Ok to use A2 stainless for brake disc bolts?

Reppyboyo · Nearly there... Joined: 15 Sep 2006 Karma :

Hi,

Im thinking of getting new disc brake bolts (and possibly caliper bolts) as my current ones are getting rusty/furry.
I`ve heard however that A2 SS doesn't have the tensile strength for these areas?
Is that true?
Also heard the same for Titanium but found much less on that.

Any one know for certain?

uniuk2000 · Borekit Bruiser Joined: 11 Jun 2008 Karma :

Ichy · World Chat Champion Joined: 15 Jul 2005 Karma :

On paper A2-70 stainless has less tensile strength than plain old carbon steel. The highest rating I've seen is 80 which is comparable with 8.8 in steel. There is an 100 grade but never seen any for sale. No idea what the current rating of your bolts are?

Ignore the A2-A4 bit, that just relates to corrosion resistance.

I have problems with stainless bolts. A lot of cheaper bolts are sourced from Bulgaria or similar so quality control is a bit random. The other problem is that while steel stretches and bends stainless just snaps. Which brings the last bit in to context. For all the talk about tensile strength caliper and disc bolts work in shear stress. Might be a bit out in my figures but the shear strength of stainless is 14% of its tensile strength while steel is about 84%.

Titanium is great if you are sure of the quality and have the money to pay for it!
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s44678 · Guest

Whip the tatty ones out, put them in a electric drill (not too tight) then spin 'em up and wet/dry the crap off them first, polishing them up with autosol after. Unless they're properly corroded they should come up pretty nicely.

Saves the cost of replacements and the worry of getting the wrong grade of bolt.

motobiker · Nitrous Nuisance Joined: 12 Dec 2008 Karma :

Owners have been fitting stainless bolts to the front discs on the GS for years.. A4 though - mainly because its completely impervious to road salt. (or indeed a dip in the sea)

In fact for the GS most stainless kits will be A4 - cause we don't do things by halves. Laughing

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Sooner or later opinions fade and the name on the tank matters not. I think that happens somewhere between 3rd and 4th gear. Enjoy the ride... everything else takes care of itself.

sickpup · Old Timer Joined: 21 Apr 2004 Karma :

The problem I have with Stainless disc bolts it that the originals are shouldered with a shoulder wider than the threaded area. Most replacement ones aren't, they are just standard bolts which don't supply the proper support to the disc.

Note the price of a single disc bolt on the Inox site is around £4 with a wide shoulder. Compare that to the price and pictures of the ones on Ebay, quite shocking.

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

I for one wouldn't risk it.

The torque settings for a bolt working in tension are all worked out to stretch the bolt by a certain amount. When stretched by this amount and holding two pieces of metal in compression, they won't undo.

The grade and number of bolts is selected to make the degree of compression such that the two pieces of metal behave like one piece. The bolts are NOT acting in shear, they are acting in tension.

With a grade 8,8 bolt, they should be tightened to 80% of the plastic phase on a material with a tensile strength of 800N/mm^2.

With a 12,9 bolt they are tightened to 90% of the plastic phase on a material with a tensile strength of 1,200N/mm^2.

In other words, when properly torqued, a 12,9 bolt is 90% of the way to being broken.

So, how tight do you do up your stainless bolt so it a) Won't come undone and b) Compresses the metal enough so it isn't acting in the shear plane?

I sure as hell don't know and two things I am absolutely certain about are a) I don't want my disc or calliper bolts coming undone and b) I don't want stainless bolts acting in shear.

The strength of the material it is going into is largely irrelevant to the choice of bolt. The thread pitch selection and hole depth on the wheel hub ensures there is sufficient metal "inside" the threads to take the required forces.

Calliper bolts screw into the calliper bracket, not the fork leg. The fork leg just gets compressed between the bolt and the calliper bracket.

Make your own decision.
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