12.9 vs A2.70 bolts

[moribundman]

Can I risk replacing M5 12.9 high tensile steel socket head bolts with M5 A2-70 stainless steel socket head bolts?

A2-70 has 700 N/mm^2 tensile strength.
12.9 high tensile strength steel is rated 1,200 N/mm^2

I have no idea if I need the high tensile strength bolts in this particular application. It's a bottom bracket from a bike. Stainless steel would be nice, if it's durable enough. As you can see, the bolts really only hold the cap on. The cap fits pretty tightly on the bottom bracket (It's really a combined bottom bracket and transmission housing). A smaller, but similar cap goes on the other side of the bottom bracket.

In the picture below, the cap is not pushed into place all the way:

 

[Pablo]

In this case, likely. Were the stock ones corroding?

 

[moribundman]

Instant corrosion by the sea! I live right by the beach. Anything that can rust will rust within about 3 minutes.

 

[Papa Bear]

Get a chain ....

 

[XS650]

Originally Posted By: Pablo
In this case, likely. Were the stock ones corroding?


+1

The originals were prolly high strength because that's what socket head cap screws normally are rather than because the strength was needed.

If they speced a torque, check it to be sure it isn't over torqueing the stainless screws.

 

[moribundman]

I have already used the A2.70 steel bolts for over one year without noticeable issues. I just wanted confirmation that they were suitable, so that I can decide on permanent replacement of the high tensile strength bolts with a bit more peace of mind.

The manual does not call for a specific torque. Since the bolts thread into Al (very hard alloy, though), I am obviously very careful when tightening them. I do use blue thread locker there.

 

[moribundman]

Originally Posted By: Papa Bear
Get a chain ....

Every 3 weeks?

 

[XS650]

There is a galvanic couple between aluminum and SS, you should consider anti-seize.

The aluminum is more active, so the aluminum thread will corrode away to protect the stainless screw. Probably not what you want.

 

[moribundman]

I'm not sure anti-seize would prevent the bolts from coming loose. Won't thread locker prevent galvanic corrosion issues at least somewhat?

A2 steel is barely SS, do you really expect much galvanic corrosion?

The other solution would be to use the high tensile strength bolts, but to paint their heads after installation.

 

[XS650]

I didn't know you had a lose fastener problem.

If you don't have one a lose fastener problem now, I doubt that anti-seize will cause one. Just check the screws for tightness daily for a few day until you either find a problem or find there isn't one.

A generous application of weak thread locker works too. Weak because you don't want to abuse the aluminum by using strong locker every time you do maintenance

If you Google around you can probably find out what the galvanic couple is between your SS and an educated guess at what the aluminum is.

 

[moribundman]

I don't have a loose fastener problem -- maybe because I do use thread locker? The blue stuff is weak enough to not cause harm to the threads in the Al, I believe. At least it hasn't been an issue so far. The bottom bracket is not an area where I really can afford a bolt to come loose. I wish there were more fasteners than just three, but I have to make do with the design as it is.

 

[mechtech2]

Stay away from brass.

 

[moribundman]

Where do you see brass?

 

[tom slick]

regular SS fasteners are terrible, they like to gall and are seemingly "soft". since it is fastening aluminum it will probably be okay.

is tensile strength really an issue with that assembly?

 

[moribundman]

I don't believe relatively low tensile strength to be an issue there, but I am not an engineer. Should I got for Ti bolts?

What would be the ideal material?

 

[tom slick]

Ti is a worse material for fasteners. SS will probably work just fine for your application but it is not a great fastener material unless it is aerospace grade stuff. standard steel is the best fastener material.

 

[moribundman]

What SAE standard do you consider to denote standard steel?

 

[XS650]

Originally Posted By: tom slick
regular SS fasteners are terrible, they like to gall and are seemingly "soft". since it is fastening aluminum it will probably be okay.


The A2-70 he is using is supposed to be 700 N/mm^2 tensile strength. That's equivalent to an SAE grade 5 fastener. At least it isn't the shiny chewing gum that you normally get when buying shiny bits for your car.

 

[Papa Bear]

"What SAE standard do you consider to denote standard steel?"

HRS ...

 

[moribundman]

For the car I use at least SAE grade 8.8 hardware. Hence my concern about the SS bolts (Uh, I should phrase that better) on the bike.