how flat is flat enough?

[85holiram]

Sooo....chev 454 notorious warped leaking exhaust manifolds.  Had blown out gaskets. Came off no problem, except one A.I.R port ps front,yay me.     I decided to try flattening them myself, took a stationary 36" belt sander to get all pits out. Quite warped, think ends curling away from heads.  right now I'm .009 gap at outer ends by mounting holes, both ends surprisingly. I also cut between ports as described in Chev motorhome chassis manual. what say ye, take it down some more ? right now I'm using 24 grit what should my final finish be? Oh and btw it's a 85 holiday rambler but is on an 84 P30 chassis. Thanks in advance

[tmsnyder]

There's probably a spec for that from chevy, but 0.009" is definitely too much.  Especially if you mount these metal on metal with no gasket as intended by GM. If you mount them with a gasket it acts as a layer of insulation between the block and the manifold, the block is supposed to cool the manifold so it doesn't warp.


How are you measuring its flatness btw?


I'd put it on a mill and cut the surface flat, or buy a new one from Dorman.


If you put it on with a gasket, and the surface isn't flat, you're going to be putting stress on the manifold and it may crack on you.  It's also going to warp worse and start leaking again.   


If you want to just throw it together b/c your stuck on the side of the road and don't mind revisiting this later on down the road, then sure, put it back on with one or two gaskets stacked in there but just my opinion it probably won't last very long.

[Rickf1985]

Are these the cast iron manifolds or the factory stainless header type? As far as gaskets, that is the specified repair for a leaking manifold. There is little cooling being done by the head to manifold interface due to carbon buildup and dissimilar metals. The factory stainless header type manifolds would warp quite badly and bend up on the rear end usually. One of the fixes was to cut through the tie bar between the tubes so the tube could have a little movement on its own.

[BamBam]

Just remember that the more you take off, the thinner the flanges qwill be and make them more susceptible to cracking or breaking when torqued down. "Remflex gaskets are thick and do a good job of sealing. Good Luck

[tmsnyder]

The head has water recirculating through it, it's running at just over the thermostat temperature. 220, 230, somewhere in there.   Manifold running a touch red is 1000F.   Bolting something that's 1000F to something at 230 makes a huge heat sink for the manifold.  If you separate the two with the typical exhaust gasket which is something like an asbestos gasket with just a sliver of a ring of steel crushed to make the seal, now the joint is insulated. Not much heat is going to be able to transfer to the head from the manifold and the manifold is going to run even hotter and warp even worse.


About removing too much and weakening something, he's not going to remove enough material to cause thinning and warping, we're talking 0.010 max removed from something that's 0.750 thick

[Rickf1985]

Average exhaust gas temperature should be around 500-650 if the AIR system is disabled. If the flanges are warped and you are machining them flat then you are going to have a flange that varies in thickness so it is still going to warp again, just a matter of how much and how fast. I have never seen a machined set of cast iron manifolds last more than a year before cracking. The stainless steel ones will usually just warp again or I have seen several, one on mine, crack around the tube weld to the flange. Something has to give somewhere. Keep in mind that the metal to metal interface didn't work the first time since they warped without the gasket. I actually have a double gasket on one side of mine. It has been quiet for two years. And it was not even close to .009, more like 1/16" out!

[85holiram]

Well,I finally got the manifolds back on, used Remflex gaskets, all is quiet for now. hopefully I will be able to get headers next year. I decided to eliminate the A.I.R.. Out of curiosity I took the pump apart and the bearings were literally flat, no wonder I was throwing belts. Thanks to all the info here I was able to get the right belt immediately. Next is to address the timing and adjustments necessary due to removing the A.I.R.Gotta go back and look at all the posts I read here on that subject.Thanks again all!