Brake caliper bolt lengths - longer calliper bolts?

ive tried a multitude of combinations with different length nuts and different callipers, the problem seems to be the fork.
The drilling is 8mm from the back as it should be for recessed and 6mm at the front for the bolt of the calliper. the problem is that the long nut I have is so long it hits the 6mm diameter part of the fork and so doesn't fit, and the short one doesn't reach. not to mention the fact that on both callipers I tried the bolt and nut wouldn't connect regardless of combination. I think it might be because of the thickness of the carbon walls on the forks?

looking at it i think i need a brake with a longer bolt attached and just a standard length recessed nut, but i don't know where i could get a longer bolt or a new callipe with a long one?

You're not trying to use a rear brake are you?

How long is the brake's bolt and how long are your nuts (snigger)?

haha, well i wondered that but one i tried (the dura ace) is definitely a front,

campag mirage calliper is 12.5mm bolt and 25mm nut,
dura ace font is 20mm bolt and 20mm nut

There's your problem: the Mirage is a rear, so without replacing the bolt it won't fit.

Both your nuts are on the long side (guffaw): just checked some of mine and they around 12mm to 15mm.

Try your LBS for something shorter to use with the DA.

haha, the joys of nuts!
the problem is though im using the dura ace as we speak with the 20mm nut and it doesn't reach.. then I try the 25mm and I cant sink the whole nut in because it meets the front wall of the fork... this is really confusing me...

thanks for all the help so far though!

The 20mm nut doesn't reach the calliper's bolt, but the 25mm hits the front wall? You're doing something wrong, because those numbers don't add up.

If the 25mm is hitting the front, then the gap must be less than 25mm; if that were the case then the bolt would almost be poking out the rear.

If the 20mm nut doesn't reach the bolt, then the gap must be more than 40mm (20mm bolt + 20mm nut).

Try turning the bike upside down, so you can see what's going on.

yeah, this is the problem, I think the gap from the outside surfaces is about 40mm, but the forks are aero carbon and the walls are really thick. if I turn the forks over then the hole you would normally look through is just big enough to poke one finger through if that gives you an idea of the thickness? so I kind of end up with the 20mm bolt on the calliper sitting in a bolt hole which is 6mm in diameter, and no way to attach a nut.. not sure if what im trying to describe is clear or not

I've got a feeling you've essentially got two rear brake calipers ..

You've measured 7.5mm difference in pivot bolt length? The difference in bolt length between front and rear according to this tech document for 7800 series is 14.7mm

http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/BR/EV-BR-7800-2249A_v1_m56577569830609052.pdf

Part 4 : pivot bolt 52.5mm for front and 37.8mm for rear

Eg. if we say the rear pivot bolt is 15mm then the front should be 30mm .. I'm only guessing these numbers as I can't be bothered to go measure a pair.

I realise that you have a Dura Ace and Mirage to compare but even across brands there will be only small differences in the lengths. One scenario is someone's taken a saw to the Dura Ace pivot bolt .. I'd hazard a guess to make it fit better in a steel fork crown or a track bike brake adapter?

looking at it I think you're right, seems like the dura ace has been sawn...
I guess I will just buy a new front calliper.

thanks for all the help guys

SJS have spares, although the DA bolt kits are a bit pricey (relative to the cost of a new calliper).

I guess you could use Sora or Tiagra etc, but Shimano is a foreign country to me so seek advice before shelling out.

You'll also find recessed nuts in every size you could ever wish for!