Metric bolt sizing.

Very quick query: I need a allen bolt with a 4mm socket with 10mm of thread and just need to check it is a M4x10 that i order.

Mucho thanks

no.

m4 relates to the diameter of the thread – somewhat unrelated the the allen key size.

HTH

4mm hex will be a M5 bolt

Sounds like m6x10 bolt you req (also you may need to check thread pitch?)

What’s it for? Most bolts on bikes are M6, but often a bolt with a 4mm head will be an M5.

You need to measure the thread diameter. a 4 mm thread diameter is M4. 4 mm allen socket head is probably M6 but not always

Almost all metric bolts are the same thread although there are two sizes – fine and normal

Yay!
4mm diameter metric, so M4 x10.
2 lovely (!) new Ti bolts courtesy of local fastener company