I’ve tried google-a-saurus-ing this a number of times, but am still lost. I just want to know how wide the actual thread is.

As I take it – an M5 allen bolt, for example, is 5mm diameter at the thread bit. Is that correct?

So, an M4 would be 4mm across at the threaded bit, yes?

Or is that entirely wrong?

Ta! 🙂

M4 = 4mm

M5 = 5mm

etc.

(thread diameter)

M5 = 5mm dia, uses 4mm allen key
M4 = 4mm dia, uses 3mm allen key

More here at Roymech

It’s a bit more accurate than that, everything from the diameter of the bolt without the thread, with the thread, pitch of the thread and even the shape of the thread is defined to a tolerance.

But yes, broadly, take a 4mm rod, cut a thread on it with an M4 die, and it’ll fit into an M4 hole.

That’s a very useful link there, Bandito, thankyou. :P)

Cheers! …I was getting completely swamped by thread pitches and this and that.

Obvious and simples!

<Edit> You see, there you go – Rorshach. Bloomin’ engineering types! Just want a simple answer so I can measure a bolt with a ruler and order the correct one! 🙂

M5 = 5mm dia, uses 4mm allen key
M4 = 4mm dia, uses 3mm allen key

Except the thread size doesn’t define the size of the tool used – I have M5 bolts which take a 3mm AK (not to mention those with screwdriver slots).

Except the thread size doesn’t define the size of the tool used

This.

As an aside… many bolts on a bike are M6.

Bottle cage, shifter and brake lever clamps more commonly M5.

I have M5 bolts which take a 3mm AK

those dims up there ^ are for standard socket head cap screws. Button head screws <spits> tend to be a size down.