My M900 Project...

[Mr Gazza]

Ibanezlynx, Please don't be offended by our comments about your bolt heads..
It's a smashing re-build in every way...I think we've been quite rude.. Sorry.

Apologies also for hijack.
I personally wouldn't use those A2-70 bolts Darren. I tried A2-70 button heads on my discs, but chewed the heads out before I reached 25nm.
I asked the supplier if I had chosen the wrong bolts for the job.
They said this;

"Hi Gary

Thanks for your email and I'm sorry about the problems you appear to be encountering with the A2-70 bolts.

Firstly, may I express a word of caution in using stainless steel bolts for strength-critical applications such as motorcycle brake discs.
As I'll outline below, the tensile strength of standard stainless fasteners is significantly inferior to carbon steel fasteners.

I've just looked up the torque settings for M8 A2-70 bolts which is 17Nm, so you would be very likely to round off the sockets if torqing up to 25Nm.

There is also a common misconception that A4 stainless fasteners are stronger than their A2 equivalent and have a similar strength to 8.8 carbon fasteners.

The difference between A2 and A4 stainless is not its mechanical properties but its chemical composition. A4 is generally for marine applications where its extra corrosion resistance is required for the constant splashing of corrosive salt water. A2 provides sufficient corrosion resistance for most applications.

Our A2 fasteners are A2-70 grade. The 70 signifies that the Ultimate Tensile Strength is at least 700 N/mm^2. More importantly: the stress at which A2-70 begins to yield is at least 450N/mm^2. A4-70 has exactly the same ultimate tensile strength and yield stress.

Standard carbon steel fasteners are generally 8.8 grade. The first figure signifies that the Ultimate Tensile Strength is at least 800N/mm^2. The second figure signifies that the fastener will begin to yield at 80% of the Ultimate Tensile Strength, ie at least 640N/mm^2.

To get similar strength stainless fasteners you would require A2-80 or A4-80 where the yield stress is 600N/mm^2. There are significantly fewer of these on the market as A2-70 are sufficient for the majority of applications. In practice fasteners generally fail due to the excess load applied to them when they are assembled not when used in operation.

So, if you are replacing 8.8 carbon steel fasteners and you are certain you want to use stainless products, then I would recommend that you purchase our A4-80 fasteners for your disc bolts.

I hope that this makes the situation clear. Please email me if you have any further questions.

Many thanks

Matt Naish "

I couldn't find A2-80 button heads. He came back with this;

"Hi Gary

No problem at all.

I guess that's the main problem you'll face:- socket button screws are the best fastener type for the job but they're very hard to come by in A4-80 (A2-80 is almost impossible to find).

As you suggest, this leaves you with cap screws as the only real option from us (I'm not sure if the heads will clear the fork/brake assembly on rotation?) or you can risk using A2-70 or A4-70 as many other motorcyclists do!

I'm certainly extremely scepticle of doing this (hence my previous email!) and I'm not going to recommend doing this, but it's not at all uncommon.
In fact Inox Fasteners advertise A2-70 specifically as brake disc bolts!

My best suggestion would be to try specialist motorcycle fastener suppliers to see if they have sources of the less common bits like A2 or A4-80 button heads, or order the cap heads from us to test them as you can always return them for a full refund if they're no good.

I'm sorry that I'm not able to help more.

Many thanks

Matt Naish

Westfield Fasteners Ltd
www.westfieldfasteners.co.uk
enquiries@westfieldfasteners.co.uk
01844 201133"

In the end I used high tensile steel 10.9 BZP button heads.
With a light polish they look like s/s.