I wasn't making boost and had a terrible oil blow by issue. I couldn't figure it out so i decided to take the cartridge out. The cartridge was brand new. I got it off of ebay 10 years ago or so. I think it was Chinese b/c I didn"t see a garret label on it. Anyway, it looked brand new inside. but shafts were not seized there was no oil coking all. The shaft broke at the area where it goes from a thin shaft to a thicker shaft. It seems it was a metal fatigue issue. My feeling is the metlurgy was off. If there was any evidence of obstruction I couldn't find it.
Thoughts????

 

I think you hit the nail on the head.

 

That, and/ or poor machining... ie no radius where it steps up.

 

Some other common causes of shaft breakage are:
Bearings got loose and compressor wheel hit the housing,
Foreign object hit the compressor wheel,
Compressor surge,
BOV to small or misadjusted.

 

I will look at it more closely. I do not remember a radius. My BOV seemed to b blowing off just fine. I used this type of turbos without a BOV on a manual stang and had no issues. Even a poorly functioning BOV is better than none. I do however see your point

 

Everything ive heard of the Chinese t3 small shaft turbos is they self destruct, in one way or another. The t4 frames seem fine tho, think thats due to the fact those are usually made for big truck replacements over there and get better balancing and materials for better reliability.