M6 Screw, Cap Head Allen Socket Screw

We make M6 screws in stainless steel with a socket head, quoting length, thread and engagement to the drawing. M6 is the size most equipment joints land on, and the limit is usually the engagement and the plate, so the first article is tightened in your joint.

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ISO 9001:2015 · Manufacturing since 1998 · 8,678+ customers · 89 countries and regions

Made to order – there is no stock catalogue. Each part is produced to your drawing, and production starts after drawing review and first-article approval.

What an M6 Screw Is

An M6 screw is a metric machine screw with a 6 mm nominal thread, made in the socket head cap form on this page with a hex key drive. The part recorded here is stainless steel left plain, quoted with the length made to the drawing.

6 mm thread. A size that sits between the small fittings of an instrument and the heavier hardware of a frame.

Socket head. A cylindrical head with a hexagon inside, driven on the axis.

Stainless body. No coating to chip at the socket, and no rust where the joint is handled or washed.

They are used as the general-purpose fixing of equipment assembly, from brackets and covers to frames and fixtures. That breadth is why an M6 screw is often bought in more than one length and one head height for the same project, and why the drawing has to say which one belongs to which joint.

Why M6 Became the Default Size

An M6 screw sits at a practical point in the metric range. The socket in its head takes a 5 mm key, which is large enough to grip comfortably in a workshop and small enough to fit in a compact assembly; the head diameter gives a bearing face that spreads a useful clamp over a bracket or a plate; and the thread takes enough turns in a few millimetres of engagement to carry the loads most equipment joints see.

That combination is why so many drawings land on M6 without much deliberation. It is not that M6 is optimal for everything; it is that for a bracket, a cover, a plate and most internal frames it is sufficient, available and familiar to the people who assemble it.

Where an M6 screw stops being sufficient is worth stating, because the failure is usually not the screw breaking. On a cantilever the bending load grows with the distance from the joint, and an M6 head and thread can hold a load that the bracket around it cannot. On a thin plate the joint fails by pulling the thread or deforming the plate rather than by shearing the screw. On a joint that needs a high preload for fatigue, the engagement in a thin part is too short to develop it.

So the useful question is not whether an M6 screw is strong enough, but what it is holding. When the joint is a bracket on a solid part, M6 is generous. When the joint is a plate on a thin sheet, the sheet is the limit and the screw size makes little difference.

The head and socket then set the assembly method. A 5 mm key gives good control of torque, which makes a set preload possible; that in turn means the joint can be specified rather than felt, provided the tool is set and the thread pair does not gall.

Thread, Head and Engagement

The part is quoted to the drawing with the thread, the head and the engagement stated separately. Custom lengths up to 2,000 mm, subject to DFM review after drawing submission. The quotation for an M6 screw carries the length, the head and socket, the thread, and the engagement in the part it joins.

On an M6 screw the engagement is the dimension that decides whether the joint reaches its load. An M6 thread in a thin plate may have only two or three turns in contact, and that is what fails first, not the screw.

On an M6 screw the head height and socket depth are paired: a socket cut deeper than the head allows takes the material that carries the torque. On a standard cap head there is room, but on a low head version there is not.

The length is measured under the head, and it is chosen so the thread engages fully without bottoming in a blind hole. On an M6 screw the difference between the right length and the next size up is often the difference between a solid joint and a bottomed screw.

What an M6 Screw Gives and Costs

Four properties decide an M6 screw of this form, and each one carries a cost:

The value is a size that works almost everywhere without a calculation: strong enough for brackets and frames, small enough for covers, and driven with a key every workshop has.

The cost is that its familiarity hides the cases where the joint, not the screw, is the weak point. An M6 screw in a thin plate looks the same as an M6 screw in a solid block.

Plain Stainless on an M6 Socket Head

The recorded finish is plain stainless steel. On a socket head that keeps the hexagon at its cut size and the bearing face flat, and it avoids a plate inside a socket that a 5 mm key has to enter.

Surface Options for M6 Screws

Inside a socket, a coating is a fit problem. A 5 mm key in a hexagon that has been plated can become tight or refuse to enter, which is why a plated socket head is checked with the key in the plated state.

On stainless parts the useful treatment is often a dry film lubricant rather than a coating. It leaves the socket alone and reduces the galling that can stop an M6 stainless thread from reaching its preload.

