How to replace [seized] rear disc?

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#1
I need some help. My rear disc is toast, but, with any eight year old well-worn dirtbike, the steel bolts have seized in the aluminum hub. How do I get these out? I can drill the heads off, but then what? Drill and re-tap with a larger bolt? I don't think I can go larger and find something that still will fit. What are my options?

Here is the bolt-heads on the "front" of the disc:
<img src=http://x5000.com/body/misc/boltHead.jpg>

Here's the back of the disc where you can see the bolts on the backside of the rotor. There *might* be enough room to throw a nylock nut on there, but I'm not 100% sure that I want to do that.
<img src=http://x5000.com/body/misc/backDisc.jpg>

By the way, the bike is a bit dirty. Didn't clean it after I grenaded it out in NM this past weekend.
 
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#2
Heat, heat, and more heat. There is permanant locking compound on the threads of those suckers, it takes a few hundered degrees to get it to release. A small propane torch should be more than adaquate.
 
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#3
And penetrating lube. I have found that beeswax works best when heat is involved. The disk doesn't appear worn out to me. Have you measured the thickness?
 
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#5
Same thing happened to me about 2 weeks ago when I tried to replace my front disk. I totallly stipped out 3 bolts and got 3 out fairly easy. I stripped it enough that I could just hammer in the next size allen key that attatches to a socket. The hardest part is getting a flat pull on them with a normal allen key. If you have a bit that fits into a socket hammer it in and you should be able to get it out.
 

john stu

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#6
if you totaly stip the heads i would use a thin grinding wheel and cut a slot deep in the head big enough to fit a LARGE flat head screw driver in it if your replacing the disk dont worry about cutting into it just make sure you make a clean sharp cut straight into the bolt head so you have a good place to use that big screw driver (straight in is the key dont twist or angle at all so you make a good sharp place to the screw driver to fit)(if you can find one use a srcew driver the has the hex head on the handle or top of the shaft itself so you can put a wrench on it for leverage then use heat and "pb blaster found at auto parts stores"
 
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#7
Thanks for all the input.

The disc was completely cupped in the middle. You can't really see it in the pictures, but the middle of the "friction" area of the disc is less than half the thickness of the disc. It is so worn, I was starting to become concerned about failure.

I tried the penetrating oil, heat, impact gun, etc. I even slotted one of the heads and tried a chisel/punch and hammer. Nothing.

I finally gave in and drilled them out and used stainless bolts and nylock nuts.
 
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#8
John Stu: if you totaly stip the heads i would use a thin grinding wheel and cut a slot deep in the head big enough to fit a LARGE flat head screw driver in it
This method does work really really well. Especially on engine-case bolts and alan/torque head bolts.

The_Monk: I finally gave in and drilled them out and used stainless bolts and nylock nuts.
Why not just re-tap the holes in the hub and use some high-tensile bolts? It'll be much-much stronger in the long-run...
 
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#9
Because re-tapping cuts even more into the mounting ears, weakening it even further. The bolts with nuts is fine. Somebody suggested high tensile bolts. Do realise, stainless steel bolts are the equivalent strength of a SAE grade 2 or metric 4.8. They are weak, but since you drilled out the hole, the new bolts are significantly larger than the old and should be significantly stronger.
 
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#10
Because re-tapping cuts even more into the mounting ears, weakening it even further. The bolts with nuts is fine. Somebody suggested high tensile bolts. Do realise, stainless steel bolts are the equivalent strength of a SAE grade 2 or metric 4.8. They are weak, but since you drilled out the hole, the new bolts are significantly larger than the old and should be significantly stronger.
True, however look at the ears on the hub... they're HUGE. The stock hole is probalby 5mm in width, you can easilly re-tap to a 5.5 or even 6mm without over-stressing the surrounding aluminum (or whatever white-metal they're made from).

What you could try is a set of easy-outs. You basically drill a small hole in the center of the bolt, tap in a special extractor, and then crank it out. They work amazing and a good set will run around 30-50 bucks. Definately good to have around.

I suppose, in-the-end, you're right with running nuts in bolts. I didn't think about it before, but if you drill and tap the holes in the hub (making them bigger) you may have to bore the bolt-holes in the brake disc as well.

Dan