Published by Tristan on 16 Apr 2009 at 07:41 am
Torx fittings – a word of warning
Torx fittings are becoming more and more common on bike components. FSA was one of the first companies to use this style of fastener and now SRAM / Truvativ, Campagnolo and Shimano have jumped on the bandwagon. It started with a few rotor bolts, now most chainring bolts are Torx, shifters are being held to handlebars with them, and bleedvalves use them.
Personally I don’t see the point: Allen head fasteners work just-fine, we’ve all got multi-tools with 4mm, 5mm and 6mm allen keys, and lugging around a T25, T27 and T30 as well doesn’t make much sense. There are upsides to Torx fittings: Both tool- and fastener-life is increased.
Putting that aside there is one caution when using Torx fittings – make sure you have the correct size tool! With allen head fasteners it’s quite clear that a 4mm allen key will not turn a 5mm head but with Torx it’s not so clear: a T27 will fit quite securely into a T30 but problems will only occur once a bit of torque is applied and the head of the fastener deforms.
To illustrate this I set up a bit of an experiment: An old FSA T30 chainring bolt, a T27 socket, and a torque wrench.
First lets take a look at how securely the different size sockets sit in the chainring bolt:
T25 - clearly a loose fit in the chainring bolt
First the T25 socket: This slides into the T30 chainring bolt but is quite clearly not the correct size.
T27 - The wrong size but it feels like a good fit
Second the T27 socket: This is where the problem lies. The T27 fits snugly into the chainring bolt but is not the correct size and attempting to apply torque to the chainring bolt will deform and destroy the chainring bolt.
T30 - The correct size
Third the T30: This is the correct size socket for the chainring bolt.
If you’re not sure what size Torx fitting you’re attempting to work on my suggestion is to start with a larger socket and work downwards in sizes until you find the correct size. This will eliminate the risk of selecting the wrong size socket and destroying the fastener.
Out of courosity I applied torque with the T27 bolt to see what the failure point would be – at around 20Nm the chainring bolt deformed. This is more torque than should be used to tighten a chainring bolt but much more torque than this might need to be applied to loosen a stuck bolt.
Chainring bolt failure at 20Nm
Related posts: