M8 A2 Stainless Steel Socket Cap Screws (Allen Bolts) DIN 912 / ISO 4762

M8 socket cap screws in A2 stainless steel, also sold as M8 allen bolts and M8 cap head bolts. The cylindrical head has a hex socket in the top, so a 6mm hex key tightens them from straight above and the head can sit down in a counterbore. Dimensions follow DIN 912 / ISO 4762.

The M in M8 is the thread diameter in millimetres, and it is not the hex key size. An M8 screw has an 8mm thread but takes a 6mm key, so check the socket size in the table before you buy a key. The full M8 cap head bolt dimensions are also in the downloadable PDF spec sheet.

There are 28 lengths, from 10mm to 150mm. We have no minimum order, whether you need ten screws or a full box, and the price per screw falls as the quantity goes up.

Where M8 socket cap screws are used

The head is only as tall as the thread is wide, so an M8 head stands 8mm proud. That makes socket cap screws a good fit for machinery, tight spaces and jobs where a hex head and spanner will not fit. They are commonly used on motorbikes, kit cars and model making, and across general engineering.

Choosing the length of an M8

Length is measured under the head, so an M8 x 20 reaches 20mm into the part and the head sits on top. The range runs from 10mm to 150mm in 28 lengths. Up to 35mm the thread runs the full length of the screw.

From 40mm the screws are partially threaded, with a plain shank under the head and at least about 28mm of thread at the tip. That figure is the standard's reference length of 2 x the diameter plus 12mm, and it is the same at every length. On a partially threaded screw the plain shank is the part that lines up with the top plate, and the thread does the gripping in the bottom one.

M8 is property class 70, like every size from M5 up. M4 and the smaller sizes are class 50.

A2 stainless steel, class 70

A2 is 304 stainless steel, also written 18/8. It resists rust in damp and in ordinary outdoor use, which is why it is the usual choice for general work. M8 in A2 is property class 70, a minimum tensile strength of 700 N/mm².

Near salt water, pools or chemicals, A4 stainless lasts longer because it contains molybdenum. The same screw is available in A4 stainless and, for higher strength, in 12.9 high tensile steel. Class 12.9 has a minimum tensile strength of 1220 N/mm², but it offers little protection against rust.

Our A2 stainless steel and A4 stainless steel guides cover each grade in detail, and A2 vs A4 stainless steel compares them side by side. For lifting or other safety-critical joints, check the strength your design needs before choosing a screw.

Questions about M8 socket cap screws

What size Allen key fits an M8 socket cap screw?

A 6mm hex key. The M number is the thread diameter and is not the same thing as the key size, so M6 takes 5mm, M8 takes a 6mm key, M10 takes 8mm and M12 takes 10mm.

Is DIN 912 the same as ISO 4762?

ISO 4762 replaced DIN 912, and both names are still in everyday use. Suppliers and manufacturers tend to say DIN 912, while engineers often specify ISO 4762.

For an M8, the thread, head, socket and thread length figures in the table are the same in both. The standards differ in some tolerances and secondary dimensions that the table does not show. Our DIN 912 guide goes through the standard in more detail.

How long is the thread on an M8 socket cap screw?

Screws up to 35mm are threaded along the whole length. From 40mm, the thread is at least about 28mm long at the tip, and the rest is plain shank. The 28mm is a reference figure (2 x the diameter plus 12mm) and does not change with length.

How do I measure the length of a socket cap screw?

Measure from the underside of the head to the tip, with the head left out. An M8 x 20 is 20mm under the head, and about 28mm overall once you add the 8mm head.