To Heat or not to Heat....

Tony Wells

Tony Wells

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Ok...I own a Jeep with a 4.0l 6 that has the usual cracked exhaust manifold. So I decide it's not going to fix itself, nor pass inspection this time around. So I buy a nice ss header/manifold to replace it, and hoping it won't crack again any time soon.
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Sounds like fun, eh? Well, I decided to check the mounting flange for flatness and to my surprise (not), found it to be a little out. Like 3/16", approximately, with a twist. In the old days, when I had my engine service equipment, about 2 minutes on the belt sander would flatten it acceptably. Done many, many heads and manifolds, in addition to various and sundry other less than flat surfaces. No longer having that machine, I can take it to a shop I know that has one, but another thought popped into mind. I'm sure the flange was plasma cut, and jigged up to weld all the tubes in place, and what I see is a combined result. Now it crossed my mind that the jig was probably ok, and this thing sprung or relaxed on the boat over here. Before I start carving or sanding on it, I begin to wonder if it will relieve itself somewhat at normal operating temperatures. Which makes me question doing anything prior to installing it. If that's going to happen, then fine, it will relax and be flat again. If not, I'm asking a lot of the bolts and gasket. But then again, if I make it flat now, and it relaxes, it's going OUT of flat and will bring in basically the same forces against the gasket and bolts. Soooo, I question what effect, if any, running it through a stress relief process will have. It is T304. I have not looked at the temperature curves yet to see what the critical temperature is. I can cook it in my propane grill, or I can send it to a heat treater. Or maybe bake it and then flatten it.

What say you, engine guys in particular?
 
Tony,
I'm not an engine guy, but, could you do it halfway, and see if/which way it jumps?
 
Removing as much as 3/16" off the flange seems a lot to me, so I'm really looking for a way to avoid just machining it flat. I could do that, if I wanted to risk the stress relief coming from the engine heat. I'm sure it will get hotter there than I can get it on my grill. But if I new for sure what the current manifold temp is, which I could measure, I would know what to tell the HT guys. Problem might be it going even more out of flat and then having cooked it, having a problem returning it. I probably wouldn't want to remove any more from it than where it stands now.

Naturally I could send it back now, but I doubt I'd get anything better or different on replacement. After surfacing it or cooking it, return would probably be out. I've got time to study on it. It's not a huge job, and I'll have to take it a little at a time anyway, being I can't stand too long at a time, much less lean over a fender for a few hours. So I'm just kicking around some ideas.
 
I think I would try bending it back to straight. I realize the flange is thin and the bolt hole pattern is not that uniform. So grabbing onto the thing to straighten it would be difficult. Could bend the flange in the wrong place, trying. You would need to attach the manifold to some kind of mechanical advantage. Maybe a couple of 2x4’s? Or steel bars?
 
How stiff is it Tony? It looks pretty flexible, if it were to pull flat on the head fairly easily, I'd leave well enough alone. Im thinking they all warp from welding and they expect them to pull flat.

Greg
 
I don't really know how stiff it is. I'll clamp it down on the mill table tomorrow and see how much it takes. The flanges is almost 1/2" thick. Another concern is that the exhaust manifold shares a few bolts with the intake, so if the thickness is very different, I'll have to compensate for the uneven thickness if I cut it down.

The only reviews on Amazon were not related to (directly) flatness, but maybe a hint. Several people said toss the gasket that the vendor furnished and go OEM. Might be a sign that they are all warped and perhaps warp even more after getting up to temp.
 
Those headers get pretty hot in operation. If it can be bolted it up, I would think the first high speed run would heat it enough to to correct any misalignment. the Jeep is the best alignment jig you have.
 
I think I'd try clamping it down but had a washer or two under the leg that's off. By clamping it the pressure won't be as much to cause kinking or splits. You can watch as you put the pressure on the bolts . I'd wave a propane torch over the distorted part . Just what I'd try . When you loosen it you'll know if it took any or all of the tilt ,,, bend.
 
I'm sure the intention is for the average DIY'er to simply bolt it up and go about their business. Standard procedure would call for a retorque of the bolts after some run time anyway, and I would imagine the CI head would be safe from stress. I am just concerned that the welds (although they look pretty decent) would stress crack. It is 304, fairly ductile, so perhaps the first heat from a run would relax it to preweld alignment. I guess partly the curiosity of how it would move if stress relieved is part of what is driving me.
 
Isn't it our nature to over think things Tony

Greg
 
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