I’ve almost completely rounded my Headset Bolt (The one which goes through the top cap) does anyone know how I get it replaced ?

There are various options around but how do I find out which is compatible ? Are the all a standard size ? Mine requires a 4mm Allen Key but other than that how do I find out what type of Bolt it is ?

This may sound like a Beginner question, but that is exactly what I am when it comes to Mechanics.

Are you sure it takes a 4mm allen key? Normally it’s an M6 bolt that takes a 5mm allen key.

Try a brake caliper bolt, I’ve used one before in a pinch.

Are you sure it takes a 4mm allen key?

I’ve got both on different bikes at the mo.

£1 Halfords including the cap.

It’s generally an M6 thread, usually 25mm long, and if it takes a 4mm hex-key, it’s probably a countersunk type. Probably better to get a stainless one so it doesn’t rust up, and coat the threads and under it’s head as they tend to stick,causing rounding-out , if you don’t.

Yes it’s a 4mm

My other Bike is a 5mm so it just shows that they vary

More to the point, why/how did it round out?

That botl should be used for preload *only*, ie: not very tight, if you’re doing it up so tight as to round it then you’re either working against an already tightened stem (ie: doing it wrong), or you’ll be putting so much load on the bearings you’re actually likely to be damaging them.

Anyway, unless it’s a fancy proprietary part like for a Hope Head-Doctor or a weird carbon bung then it’ll be a bog standard M6 thread. Normally with a 5mm socket cap head, but if your existing one is 4mm then possibly you have a top cap with a countersunk type fitting like Specialized/Canyon/others are fond of fitting?

Best bet if you’re not sure is take the old bolt with you to the shop to compare, or post a pic…

wot amedias said.  It probably needs as little torque as anything on a bike, save maybe the dust caps!

best ever

Whats the betting that the Local Bike Mechanic has had a breaker bar on it to “adjust the bearings”.

For heavens sake! this is STW, therefore you will be needing a titanium one, possibly artisan produced,

http://www.competitionfasteners.com/

Bolts operate in both directions, if it’s “stuck” on the countersunk interface, as often happens with them (see my previous reply), then the relatively shallow hex is very easily rounded off when trying to loosen it, no overtightening needed in that case.

Not sure how I rounded it. Using a worn-out multi tool did it I think.

Just gone and bought a couple of M6’s they were just £1 each from Halfords. Not had the chance to fit yet because the weather is appalling.