Snapped stainless head bolt

[Livid]

I was tearing down a 2cyl Yamaha/mariner 40hp (6e9) and was gonna take the head off to get skimmed, but I've snapped 3 of of the head bolts. Has anyone got any advice on how to get them out? I've got a pretty chintzy mig welder so could weld a nut on them to try and wind them out, or left hand drill bits to try and drill them out. I've got the head off, the bolts snapped basically flush with the head.

All but 1 of the other bolts to get the power head and leg off came out without a fight, it had to be the head bolts that decided to break on me

[beamishken]

I usually find welding a washer on then a nut gets better results as you get more penetration with the thin washer. If they are corroded in the left hand drill is unlikely to yield much success. If the welded washer fails I usually grind the stud flat & drill out the bolt, sometimes you can run a tap down & clean out the threads but sometimes you need to revert to a helicoil or timesert type insert

[Matt]

Yeah, a common nightmare. I have managed to get down to M8 or so size carefully drilled out by hand using a sharp set of cobalt drill bits. Never bother with LH ones since they never unwind anyway, but I start with say, a 3mm drill and increment by 0.5mm until things start to move - and then review the thread to decide whether you need to helicoil it or not.

Smaller than that I find a bit of a struggle though, you'd probably want to make up a jig. I'd start with Ken's welding recommendation and see where you get to.

[Pikey Dave]

If all else fails try taking it to a company that does spark erosion. Not overly expensive & quick.
https://www.erodatoolsltd.co.uk/spark-erosion/

[beamishken]

Originally Posted by Pikey Dave

If all else fails try taking it to a company that does spark erosion. Not overly expensive & quick.
https://www.erodatoolsltd.co.uk/spark-erosion/

That's a good suggestion👍 opinions on drilling stainless differ & some folk struggle with it, but let's not go down that (rabbit) hole!

[Matt]

Depending on how the last job went, my opinion varies between easy, hard and impossible.