Gearbox undertray sheared bolt

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: NB2.5
  2. I’m based near: Glasgow
  3. I’m looking for technical help or recommendations on:

My first attempt at changing the transmission fluid seemed to be going well until I tried to take off the second aluminium undertray. Liberal squirts of penetrating oil beforehand; 3 of the 10mm bolts came off fine, the third has sheared off with half left in the mount and the mount has twisted.

What’s the best way to fix this? Drill out the remaining bolt-end and try and re-fit another same-size bolt, find another mounting point? All suggestions and advice gratefully received, thankyou.

Photo 11-05-2024, 10 38 30
Photo 11-05-2024, 10 38 301920×1440 146 KB

Photo 11-05-2024, 10 40 41
Photo 11-05-2024, 10 40 411920×1440 143 KB

2

I’d give it a good clout with a centre pop , drill and then use an ‘easy out’ or similar extraction tool / bit.

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3

Thanks for that. I’ve got hold of a small extraction tool set and will give that a go.

4

Good good
don’t be afraid of giving the extraction bit a good clout either to ensure it’s seated tight in the drilled hole.
Gives it a better bite when attempting to remove bolt…which is fairly tight I guess :rofl:

5

My appraoch to that issue is, as follows;

Apply copious amounts of a good penetrating fluid such as Plus Gas or similar.

Then obtain what is known in the trade as some “easi-outs” online, such as these on amazon;

You will also need a tap wrench, with which to apply torque to the easi-outs.

Drilling down the centre of the sheared bolt, using the correct size of drill, should relive the stress in the bolt and allow the penetrating oil to seep down the thread’s

You may need to use a blow torch to heat up the area and then by using the the tap wrench and apply torque by hand n the oppsite direction to doong up a bolt, ie anti-clockwise and if you are lucky it should wind out.

It has workeds 8 out of 10 times for me but is not infallible.

Good luck🤞

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6

If its been rusted solid in there I’d be tempted to just drill and tap.

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Success! Tried the bolt extractor but it was locked solid, so ending up drilling out with increasing bit sizes until it was close to the original bolt width (minus thread). Then I took a sacrifical bolt from the I’ll-definitely-need-these-one-day drawer, liberally slathered it in copper ease, wound it through the mount and that cleared out the threads. Just need the right size replacement bolt now.

Thanks for all the suggestions, much appreciated.

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8

I always find that if a bolts sheared through overtightening or just snapped due to movement an easy out normally works, but for rusted stuff - if it’s in there well enough to snap a head off a 6mm bolt then to come out with an extractor is a big ask. :+1:

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