NA Crank Cover bolt sheared

my MX-5 is an NA, 1991 and I’m based near Guildford, in Shamley Green.
I’m looking for technical help with the driver side bolt holding the crankshaft cover in place. It has seized and snapped off. I would like to know, please, how long the bolt is or how far in to the housing it penetrates. I am trying to extract it but don’t want to drill in too far and risk going through into the oil system which would introduce swarf. If I can’t get it out cleanly I am thinking of setting an M6 stud in to the hole with epoxy resin, using the new cover to get the position and alignment correct, then putting a locking nut on it to hold the cover.
The worst scenario would be if the bolt goes right through and in to the bottom of the engine as that would mean it would need totally drilling out if the oil pump ever needed replacing. The car is on £103k miles, I’ve had it about 6 months and there was no record of the cambelt ever being done, so I am doing it myself. The bolt had already been sheared off by previous owner and not dealt with; legacy issue. :frowning_face:

If there is any of the stud sticking up from the surface, and if you can find someone with a portable mig welder, ask them to weld a bolt or nut onto the stub.
Two things happen now, the heat will have made the local ali of the cambox expand a tiny bit more than the steel of the bolt, and it is likely to have cracked off the corrosion.

With patience you should now be able to undo it with a gentle little impact driver (I use a little old Bosch GDR) to continue working on weakening the corrosion seal without twisting off the remaining bit of bolt. Don’t apply big force from big spanner!

Over the years I’ve seen this done and then (monkey see, monkey do) done it myself a couple of times with exhaust studs.

Good luck

Thanks for your suggestion Richard but the bolt is now partly drilled out so not enough to weld on to. If I was able to, I’d use your idea and get the welder out.

Drill it out with a smaller drill bit, if the threads do get a bit buggered in the process then Helicoil or similar.

Can’t you get the depth by undoing one of the others next to it?

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Thanks Duratech; Helicoil, that’s another new one for me, good idea. The other cover bolt hole goes right through the housing and does not back on to the engine, ie. I could insert the bolt from back or front. So it doesn’t necessarily have the same length. Ideally I’d be able to look at at an oil pump off the engine or get the bolt length from someone that is currently in ‘dismantled’ state, as I am. I will try drilling and screw extractor again. Thanks.

I’m not quite sure which bolt it is but perhaps you might find a dismantling video on youtube that might give you a clue. If you can drop a pic on here I can also possible go and take a look at ours.

Thanks, that would really help if you could. Picture attached, it’s the lefthand bolt circled. The corrosion broke apart the cover round it. Got a new cover to go on, from mx5parts.

I think I took the correct one out! I photographed the hole too so you can match it up!

Ours is a 1990 Eunos BTW

That’s fantastic, very kind. I’m off work tomorrow so will be trying to finish it off. Thank you.

No problem hope it helps.