The driver side stud snapped on my car recently while reinstalling the turbo. It broke a few mm below the head surface and I was being lazy and didn’t feel like pulling the engine. I can confirm that it’s a pain but not impossible to drill out the broken stud. You’ll need a right angle drill, drill bits ranging from 1/8 to 21/64 in 1/64 increments, a mirror, and a shit ton patience. I did try to drill then extract the broken stud with both straight and spiral flute extractors, but it didn’t budge and I wasn’t going to risk breaking off an extractor. I ended up just drilling the stud out completely, then tapping for a M8x1.25 Helicoil.
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There’s a decent amount of room once the cowl trim is removed
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It’s important to center punch the broken stud to prevent the drill bit from walking. Doing this properly is a critical first step.
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Next up is starting with a 1/8” drill bit and plenty of tapping oil. Use slow RPMs and take your time here. If you feel any resistance, reverse and clean the shavings from the bit. Also make sure the drill angle is correct, again a mirror here is key. Step up to the next 1/64 size bit and repeat until the stud is drilled completely out. The Helicoil will require drilling an oversized hole to the insert to thread into after tapping.
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Here’s what the final result will be with Helicoil installed. At that point I installed a new stud proceeded with the turbo install.
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Welding a nut onto the stud is another option. It involves using a flux core or mig welder to build up the stud and then placing a nut over the now protruding stud. I’m not sure this is possible with the engine still in the car, but it seems like a decent alternative to drilling. It’s also a good option if the passenger side stud breaks, because there’s very limited access to drill on that side due to the head design.