Stainless Disc Bolts

[Snowman]

I am in the process of updating and rebuilding my front brakes on my '93 K1100LT. Is anyone using stainless steel fasteners to secure their brake discs to the wheel??
Motobins in the UK only sells SSteel for K16V disc bolts. Doing some research on the web (must be true) some frown on using SSteel. Obviously, there are others that swear by them and have done so for years with no problems.

[The Mighty Gryphon]

Stainless fasteners are not as strong as steel, especially hardened steel fasteners.  Shock loading in shear can result in breakage.  I have had problems in that type of application, and would be wary of using stainless for things like brake rotors and caliper mounts.  Stressed parts like shock mounts, engine mounts, and drive line assembly are places  I would never use stainless.

For things like assembling fairings and engine cases I have no problem using stainless.  One thing to consider is that stainless in aluminum can create some corrosion issues making disassembly difficult.  I always use a copper anti-seize when putting things together.

Besides being weaker, stainless also is subject to a thing known as crevice corrosion that results in weakening in places where it can't easily be seen.  This can result in parts becoming even weaker still without the owner knowing it.

[Snowman]

Thank you for the info. It's interesting that a vendor such as Motobins offers them for this application. It seems it divides opinions with some owners swearing by them. I have read in other places the very points that you bring up so I'll stay with a new set of factory fasteners.

[Martin]

Snowman there used to be a chrome paint available in pressure pack. I did try it about 15 years ago and was disappointed with it, it didn't seem to dry and would rub off. Whether that was the nature of the product or I had a bad batch I don't know. It did look pretty good on my mesh screens, which I replaced with aluminium. At the time I was going to try clear coat over the top, but I don't know whether it would have solved the problem. Might be worth a look, experiment on something. Do let us know if it is successful.
Regards Martin.

[Snowman]

I might try that Martin. I'll let you know when it's all done. Waiting of forks to come back from rebuilder.

[Glacial]

This topic intrigues me. There are many grades of stainless steel, each formulated for different levels of anti corrosion and toughness. Stainless is used extensively in heavy marine engineering applications so I would have thought there is something suitable for motorbikes. I have a lot of grade A4 fasteners on my K75 and I'm steadily working through replacing as many as I can. I would welcome a definitive guide from someone with a solid metallurgy/engineering background that says whether I am being an idiot or not.

[The Mighty Gryphon]

Nothing wrong with stainless, but you have to be careful what grade you are buying.  The OEM hardware looks like steel with a zinc/aluminum.  The tensile strength of these fasteners is 170,000 PSI.  The typical 18-8 stainless known as A2 that is sold in hardware stores and on the internet has a tensile strength of 70,000 PSI or about 40% of the OEM strength.

It is possible to find high strength stainless fasteners in 17-4 PH stainless.  They have a tensile strength of 130,000 PSI or about 75% of the OEM strength.  There is a grade of stainless A286 that has a tensile of 160,000 PSI, but the prices are about double of other stainless fasteners.

It can be hard to find high strength fasteners, and even then, can you be sure of what you are buying unless or even when you pay premium prices.   Personally, I would only buy from a reputable industrial supply house for any highly loaded hardware on my bikes.

All of the data above is from the McMaster-Carr Industrial Supply catalog.

[bocutter Ed]

The tensile strength of these fasteners is 170,000 PSI.

Of course it goes that if your fasteners are not OEM, and of lesser tensile strength, you'll have to rethink all the torque specs in the manuals ...

[Glacial]

Of course it goes that if your fasteners are not OEM, and of lesser tensile strength, you'll have to rethink all the torque specs in the manuals ...

I don't see that logic. If the bolt can't take the recommended torque (with a sensible margin) then it should definitely not be used as a replacement for the OEM. Whatever the material of the fastener, you still want it to clamp the two parts together with the same force surely?

[Filmcamera]

Whenever I use stainless I make sure to use lots of copper anti-seize, especially if going into aluminium.  I do still use the same torque settings though, I agree that the correct level of tightness is the correct level - no matter what fastener is being used.

[Snowman]

Torque settings are calculated for the type of material the fastener is made from. This is based on how much the metal stretches before breaking. A stainless steel bolt will have a different torque setting than one made of regular steel.

[Filmcamera]

Torque settings are calculated for the type of material the fastener is made from. This is based on how much the metal stretches before breaking. A stainless steel bolt will have a different torque setting than one made of regular steel.

Most of the torque settings on my bike, especially on things like engine covers, swing arms etc are very low values, often in the 5-12 foot pound range, seldom more than  30.  I really don't think stainless bolts are going to be suffering at these levels. Sure 150 ft pounds for the clutch nut I wouldn't go stainless but for most of the external visible  bolts I think it is pretty safe.

