Better 4.0 header??

skyhighZJ

skyhighZJ

Gov retirement < needs to live
Joined
May 31, 2012
Location
Aberdeen, NC.
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So I'm wondering if these "flex pipes" will help in the cracking issue that Jeep 4.0 headers always seem to have. Thoughts?
 
I guess I hadn't looked much before due to the cracking issue and didn't want to waste the money but saw this one and thought it might make sense. Another thing I saw was a guy cut the flange in between #1 & #2 and then again between #5 & #6. Any thoughts. @Fabrik8
 
Cutting or slotting the flange is pretty common on many inline engines because of the header length. You change how the header is supported, how the gasket is compressed/loaded, and how the expansion is constrained on the header tubes when you do that though, so it's not something to do without some consideration. For example, on some engines you could cut the flange and then have sealing issues, because you change how the expansion is constrained between the flange and the header tubes. So you might prevent the manifold from cracking, but you might need to change the flange design.

That's such a long header though, so expansion is going to be fairly large.
 
Pretty sure that the stock manifolds have flex joints in them just like that.

Didn't drive a Jeep today, or I'd go take a picture.
 
shawn said:
Pretty sure that the stock manifolds have flex joints in them just like that.

Didn't drive a Jeep today, or I'd go take a picture.
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They started doing bellow-style flex joints around around '99 it seems; I just researched that after you made me curious. Some variants (TJ maybe?) got 2-piece manifolds at around the same time it looks like.
They both solve the problem by making the rigid length of the manifold shorter.

Has anyone figured out whether this is a thermal expansion problem, or a torsional loading problem from the engine movement and exhaust system weight? That's an important distinction, as one can be solved with a different header design, and the other could likely be solved with a flex joint after the header... I really don't know anything about the history of the problem or if anyone has done a good failure analysis.
 
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My 99 TJ had a one piece that cracked. I put a stainless one off from ebay on it about 4 years ago that was a one piece, so far no more cracks.
 
I have a 98. Replaced the stock manifold with a Hedman header, wasn't very expensive. After 10+ years no problems or cracks.
 
2000 4.0 TJ's have a cast manifold for cylinder 1-3 and another for 4-6. The rear one was cracked within the first year or two of life, the front has never cracked to my knowledge.
 
I've seen the ones with flex joints crack after two years. I welded my stock manifold up after it cracked and that was three years ago. YMMV. I think it is a composite issue of thermal expansion and also torsional loading from engine mount movement.
 
After the stock one piece cracked I tried a one piece Borla knock off and it cracked. I went to the flex pipe a few years ago on my '99 and it hasn't cracked yet. Don't get it from rockauto though. Those did not fit correctly.


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I'm in the market for one now. The last one I bought from eBay needed only slight modification but I owned the vehicle for 6 more months so I don't know how long it performed. I'll buy something this week and review it when it gets here.
 
I put one on a customers vehicles from a local salvage yard, brand new in packaging $75, it was only run a year before the Jeep got rolled but it hasn't cracked in that length of time.
 
I bought the one pictured from eBay for $75 I'm gonna give it a try as it had good reviews. Will report back on fitment and actual quality when I get it and install.
 
skyhighZJ said:
I bought the one pictured from eBay for $75 I'm gonna give it a try as it had good reviews. Will report back on fitment and actual quality when I get it and install.
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Make sure to check clearance between intake and exhaust. The last one i bought needed to be heated and a indention put in to keep from rubbing.
 
Oh, you have the motor out. Mine is still in the Jeep. Can you actually torque the bolts with the mirror out?
 
My flex plate fractured so I pulled the motor and gonna replace with a lower mile unit. I may torque the manifolds on the stand so I don't have to deal with it once installed. It's a proper be-otch!!
 
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