12 replies to this topic

[TheJack]

So after some research for me to tap a 5/16 UNF I need to drill a hole with an "I" drill bit. Obviously not an easy thing to buy in the UK, and after a bit of translation it comes back as 6.91mm. So if I drilled the hole with a 7mm bit, would the 0.09mm difference be too much and not be able to get a good tap?

[Dusky]

A good drill bit will drill a little undersize anyway. Unless it's a very high strength application you won't notice it.

[TheJack]

A good drill bit will drill a little undersize anyway. Unless it's a very high strength application you won't notice it.

As I put this up I found a 6.8mm which I'm guessing will be fine.   

It's for the water pump, bolt snapped off so I've drilled it out ready for it to be tapped.

[nicklouse]

9/32” for steel.

[GraemeC]

I’d certainly go larger rather than smaller if you’re tapping the block.

6.9mm bits are easy enough to get, but the letter I aren’t difficult.

 

Nicks suggestion of a 9/32” will work, and will make the tapping easy, especially if you haven’t got a full set of 3 taps.

[TheJack]

Thanks guys, will use a 9/32" 

[absx2]

Hopefully you haven`t drilled any holes yet.

With a bit of skill/luck you can drill the stud out without the need to re-tap the thread.

Just start in the center with a 2 mm drill bit and work your way up through the sizes while making sure you are keeping center and by the time you hit 6.7/6.8 mm if everything has gone well you will be able to bend the top of the thread with a scriber or small screwdriver and pull the tread out with a long nose pliers like a spring. I`ve probably done hundreds !

Don`t be tempted to make a jig as they never work as well as your eye.

Good luck

[absx2]

The trouble with drilling and tapping is that a large percentage of the time you will break into or out of the original threaded hole thus not producing a good thread.

Edited by absx2, 18 August 2021 - 07:28 PM.

[Spider]

Some really good suggestion here guys.

 

Another option is to find some left hand drill bits and for a bolt of this size, I'd use something around 6 or 6.5 mm. Usually as some point when drilling, it will grab the old bolt and run it out.

 

Before doing that, get the old bolt really hot, red hot if you can and then let it cool. If it's rusted, that will usually release the rust's grip on it.

 

Yet another good method I've used is to get some thin sheet metal (say 0.5 to 0.8 mm) about 150 mm square, drill a hole about 7.5 mm in this case in the middle of it, then grab a M8 or 5/16" nut, I then drill through the threads with a 9 or 10 mm drill, but leave a base in it about 1 mm thick. Place the sheet metal centred over the broken bolt and then the nut, thin side down, then MIG weld the nut to the bolt, allow it to cool and with the heat that it got from welding. The sheet metal is used in case of splatter from welding. The broken bolt then should come out easy, you've got a hex there to land a spanner on if you need to get it started.

[Ethel]

You do really need to be bang on centre anyway. If you're not the drill bit will wander when it hits the threads.

 

https://www.theminif.../331577-eurika/

[Homersimpson]

I tend to weld a nut onto a broken bolt, even if its snapped off flush it generally still works although it can take a few goes.  Sometimes putting a nugget of weld on first then welding the nut to that works better as you get more heat into it.

 

I recently removed a broken bleed screw from a Jaguar 420 calliper using this method, it took five nuts before it came out but in the end it came out clean.

[Magneto]

I've been successful drilling them out, even in aluminum but you do have to be very careful, and if you're trying to do it with the engine in the car, your chances just went down significantly.

 

Welding a nut to it would be your best bet in that case.

 

Here's one I did just the other day in an aluminum Sprite transmission case - I got very lucky on this one! If you notice all the rust in the third pic you can see why it broke, tho they don't usually rust in an aluminum block! The good news, the new bolt torqued up to spec just fine.

 

One of the best purchases I ever made was a good set of left hand drill bits, I've gotten a ton of broken bolts out with them

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Edited by Magneto, 18 August 2021 - 11:55 PM.

[Spider]

Here's one I did just the other day in an aluminum Sprite transmission case

 

Nice work !