi Size is 1.08 And Jet #47 from a 72 bronco with the 302

 

[slomo]

Been running a 600 Holley 4bbl on the Offenhauser aluminum intake since 1998. Wouldn't trade it at all.

Weber carbs, really guys? Those are for 1800cc air cooled engines. My darn 0 turn riding mower has a larger carb than that Weber. LOL

slomo