Do you have a brace on the exhaust to support the weight of the turbo?
I've seen these bolts lock-wired on rally cars to stop them coming loose.
I have to agree about the exhaust dump being SECURELY braced to the block ( as per original exhaust brace). It can take a bit of creativity to fabricate
essentially a cup style brace with a stainless Tbar clamp to hold onto the exhaust. I have been racing/rallying for YEARS... and EVERY time there is a loosening
off of bolts/breakages etc... it can be traced directly to a lack of exhaust/block brace. I run dynos, and even under a short period of load, the turbo/manifold/exhaust
glow yellow/orange and the metal becomes "plastic" like. Think about what is hanging off the turbo- a rather heavy long bit of exhaust!! Swinging away and stretching
"plastic" like turbo retaining bolts. When it all cools down, no tension exists and gaskets blow out. Think about the additional loads then placed on that fastener doing
work it wasnt designed for. This also applies for every other type of car, be it a Holden,Ford, turbo or N/A . I have had a running battle with "guru" exhaust shops over this
problem for bloody years. And when they do eventually put a brace on, they weld it on !! DONT !!The weld will crack out.
For the record, I only use genuine Mitsubishi turbo bolts. Expensive, but I never seem to have issues with them in our competition car. The main reason why the broken bolt
is stuck and hard to remove is the threads have stretched and deformed... and lock in. Good luck, and after the thread repairs, use genuine bolts and make an exhaust brace.
Mike