I'm certainly not defending Merc or CMI, but CMI has told me Merc spec'ed a higher quality stainless than what CMI uses. So who knows what to believe.

i have tested the material on various CMI headers....with a PMI machine including the ones on 525s
Positive Material Identification , it gives you the chemical composition of the material in a % value
CMI uses 304 grade stainless steel quite cheap actually.. but more than adequate
in the 525 application they cut corners on the anealing process/post weld heat treating, by either not doing it at all or not doing it properly and it doesnt help that the work on the headers themslves is probably outsourced out of country where materials and quality control might be questionable
an other possibility is that the welds were not backpurged.....
this causes oxidation of the metal opposite the weld zone and leaches the nickel from the stainless making it super brittle
warming up and down will eliminate 75% of failures and as much as 90%
definitely cost saving measures....304L has the best heat qualities of all the common stainless grades
the metalurgy is actually quite intricate. half a dozen different filler metals can be used.......for specific applications
and a dozen more variables to each each filler metal to be considered

Wow. That's what I thought. :D :D

Now I'm thinking of not even testing my existing 500EFI headers, and just buying new ones. I'm doing a complete rebuild and these are the original CMI headers. Does that sound rediculous or should I see what condition mine are in before buying new?

Stellings +1. If not stellings, i'd go to a stainless marine manifold setup or keith eickert manifolds.

Got enough to worry about with the engines, now you have to warm up and cool down your headers so they dont spring leaks??....F&CK THAT. I'll be damned if im gonna drop 8-10k for some headers and have to worry about that every time i fire my motors up.

NORT
pressure test your existing headers with water NOT AIR no more than 20 PSI using good guages for accuracy for one hr if there is a pressure drop wich can happen because of normalising and heat variables both in water and metal temp bring the pressure up again for an other hr if there is a drop in pressure then you have your answer try to use room temp water to avoid condensation so you dont think the headers are falsely leaking
05/06 CMI headers for 525 were the problem yrs i think those after are less prone to problems
PM me if you have any more ?
THE DISCLAIMER
i am in no way involved with CMI or MERC
just a simple tradesman with a thirst for water and a need for speed!!!!!!!!!

MILD THUNDER
no disrespect mate....but everyone and i mean everyone cuts corners
in my industry customers have an inspector on fabrication site for the last 50% of the project in some applications to make sure that testing and procedures are followed to the letter
if i had a tube bending machine at my disposal i would make my own headers a roached motor because they are paying some poor immigrant substandard wages and pushing for more production with threats
galls me to no end

Why not air?

air in a pressure test is dangerous....... its the volume of air needed at any given pressure
if there is a total failure the volume of air is instantly released
most of the welds i work with in a pressure application are 100% and proven as such by XRAY or ULTRA SOUND and we never exceed 5PSI if we use air
water on the other hand if there is a failure just goes fart!!!!!
much safer
there was an accident in a shop where i worked some yrs ago using air to test a guage failed luckily no one was hurt but we had to cut a pressure vessel in half to repair the damage
uglyyyyy to no end and very very costly........

Do you think CMI uses air to test them? My point was.......since they are so old, even if they tested okay today.......next week they might leak and wreck my new rebuild.

My stellings are 10 years old and they still pressure check perfect and I expect to get quite a few more years out of them.

You get what you pay for, but I also had CMI's from CMI and not merc and they are a better product than the merc units.