A question about your nuts...

[Nick Calne]

Well bolts actually... but now I have your attention...

Why on disc brake rotors do they install Allen head bolts to fix to the hub?

It seems every time a rotor has to come off the bolts are seized on with lock tite and brake heat. Trying to get the bolt out involves the risk of rounding the head. At least one of those tiny little surfaces gets rounded. Today it was 5 out of 12. Which, keeping on the nuts theme, was a bollock.

Why don't they use normal bolts you can get off with a socket?

[London_Phil]

You need heat on the bolt to help break down the loctite, but even then its hit and miss.
I did my Steamer years ago, and used a heat gun to get the bolts hot, but I made sure I hade a top quality Allen hex on a socket, so I could put downward pressure whilst turning. (Wheel on floor, set between  some 4x2's.)
Still ruined a few, but at least they were out.
I think they are like they are for clearance, and aesthetics too.
Its all a labour of love, so keep calm, and carry on.....

[Timbox2]

I think KTM realises this which is why they use twin drive heads, torx and hex head in one, nifty

[London_Phil]

We used to have those in Kodak Copiers from 20 plus hears ago... Damn good idea too

[Nick Calne]

Thanks for the responses guys. So there would be nothing wrong with using a standard hex bolt then? Other than it might look a bit gash.

I think the one I have in mind is called a washer hex head.

[Timbox2]

Thanks for the responses guys. So there would be nothing wrong with using a standard hex bolt then? Other than it might look a bit gash.

I think the one I have in mind is called a washer hex head.

Flanged Hex is what I would call them: Similar to this?  With a dab of Loctite 232 (Medium)

[Nick Calne]

That's them. I ordered some from Ebay.

[Sin_Tiger]

I know this is an old thread but I only just spotted it and feel it's sufficiently important to refresh it.

Common disc bolts are close tolerance shouldered bolts. to ensure the torque load applied to the brake disc is transmitted evenly across all of them to the hub and the disc should not move.

The ones pictured are flanged, serrated machine screws, do NOT use those.

1) They will not locate the disc sufficiently accurately which will allow radial movement and possibly a disc coming loose.
2) The torque load applied to the disc will be transmitted through the thread and if there is any movement at all the thread will have a cutting effect like a chisel.
3) The shear load area is greatly reduced, effectively the root diameter of the thread compared to a shouldered & fitted bolt.
4) The serrations will chew into the the spot faced contact areas on the disc, so even if you change back to standard bolts the disc will now be compromised.
5) Those will be a lot heavier than the proper bolts and weight can vary a lot with that type compared to the proper bolts.

If you want to give yourself the best chance of getting a stubborn bolt out without damage. Use the best quality hex bit you can find, I keep a really expensive one that is used for no other purpose. Warm them up as already suggested just before you start removal. Give the bit a solid whack to ensure the bit is well seated, any remaining debris is compressed and it also helps to disturb the locking compound hold. Use a bit with an extension and a T bar on the end so that you can comfortably apply force evenly without uneven angular effect.

NEVER use an Allen Key, NEVER use a Ball Head 

[Nick Calne]

Too late now.....

[Sin_Tiger]

If you get a really chewed one that just isn't going play and has you looking for a grinder, there is another option.

Weld a mild steel nut onto it. I think an M12 is about right, drill the thread out of it to give you a fighting chance of getting a decent weld and so that it fits as close to the outer diameter of the disc bolt flange. Grind the chrome off the disc bolt (you won't have a chance of getting a decent weld otherwise) but don't take any more metal off than you absolutely have to, just until you get proper sparks.

Degrease the bolt head and nut but cover your disc if you hope to reuse it, either proper weld anti spray the disc or a light smear of copper grease on the disc will also work. Place the nut over the disc bolt and plug weld inside the nut onto the disc bolt. I've used stick on larger items but there's too much risk of slag getting in the way of a good weld so MIG is the best option, you can use TIG but if you're that skilled you've probably used this technique before. The heat will help soften the thread lock, you want to wait long enough for it to cool to the point where it's not going to chew but not long enough for the heat to dissipate into the hub, before you try to remove it.

I've only had to resort to this once many years ago but have frequently used this technique to remove sheared studs, chewed bolts, seized clevis pins and even a knackered diesel injector on one occasion.

[ghulst]

If you get a really chewed one that just isn't going play and has you looking for a grinder, there is another option.

And another option is to get a slightly oversizes torx bit and put that in gently... While using a big hammer, usually. So far, that has always worked for me.

[mat-tiger1]

These are useful! 
Around £12 from Screwfix. 

[PeteH]

All the bolts came out ok when I  changed the discs on my old girly, I even went and bought her new disc bolts....because she was worth it 

[Lee337]

There is a school of thought that says the bolts should be replaced every time as they stretch when torqueing them down.

While I've never done that myself, I can sort of see the sense in it.

[Sin_Tiger]

They're not designed as a "stretch" bolt as the shoulder comes into play. Having said that, I don't reuse disc bolts that I haven't removed from a wheel myself. I don't buy used bolts and don't sell them either, I just couldn't sleep at night if anything went wrong.