The bolt is sheared around 10% of the way down the thread, so I have the bolt head and a bit of shank, and maybe two threads. So it's unusable and there's nothing to get an extractor onto. I've put the cap back on the filter housing and the turbo is out at the moment, so I have clear access to drill without any risk of metal bits getting in the filter.
My plan is to drill a 1.5mm pilot with a cobalt drill, then try a bolt extractor so see if the remaining shank will break free and unscrew.
If it doesn't I'll drill a 3mm hole, and try the above again with a bigger extractor.
If that doesn't work, I'll drill out 6mm and just put long M6 through to take a washer and nut.
There really doesn't need to be much torque on these bolts, as the O-ring provides the seal, so the M6 through should provide enough tension to do the job.
What do you think?
Big Jock I have been restoring vintage motorcycles for many years and have had to remove many screws,studs or bolts sized in alloy castings my recommendations are,
You will at some point be drilling the screw so be sure to center punch the screw to get the drilled hole in the center of the screw
Left hand drills tried them no success they just drill a hole as with right hand drills so I don't bother trying now.
Cobalt drill I would not recommend as it is not as efficient as a HSS drill at cutting steel and liable to wander off center and damage the thread you are trying to save.
If you meant a carbide drill which is better at cutting than a HSS drill I still would not recommend unless you have experience of using them, they are very brittle and used in a hand held drill has a tendency to break if you don't keep it perfectly in line when drilling. Then removing a broken carbide drill "you dont want to go there".
Using the tapered stud extractors is very successful but as said can when in use expand the screw you are trying to remove making it impossible to extract. Use the smallest extractor first to try to remove the screw use your watchmakers skills with a little more care this time because you don't want to break the extractor off inside the screw, better to give up with the extractor if the screw shows no sign of moving.
Heating the alloy and using a lubricant when hot to try a free the screw also works but you can't always heat things up with a torch as a gasket or seal somewhere else on the alloy mounting maybe damaged.
In this case I use localized heating by friction so in your case drill partway into the screw with say a 4mm drill then either blunt the drill on a grind stone by removing the cutting edges or simply run the drill backwards using the same action as trying to drill out the screw this will cause friction and quickly heat up the screw and localized to the screw the alloy casting again using a lubricant then try the extractor again.
Now if it is still stuck you have a tap to create the correct thread, drill it out to the correct size and re-thread it.
If after all this work the hole is so bodged you cant create the correct thread you can use a thread repair insert such as helicoil.
As a last resort drill a through hole and use as you suggested a nut and bolt on this application it would work but not on all broken studs could you use a nut and bolt.
Good luck with your repair.