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[Glacial]

Torque settings are calculated for the type of material the fastener is made from. This is based on how much the metal stretches before breaking. A stainless steel bolt will have a different torque setting than one made of regular steel.

OK, understand that.
I have been getting my bolts from the following UK supplier: http://www.westfieldfasteners.co.uk/
using their A4-80 grade which has a tensile strength of 800 MPa or about 116000 psi which puts them at 68% of OEM on that measure. So I am going to be more cautious about where I use the stainless.


Reading further on the subject, I see that elongation and yield strength are treated differently between stainless and regular steels. That presumably makes it hard to just scale torque figures based on tensile strength.


Thanks to everyone for their contributions to this thread. I too am going to stick with OEM for things like disc bolts.

[D.Bachtel]

Torque is nothing more than an estimation of the rotational force required to reach an engineered percentage of yield.
Yield is the point at which a fastener under load will not return to it's original referenced length when unloaded.
 That is 100% of Yield. Ultimate strength is the point during rotation force at which the fastener will break.

Most assemblies require an engineered load of 40 to 60%. Some, like drive components, a little more. Some much less.

KDPYSTAS is the formula for all fasteners but there are tables to determine the correct values to imput for any given material.

[Snowman]

I agree. No problem using SS fasteners on low or non load bearing assemblies. High load applications such as braking and suspension might best be left alone.

[wmax351]

Also keep in mind that marine, etc applications that have load bearing stainless hardware are going to have the lower strength factored in when designing the parts.


Much of the original BMW hardware is cadmium coated. Not sure if this persisted to the K bikes. But either way, they are high strength with a coating to prevent rust. You could add additional coating, such as chrome or nickel plating, to the relevant parts, such as the heads or nuts. It's actually pretty easy to do DIY electroplating.

[The Mighty Gryphon]

Regarding the anti corrosion plating of factory fasteners, I suspect that an aluminized/zinc coating was used on the K bikes.  Rated to be about 10x better than zinc. 

If I recall correctly, cadmium plating has a yellowish tint to it.  Seems like I have seen it on alternator and starter terminals.   Cadmium is pretty toxic, so I would guess that it is used as little as possible. 

[Snowman]

Ordered a dozen new BMW bolts for my rotors. They had been off & on a few times so installed new ones, I did spray the heads a bright silver and my disk carriers gold just to be different. I'll send a pic once the bike is back together.

[alexis291]

To answer the original question, I am using stainless bolts on my brake discs. Bear in mind that the load on these bolts when in use is shear, they are not loaded significantly in tension other than that created by the torque in fitting them. As they are fitted into aluminium, the torque setting is low. Another consideration for those that lubricate the bolts is that the load on the bolt (and the thread in the wheel) is increased when using oil, copper slip etc because of the reduced tightening friction. Torque settings should  properly be given with an instruction whether to lubricate or not, and with what.


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[technostructural]

I like to stick with what my uncle (who has taught me a lot about mechanics and carpentry) said about stainless steel:


"Stainless is great for not rusting, but it's pretty f***ing terrible for everything else".


The high heat application in this case may be a cause for concern as well. There are others on this forum that are professionals in these sorts of fields, but I believe that stainless bolts/nuts have a tendency to kind of "weld" themselves together.

[Martin]

I used to work with stainless steel tube fittings (Swagelok) going into stainless steel and we had to use anti galling compound to stop the galling. If you didn't use it, it would be impossible to remove the fittings without destroying them.
Regards Martin.

[alexis291]

I like to stick with what my uncle (who has taught me a lot about mechanics and carpentry) said about stainless steel:


"Stainless is great for not rusting, but it's pretty f***ing terrible for everything else".


The high heat application in this case may be a cause for concern as well. There are others on this forum that are professionals in these sorts of fields, but I believe that stainless bolts/nuts have a tendency to kind of "weld" themselves together.

High heat? Not really.
The application is stainless into aluminium.


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[Laitch]

High heat? Not really.
The application is stainless into aluminium.

I've lost the train here. What is not really hot?

[alexis291]

To clarify my comment about temperature:
Unless you are on a race track, brake discs will typically operate at 100-200 deg C in their hottest areas (i.e. where the brake pads touch them). They will be cooler in areas where they are bolted to the big heat-sinks  that are the wheels. This temperature isn't significant for a steel bolt, stainless or otherwise. After all, piston rings operating at 800-900 deg C can be made of stainless steel.


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[Laitch]

To clarify my comment about temperature:
Unless you are on a race track, brake discs will typically operate at 100-200 deg C in their hottest areas (

A mountain descent can double those values, too.