Where M6 Screws Are Used

Three assemblies where the size is the default choice:

Why Shi Shi Tong for M6 Screws

Four reasons that matter on a size that is often chosen without a calculation, each tied to something the drawing or the first article proves. All four show in the sample.

How M6 Screws Are Made

These screws are turned or formed from stainless bar and wire, with the cylindrical head formed and faced, the hexagon broached, and the thread cut; the parts are cleaned, passivated and inspected. The first piece of every run is measured on head diameter and height, socket across flats and depth, thread and length.

At M6 the socket is large enough to be cut with a standard broach, and the operation is well understood; the part that decides quality is the depth relative to the head height. A socket that is a fraction too deep leaves a section under the hexagon that distorts when the key is loaded hard.

The head face is finished square to the thread, because it is the surface that bears on the plate or the counterbore. On an M6 screw the head is big enough that a face out of square shows as an uneven mark rather than as a wobble, which makes it easier to miss in inspection and more important to measure.

The thread is cut rather than rolled on stainless parts to keep the crest clean and the major diameter predictable, and the parts are passivated afterwards. Every batch is checked with a key in the socket, since that fit is what the assembly depends on.

Inspection: Socket, Head Face and Thread

What decides whether an M6 screw passes is the socket, the head face and the thread. The inspection agreement covers those, and the criteria come from the drawing.

Head and socket on the projector	Thread and start checked
Material checked	Salt-spray testing where plated

The socket is measured across flats and for depth, and checked with the key on a sample from each batch. A socket that measures right but feels loose with the key is usually too deep, and one that will not take the key is usually a burr at a corner.

The head is checked for diameter, height and flatness of the bearing face. On a panel joint that face is what leaves the mark, so flatness there is a cosmetic and a functional dimension at the same time.

On an M6 screw the thread is gauged and a sample is tightened to the stated torque in a test block of the customer’s material thickness. That test is what shows whether the engagement is enough for the joint, which the screw’s own dimensions cannot tell.

Certificates and Material Documentation

Every batch of M6 screws is traceable to the bar or wire it was made from and to the passivation lot that finished it. The grade and heat number appear on the material certificate supplied with the parts, and the dimensional results are recorded against the drawing revision.

ISO 9001:2015	RoHS test report
Registered trademark	Patent: security screw and driver

With each shipment we supply the material certificate, the inspection record for the dimensions on the drawing, including the socket and head, and the passivation record. The records carry the drawing revision and the batch number, so a repeat order of M6 screws takes the same setting.

Customers and Projects

More than 8,678 customers in 89 countries and regions have ordered from Shi Shi Tong; 87% of orders come from repeat customers and 56% through referrals. Most projects follow the same route: drawing review, first-article approval, then batches checked against that sample. On a size used this widely, the first article is checked in the customer’s own joint rather than on its own. Buyers visit during the process to review samples.

Repeat orders of M6 screws are checked against the same first article, so a joint proved once is reproduced across batches. Buyers and quality engineers visit for sample review, process discussion and supplier audits; the visit is arranged around production so the machining cell and the inspection bench can be seen on the day the parts run.

Customer group visit – sample review	Customer group visit – process discussion	Customer group visit – factory tour
Customer visit – project kick-off	Danish customer – drawing review	Industry exhibition – meeting customers

Related Products and Guides

Technical Guides

Choosing Washer Head Screws When a wider bearing face is worth the height it adds. Read the guide

Thread Gauge Failure After Plating Why a thread that gauged before finishing can fail afterwards. Read the guide

Products

M10 Shoulder Screws The special hex head version in stainless. M10 Shoulder Screws	Hex Socket Shoulder Screws The low head socket version in steel. Hex Socket Shoulder Screws	Hexagon Socket Shoulder Screws The hexagon socket head shoulder version. Hexagon Socket Shoulder Screws
Stainless Socket Shoulder Screws The stainless shoulder screw with a socket drive. Stainless Socket Shoulder Screws	Metric Precision Shoulder Screws The precision metric shoulder fastener series. Metric Precision Shoulder Screws	Socket Head Shoulder Cap Screws The knurled socket head cap screw version. Socket Head Shoulder Cap Screws

M6 Screw FAQ

Order M6 Screws

How an M6 screw project runs from drawing to repeat delivery:

Each step is confirmed with you before the next one starts; the drawing revision from the first article is what repeat orders are checked against.