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Discussion Starter · #1 ·
Looking to replace the factory exhaust manifold(gasket is leaking :puke: ..is there a header made for my 4.2 that does not come with that flange for air intake? On top of the gasket leaking I get additional exhaust coming out of that flange.
 

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None of the headers have the flange you are talking about. It is known as a heat riser, designed to give the carb warm air on a cold morning start up.

If you are getting exhaust fumes from that flange, then the manifold is cracked. There should be no exhaust there, it is just designed for the heat.

If you plan on purchasing a header for a 4.2L, then make plans for a custom exhaust or at least custom work on the down tube from the manifold to the cat. The stock down tube will no longer work with a header. Even a shorty header doesn't fit with the stock down tube. I tried.

I ended up with a stock manifold that I ground the insides of the ports out to open it up and let it breathe better, and then a custom exhaust. The headers were too long and conflicted with the front driveshaft.
 

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Discussion Starter · #3 ·
None of the headers have the flange you are talking about. It is known as a heat riser, designed to give the carb warm air on a cold morning start up.

If you are getting exhaust fumes from that flange, then the manifold is cracked. There should be no exhaust there, it is just designed for the heat.

If you plan on purchasing a header for a 4.2L, then make plans for a custom exhaust or at least custom work on the down tube from the manifold to the cat. The stock down tube will no longer work with a header. Even a shorty header doesn't fit with the stock down tube. I tried.

I ended up with a stock manifold that I ground the insides of the ports out to open it up and let it breathe better, and then a custom exhaust. The headers were too long and conflicted with the front driveshaft.
Thank You..I knew there was a proper name for it. Lol I'm a Jeep newb so I'm learning..I'm going full Borla header and exhaust system...while we're on this topic, is there a better bigger air intake(cleaner) for the 2 bbl weber carb I have? Have small square k&n deal as of now.
 

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Discussion Starter · #4 ·
Nevermind on question. Just found a 4 bbl intake, so I'll be converting to that inlieu of keeping a weak 2 bbl. 💪
 

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Nevermind on question. Just found a 4 bbl intake, so I'll be converting to that inlieu of keeping a weak 2 bbl. ?dcaa
The 2 bbl. Weber is plenty of air for a 258 cid engine. The engine will pull about 336 cfm max. And i believe the weber is 370.
 

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Discussion Starter · #6 ·
The 2 bbl. Weber is plenty of air for a 258 cid engine. The engine will pull about 336 cfm max. And i believe the weber is 370.
My 4.2 was freshly rebuilt and bored out .40 over. The person I bought it from just ran out of funds to properly finish it. So it's more like a 4.8 now..throwing on a 4 bbl should provide a nice kick in the pants!
 

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My 4.2 was freshly rebuilt and bored out .40 over. The person I bought it from just ran out of funds to properly finish it. So it's more like a 4.8 now..throwing on a 4 bbl should provide a nice kick in the pants!
Ahh... Im on the mobile so i apologize if that was i your sig or profile.
 

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How did you bore it .40" that's over 10mm, look like you would into the water jackets, and if you meant .040" that won't even make it a 4.3. You can use a block from a 4.0 and use the Crank and Rods from a 4.2 bore it .060" and get a 4.7, the 4.0 has a lot bigger cylinders than the 4.2 block. 370 cfm is good for over 300 CID.
 

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Discussion Starter · #10 ·
How did you bore it .40" that's over 10mm, look like you would into the water jackets, and if you meant .040" that won't even make it a 4.3. You can use a block from a 4.0 and use the Crank and Rods from a 4.2 bore it .060" and get a 4.7, the 4.0 has a lot bigger cylinders than the 4.2 block. 370 cfm is good for over 300 CID.
I said it was freshly rebuilt, I didn't do it nor have it done. The guy I bought it from did. He ran out of funds to do the rest of the rebuild properly..I'm telling you this YJ runs like a bat out of hell too even off the 2 bbl..3&4th are my fave gears too..alot of bottom end! And honestly, after this rebuilt 4.2 goes, itll be V8 conversion time!
 

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My 4.2L is bored .030 over, and I have an Offenhauser 4bbl intake on it, with a Holley 390cfm. Seems to be a good combination. Other than having to add a fuel pressure regulator, and some internal parts in the carb, it has that "plant you in the seat" low end torque.

But, anything above 2500 on the tach, is just wasting gas. A 4.2L was designed as a low end torque motor. high rpm's don't do anything for it.
 

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Discussion Starter · #12 ·
My 4.2L is bored .030 over, and I have an Offenhauser 4bbl intake on it, with a Holley 390cfm. Seems to be a good combination. Other than having to add a fuel pressure regulator, and some internal parts in the carb, it has that "plant you in the seat" low end torque.

But, anything above 2500 on the tach, is just wasting gas. A 4.2L was designed as a low end torque motor. high rpm's don't do anything for it.
What parts did you install in the carb?
 
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