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Ideas needed! Enlarging hole in chassis with limited access


L19MUD
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It has been one of those evenings. A simple job of fitting new seatbelt brackets to a Defender post chassis swap. Unfortunately I did not realise the holes in the chassis are now smaller than the m8 bolts that need to go through there. I don't really want to drop a bolt size given the critical function. How on earth do I go about making these holes bigger? 

 

I tried grinding down an m8 bolt and putting a slit through it to try and cut through the zinc but that was unsuccessful. My taps are too long. 

 

I really don't want to drill holes through the floor or take the body off!! 

 

Clearance body to chassis is about 4 cm and the angle is terrible. Ideas please

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15 minutes ago, Retroanaconda said:

Cut down an 8mm drill and put it in a tap handle?

Worth a try. Managed the front two now with a very bendy 7mm long drill bit. Back ones never going to happen using that

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12 minutes ago, Retroanaconda said:

Even if you use one hand to turn the mole grips and another to use a screwdriver or similar to apply downward pressure by levering against the tub?

Yes tried this

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5 minutes ago, Arjan said:

Smaller bolt with higher strength ?

They are standard 8.8 m8 bolts in there

 

I have the front ones sorted but could put stronger m8 bolts in the front and then m6 or m7 stronger in the back? Not sure how bolt ratings correspond to actual force? 

It will be shear strength that is important in this application

 

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According to the following link https://eurocodeapplied.com/design/en1993/bolt-design-properties

The shear strength of an m8 8.8 is 14.1kN so 28.2kN across two bolts

A 10.9 M8 is 14.6kN and a 10.9 M7 is 11.6kN so 26.2 kN across the two

 

This is 7% weaker. I would have though this would be OK? If it wasn't a seat belt mount it would have had 2 M7 bolts a side in it by now and I would be moving on! 

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1 minute ago, steve b said:

short hex head roofing screw and a ratchet spanner , wind it through and continue to clear the galv .

And then , if required a larger hex head screw - floor bolt or a coach screw 

Steve 

That's a similar idea to what I tried with the bolt I cut a groove in but I agree this would be much better as the thread is coarse and they are thinner at the bottom. Will have a hunt round for something suitable tomorrow. Thanks

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Just a thought but have you checked if there's galv build up inside there each side the hole ? As when I did mine there was and I couldn't get the captive nut plates in there so I could put the bolts in had to clean them out with a chisel , luckily I tried before I put the body on .

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1 minute ago, ianmayco68 said:

Just a thought but have you checked if there's galv build up inside there each side the hole ? As when I did mine there was and I couldn't get the captive nut plates in there so I could put the bolts in had to clean them out with a chisel , luckily I tried before I put the body on .

It's the galv causing the issue but I can't see much as it has all been sprayed black. I missed these ones when I went round, couldn't have missed anything more inaccessible! 😔

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No they're a rite PIA even to tighten the bolts up when you do manage to clean them out , put your finger inside there and make sure it's only the hole that's blocked with Galv and there's no blobs of Galv on the underside inside there as the plate with the nuts welded on that slides in there won't sit flat and you'll struggle like hell to get the bolt's in .

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Weld a flanged nut to a cut down drill bit? use with ratchet spanner as above.

File could do the same.

Is there room to swing a 1/4" ratchet? I'm sure you could get something in there if you cut it down....

For anything like this, a screwdriver wedged on top against the body will help loads.

A tap would work, but you don't want to break it off in there....

 

 

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Weld a nut on to a 7mm drill bit and use a ratchet spanner to go round and round and round, repeat for 7.5mm, and 8mm? Perhaps tapering the drill bit at the pointy end?

 

 

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37 minutes ago, reb78 said:

What about a long round file?

I'd wondered at that, I'm not familiar with the structure, so don't know why it isn't possible to drill or file the hole out from above, but I think the only access is from beneath the body, ie in that gap between the chassis and the floor above.

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I had a similar situation when I built the frame around the hole in my roof in that I forgot to drill holes in 2 of the leg plates to bolt it down

 

The curvature of the roof meant I couldn’t get a drill bit in there and the fact I welded it all in situ meant I couldn’t take it out.

 

I bought these  https://www.machinemart.co.uk/p/060220840/ and used the smallest one (8mm ) in a right angle drill.

 

Took forever but I was cutting through 4mm steel

 

if you don’t have a right angle drill, you can attachments for standard ones, you’ll just need to check the height of the body. 

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