Question about bolts

[marcmax]

I am, little by little, trying to replace various bolts on my R65 with stainless versions. No major replacement just when I am working on something if I have the same size in stainless I will replace it and the nearby ones.

My question, for all you engineering types, is there a difference in holding power, torque ability, etc. between same size bolts with different types of heads? I areas that show I like to use button head socket cap screws. Am I sacrificing safety for a detailing whim?

[georgesgiralt]

Yes, There is !
On every bolt BMW use is a rating stamped. Most are 8.8. some are 10.9 and some are 12.9.
8.8 is the weakest and is waaaaay stronger than any stainless I know.
So replacing any 8.8 rated screw by a stainless screw is a very bad idea. You can easily imagine that it is worse for 10.9 or 12.9 ....
In some case, the stainless can do the job but had to be torqued differently to meet the pressure on the joint. So it is difficult to replace without thinking about the application.
Sorry about that.

[Barry]

The bolts critical to safety should be replaced with an equivalent that has the same tensile strength. BMW uses bolts that are a minimum class 8.8 which means they have a tensile strength of 640 N/mm² before taking a permanent set. The ultimate yield strength is higher still.  That just means they stretch a bit before breaking.


Stainless bolts use a different classification. I'm no expert but I think the very best stainless bolts will just match a carbon steel 8.8. You need to look for Class A2-80.  A3 is the type of stainless and 80 is the strength class.

[marcmax]

Thanks guys. I will limit my replacements to cosmetic, low stress areas and keep the rest in place.

[Bob_Roller]

I don't know how well they hold up, but there are paint products that have an appearance to cadmium plating .

If you want to 'freshen' up your exposed tarnished, rusted hardware .

[Adrian]

Hi Marc - you could buy from Motorworks in the Uk ( who i now understand are shipping to Nth America). I bought a complete fasteners kit for my R65 as I'm rebuilding. Of cousre you can buy wjhetever you need seperately instead of buying a complete set.

[marcmax]

As long as we are talking bolts, is it allowable to reuse the bolts that hold the front rotors on when replacing the rotors or should they be replaced?

[marcmax]

The reason for wanting to use stainless hardware is that living near the coast there is a high salt content in the air and corrosion is a never ending battle.

[tvrla]

I've never replaced rotor fasteners and don't see a problem with re-using them. I don't think they need to be torqued too tight anyway as the forces applied to them aren't lengthwise but in shear. Normally they utilize nyloc nuts, which might be for added insurance they don't back out even if loose, but I've taken it to mean they're not torqued down very much in the first place.

As for the topic of conversation - there are different grades of stainless. The alloy that's the least corrosion resistant and not as shiny is the strongest. The more nickle and chrome in the alloy, the more brittle and less strong, but the prettiest and corrosion resistant. If you can choose your alloy, go with the strongest.

That said, I've used stainless bolts from the hardware store for shock mounts and panniers with no issues. I've even used stainless to retain dual discs. I've seen stainless axles, but don't think I'd go that far.

[Barry]

I've heard of stainless axels too but can't think what grade they were. I remember a metalurgist (who was also the manufacturter) arguing the correct grade was OK. Stainless is a topic all to itself.

Personally I wouldn't use one of these bolt kits without knowing what grade they were and checking it out. My guess and it is only a guess is they would be 304 grade which is the A2 designation and as you say not the most corrosion resistant and hopefully the highest tensile strength class which is 80.

[steve hawkins]

I would stick to cosmetic only and try to clean up your 'beefy' bolts.  Beefy = Things like the bolts to mount your calipers or hold on your fork yokes, etc.

I had this conversation with my local nut and bolt shop, some time back, where they recomended the above approach.  

Cheers

Rev. Light

[k_enn]

As others said, the grade of the bolt is important.  This is in two respects.  First, you want one that is of sufficient grade not to break.   Second, you don't want one that is too high of a grade that it does not have enough give for the given application.  (I used too high a grade on the wheel hub on my 4x4, and they were very prone to breakage).  I highly recommend using the OEM grade.

Also, keep in mind that the metric grading is different from the English/Standard grading.  (IIRC, a metric 8.8 is the equivalent of an English 5).  Since the bolts you will be using are almost certainly metric, that should not be a problem.   Just don't expect a metric 8.8 to be anywhere near an English grade 8 in terms of strength.

k_enn