Bolts for rear sprocket

[PEZBerq]

Getting sick of stretching the so called high tensile M8 bolts from my local nut & bolt joint when installing rear sprockets    Twice now I have had to cut them off due to stretched thread causing the nut to be impossible to unscrew. What grade of bolt are people using? My manual say to torque to 60Nm or 44 ftlb. These 8.8 grade can't handle that - they struggle at 40Nm without stretching  . Is this torque excessive. Bolts were new and clean.  Where can I get decent bolts from - I need M8 x 35 hex head bolts  - preferably shouldered. Thanks.

[Davey Crocket]

Buy  Talon ones or genuine.

[firko]

I use stainless bolts and have never had any such problems.

[Nathan S]

Countersunk heads or plain?

I've got a small pile of countersunk M8 x 35 Grade12.9 bolts - I can easily spare half a dozen if it helps you. Regardless, they should be available through your local bolt shop, even if you have to buy a box of them.

Ballards sell the 'proper' type with the bulged head that take a 6mm Allen Key, but they're fairly dear when you include postage. Your local bike shop can probably get them.

[flower pot racing]

has anyone tried the grade 5 titanium countersunk allen bolts that are available in the US?  The say grade 5 is ok for 8.8 steel applications, but what ones with higher tensile requirements ie to stop them stretching....! any experience... anyone....?

[Lozza]

Are the bolts threaded all the way or do they have a shank? Is the bolt a through bolt or captive in the hub? The force required to deform a 8.8 M8 is a hell of a lot and to deform a 12.9 bolt is 20% more.
The material has little relevence on resistence to deformation, it is about the cross sectional area. So to deform a steel v's Ti bolt will be roughly the same.

[flower pot racing]

excellent.  many thanks for the responses.  I have no issues using grade 5 ti, but when some OEMs quote more than 8.8 it is always better to be on the safe side. That said you cant ignore the physics, it'll take a huge amount of energy to damage a set of bolts.

And yes, they have a good shank length before the thread.

[PEZBerq]

Are the bolts threaded all the way or do they have a shank? Is the bolt a through bolt or captive in the hub? The force required to deform a 8.8 M8 is a hell of a lot and to deform a 12.9 bolt is 20% more.
The material has little relevence on resistence to deformation, it is about the cross sectional area. So to deform a steel v's Ti bolt will be roughly the same.

Lozano the bolts are hex head machine screws - fully threaded. They seem to deform at the nut and the nut spins without unscrewing. Maybe it's the nut that is deforming? I tighten using a torque wrench and it is easy to load these bolts to cause the thread to strip. I will try locating proper sprocket bolts and see if it is the nut that is the problem - Classic Fastners supplied bag of nylons. Cheers

[Davey Crocket]

Read the second post again.

[Lozza]

Lozano the bolts are hex head machine screws - fully threaded. They seem to deform at the nut and the nut spins without unscrewing. Maybe it's the nut that is deforming? I tighten using a torque wrench and it is easy to load these bolts to cause the thread to strip. I will try locating proper sprocket bolts and see if it is the nut that is the problem - Classic Fastners supplied bag of nylons. Cheers

The load should never be bourne by the thread on a bolt, only on the shank, also threads are undersize so the thread on a M8 bolt is only 7.8mm.The shank is on size. Using counter sunk bolts will give a greater area for the clamping force to be spread over.
Do you have a micrometer or click type torque wrench?

[Nathan S]

Is the sprocket to hub mounting face clean and flat?

The bolts' main job is to clamp the sprocket to the hub, rather than to directly transfer the load through shear forces.
Its a basic engineering rule that bolts should be loaded in tension rather than shear...

[DOK]

Hi PEzberg
You need to get a workshop manual and check torque specs. 44ftlb is way to much for an m8 thread.
You shouldnt need to go more than 20ftlb. If they are coming loose get nyloc nuts or use loctite.

DOK

[yamaico]

The material has little relevence on resistence to deformation, it is about the cross sectional area. So to deform a steel v's Ti bolt will be roughly the same.

Sorry Lozza, wrong way around. The material is what determines the deformation per unit of load.

[PEZBerq]

DOK you are spot on. I had misread the manual and was looking a the rear axle torque  Sprocket bolts are 22ftlb. Wasn't wearing my specs when in the garage.....never had to before but I need them more and more these day  I will take the advice about getting some proper bolts though just. The same. Thanks all for the info and suggestions