Curious re. exhaust Manifold cracks?

[Huck]

Just Curious! Have parted out several of these trucks and occasionally come upon a stock exhaust manifold where the cast iron has started to crack - in one case - totally thru the casting. Can kind of think it may have to do with heating and cooling especially if on the pass side due to water splash but most recent has been on a very stock original truck drivers side which still had the manifold cover on it. Are the casting contiminated from the factory and just explode slowly over time or ?? Anyone have a clue? Huck

[Longhorn Man]

I have found th ones that still have the covers over the manifolds seem to be worse off. Not sure if it is true, or just my observation.

[Fred T]

Vibration and improper torquing of bolts are the two big causes of exhaust manifold cracking. The covers over the manifold cause localized heading, which can also cause cracking. Water splash can come into play, but it is usually short enough in duration not to cause a problem.

[palallin]

The driver's side manifold on my wife's '79 Caddy broke in half between two of the cylinders. It was a cast-iron b!tch to change, too, because of the difficulty of getting to the bolts nearest the firewall. Thank God none of them twisted off!

[6869704X4]

Yea, tell me about cracks.

[Longhorn Man]

palallin, you have no idea how lucky you really were...
Caddy bolts like to rust ino place, one of the crappy things about working n one.

[jku72]

I personally think any exhaust manifold over 25 years old is bound to have cracks. Even a good shape engine runs extremes in heating and cooling, and that means stress and eventual cracks. Any inconsistent heating or cooling magnifies this.
I gave up and just went to headers. I had bought 4 drivers side exhaust manifolds from western trucks, mail order, all had some degree of cracking, I finnaly just ponied up and bought a nice set of headers.

[Longhorn Man]

Don't forget, these manifolds ARE in reproduction now...

[Project1970]

My driver's side is pretty much crack-free, but my passenger side has a few big ones starting to develop...right about the center of the Y, so to speak...hey, it'll be an excuse to go to headers and a new exhaust with a pair of Flowmasters.

[Woody]

WEll a big cause of cracked and broken manifolds is bad motor and transmittion mounts. They allow the engine to move around when you're running it. Also the manifolds get RED HOT when you drive these trucks 70-80 mph out on the highway and turning the RPMs out of it. This makes a weak spot and with all the twisting stresses from the pipes that's how alot of them get broke.

[Blazer1970]

Possible causes would be:

Poorly tuned engine (lean or retarded timing) causing the manifolds to get WAY hotter than what they were designed for. With a properly tuned engine, the manifolds will not ever get red hot.

Bad motor/trans mounts as noted above.

Improperly installed exhaust systems that do not have sufficient "give" in the hanger system.

Any or all of the above are likely to have happened over the 30+ year lives of these trucks. This is not really a design flaw in the manifolds.

[540 Stroker]

Like the guy above, I had a set of Rams Horn manifolds crack right in the middle of the "horns". No big deal because they had nearly 200K? miles on them. My $.02 is to go to headers when your manifolds crack because you will get better performance, fuel economy, and sound with a set of headers. Use a set of high quality flange and collector gaskets like Percy's copper series or equivalent and a set of non-loosening Stage 8 header bolts or ARP header bolts safety wired to each other and your headers will be as trouble free/leak free as a set of manifolds.