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I have repaired a lot of cast iron manifolds but most of them are 1980's and newer. I think it was Noel who said they were thinned down and this is correct. The thinner the part, the less material required to make the part, the less energy used to heat the metal to make the part, shorter life span thus more parts sold and higher overall profits. Bottom line is the driving force over all else. Face it, with the technology available, both logical and equipment wise, there is no reason why an engine and other components cannot be made that would last ten times as long as current stuff and use a heck of a lot less fuel too. This would cut into the overall profits of the big corporations so there is your main and only reason why this does not happen. Anyway, the manifolds are made so thin that they cannot take the constant stress of expansion and contraction and will fail much quicker than the older heavy walled ones. The second reason is because of the type of materials used for these items. Most all of it is recycled material that is basically useless for any other items because it cannot meet the requirements of yeild and tensile strength. In short it's nothing more than the left over crap that they can't get rid of for anything other use. Weld repairing older heavy wall ones made with good virgin cast iron is relatively easy because you have a consistent casting alloy. With the crap used in the newer ones, they become more costsly in time and materials than they are worth. In most cases, you can spend upwards of 2 hours doing a repair that probably won't hold more than a few heating / cooling cycles before it fails next to the weld area anyway. Thus, two hours in a weld shop on cast iron is going to run you from $100 to $175 depending on the scope of the repair and materials required. If you can buy a new one for $125, there is no point in throwing good money after bad product. Other causes of cracks is a combination of bad materials used to make the manifold and poor preparation of the part before installation. Not many companies will invest time into properly fitting and finishing the parts before they go on the engine. Small casting runs that extend into the port area heat and cool faster than the rest of the wall of the casting causing small areas of rapid expansion and contraction. This does nothin but add to the stress levels in the part. Cast iron was used for much engine work because of it's properties. It is easy to work with and machine, cheap and will handle the heat with not much of a problem. Back when, steel was more expensive than cast iron and this was the main driving reason for use of cast. Now, good cast iron is higher in price than good steel thus is the reason why you will find many newer engines with steel exhaust system components. The new steel aloys will handle much higher stress levels than the cast and are perfectly fine for such uses and will generally last much longer as well. Even these have been cut down to make them cheaper to produce. In addition to making them cheaper, they also will often design fail points into the parts so the life span can be determined in order to sell more replacement parts. It's quite obvious if you know what to look for where these fail points are at and you can bet when the part comes into my shop, I know right where to look for the problems even if it's not readily visible. There are some good brands of header pipes that are made in a manner as to reduce the stress imposed on the part from expansion / contraction and will last a long time with little to no problems. While this was not the driving force behind the design, it's a nice advantage to the consumer that was not intended. Hope this answers some of your questions.
Mark Kw
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