Originally Posted by serotoninsteve
The same happened to me, I did drill mine out without removing them, but they are a bitch. As hardened steel.
Normal drills will not last, you best need something like a tungsten drill. I used a lot of normal 6mm drills and it took me about 5hours.
Then I bought a tungsten 6mm mill and could grind them out as you see in the pic.
Thanks Steve - very helpful! I'd really like to avoid removing the downpipes - and had tried drilling to no avail. The tungsten option sounds good - but can you provide some more detail (or a photo?) of the bit you used to grind out the hardened steel stud?
I have already ground off the studs flush with the flange, and even the back of the stud also flush so I am ready to drill