Drilling out an M6 bolt

[Simpo Two]

This may seem like a silly question but I'm not sure of the answer.

I need to drill out some M6 bolts that have rusted in, and replace them with new ones (to drop straight in, hole unthreaded). Do I need 6mm drills or 7mm to allow for the bolt thread?

[Thats What She Said]

Depends if you intend to helicoil or not.

Slightly smaller if you're happy you can use the existing thread.

[Simpo Two]

I just want new M6 bolts to drop straight through. There will be a nut on the other end to hold them.

[P924]

do you have both 6mm and 7mm? I'd just compare the drill to the bolt, and probably go with the 6mm anyway.

By the time you've gone through the bolt, you will probably have broken the friction/rust free anyway.

Edit: Which way through the bolt are you going? head first, or thread first? Is this on the garden bench?

[Ledaig]

Clearance hole for an M6 should be 6.6mm, so you could try 6.5mm if you have one or 7mm will be fine in most cases.

[Thats What She Said]

Ah yes you did mention unthreaded.

In that case I'd go for slightly smaller. The remaining bolt will be easy to get out once the meat of it is out. Don't skimp on the drill bits. A good good quality one will make it a much easier job. Go slow and keep the bit lubricated.

[Big Al.]

M6 denotes the thread overall diameter, i.e a 6mm bolt requires a 6mm hole and the bolt will drop through.

You could just to be on the safe side drill the clearance hole to say 6.1mm, but in theory you shouldn't need to 6.0mm will clear the bolt.

[cml24]

Is there access available either side?

I'm imagining (but could easily be wrong!) Two sheets of metal each with a 6mm hole in and the nut/bolt holding them together.

I would grind the nut or bolt head off on one side, much quicker than drilling but you need to be careful you don't damage the pieces being held together.

[CrutyRammers]

Drilling out a rusty bolt, you're not going to be worried about 0.1mm clearances, it's often messy. I'd go smaller to start with, maybe a 4, in case you go off centre. That will often cause it to collapse anyway, if not, move up to a 5, the a 6. If you go straight in with a 6 and miss the exact centre, you'll end up with an oval hole.

[Ledaig]

Big Al. said:

M6 denotes the thread overall diameter, i.e a 6mm bolt requires a 6mm hole and the bolt will drop through.

You could just to be on the safe side drill the clearance hole to say 6.1mm, but in theory you shouldn't need to 6.0mm will clear the bolt.

Size for size will give you an interference fit, and that's provided the bolt isn't on top tolerance and the drill bit on bottom

[Simpo Two]

P924 said:

do you have both 6mm and 7mm? I'd just compare the drill to the bolt, and probably go with the 6mm anyway.

By the time you've gone through the bolt, you will probably have broken the friction/rust free anyway.

Edit: Which way through the bolt are you going? head first, or thread first? Is this on the garden bench?

Yes, this is the bench! Access in a straight line is impossible due to the design of the iron frames, so I'm getting a right-angle chuck, and ordered 6mm cobalt drill bits from eBay before thinking. Due to what an utter bd it would be to remove all the bolts I've decided to refurb the existing planks in situ as best I can; however there's one plank missing and the rusty remains of the bolts need to be removed. As for head vs thread, is there a best way?

[dickymint]

It's going to be a bd doing that with a right angled drill!!

[P924]

Simpo Two said:

As for head vs thread, is there a best way?

Best way for me would be whichever was most comfortable. I imagine drilling up through the thread will be a real PITA given the shape of the iron work.

[Private Pile]

As Dicky said, using a drill adopter will be a PITA. I'd also start off with a smaller drill bit.

Have you tried undoing the bolt? Can you add heat from a blow torch? If it's proper rusty and seized then it may well snap whilst loosening it.

[TooMany2cvs]

If this is the kind of garden bench I'm thinking of, then wouldn't it just be easiest to cut the existing bolts off with a grinder, probably with the wood still in place, but destroying it if needed? They should just tap through the holes in the frame, which won't be threaded (the frame ends are usually some kind of cast monkey metal, so a thread wouldn't last two seconds anyway)


This kind of thing?

[Simpo Two]

dickymint said:

It's going to be a bd doing that with a right angled drill!!

I know, but there's only about 6" clearance above and below.

Private Pile said:

Have you tried undoing the bolt? Can you add heat from a blow torch? If it's proper rusty and seized then it may well snap whilst loosening it.

WIth great difficulty I got about half the bolts off; the others sheared, and a few nuts are impossible to get a grip on as they're too close into an angle. Can't use heat with the wood there.

TooMany2cvs said:

If this is the kind of garden bench I'm thinking of, then wouldn't it just be easiest to cut the existing bolts off with a grinder, probably with the wood still in place, but destroying it if needed? They should just tap through the holes in the frame, which won't be threaded

That was the plan, but the bolts are rusted totally solid in the iron frame. Hence the plan to refurb the wood in place, but I just need to get the bolts out where the one plank is missing.

Worst case is that I cut/grind the bolts flat with the frame and stick the new plank on with Sikaflex!!!

[TooMany2cvs]

Simpo Two said:

That was the plan, but the bolts are rusted totally solid in the iron frame.

Get the wood off, get some decent penetrating oil on for a few days, and then get some heat on. Tappity, tappity, they'll be out.

[Simpo Two]

TooMany2cvs said:

Simpo Two said:

That was the plan, but the bolts are rusted totally solid in the iron frame.

Get the wood off, get some decent penetrating oil on for a few days, and then get some heat on. Tappity, tappity, they'll be out.

If the wood was disposable maybe, but I want to save it. And the holes through the frame are about 1.5" deep... can't use too much force or the iron frame could break. So I need to get the bolts out to get the wood off without destroying it...

[Simpo Two]

As a postscript, slightly to my surprise I managed to hammer one of the bolts out! Of the crucial ones, that's one down, two to go... WD40 stewing...

[TooMany2cvs]

Simpo Two said:

WD40 stewing...

Try with some decent penetrating oil...