Metric Socket Head Cap Screws Stainless Steel

We make metric socket head cap screws in plain A2 stainless with a straight-knurled head and a hexagon socket, quoting them from M1 to M14. Head and knurl band, socket depth, thread and length come from your drawing, and the first article is run down by hand before it is tightened with a key.

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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 Metric Socket Head Cap Screws Are

Metric socket head cap screws are cylindrical-head machine screws with a hexagon socket in the top, driven with an Allen key. These are stainless steel, plain and passivated, with a straight knurl around the head, quoted from M1 to M14 with Number 396. The knurl lets the screw be turned by hand to the shoulder before a key is used to set the load.

Cap head. A full cylindrical head with a flat top, carrying a high clamp on a small footprint.

Hexagon socket. The drive sits inside the head, so no clearance is needed around it and the key cannot slip off the corner.

Straight knurl. A band of fine vertical ridges around the cylinder, which gives a grip for fingers.

Machine thread. A uniform metric thread for a tapped hole, a nut or an insert.

Plain stainless. A2 grade, passivated and uncoated, so the socket and the knurl are as formed.

Metric socket head cap screws are the default high-clamp fastener in machinery, and the knurled version adds one behaviour: it can be run in and out by hand. That matters on covers and adjustments that are set, checked and reset before the final tightening, where reaching for a key every time would slow the job down.

Two Ways to Turn One Screw

A knurled cap head is designed for two assembly actions. Fingers run it down until the head meets the part, which is fast and needs no tool, and then a key takes over to set the load. The knurl is not decoration; it is the interface for the first action, and its pitch and depth decide whether a hand can turn it without slipping.

The two actions have different limits. Finger torque is a small fraction of key torque, so the knurl only has to transmit enough to close a joint that is already aligned. The key does the work, and the socket is what has to survive it: on a stainless cap head the socket is the feature that fails when a joint is over-torqued, not the thread.

The head geometry of metric socket head cap screws is fixed by the standard form. A cylindrical cap head is taller than a button head and shorter than a pan head of the same diameter, and the hexagon sits in the centre with a wall of material around it. That wall is what makes the form able to take torque, and anything that removes it, such as a deeper socket or a deeper knurl, takes away from the same section.

The knurl interacts with the joint in a way that is easy to overlook. The ridges stand slightly proud of the cylinder, so a knurled head needs a little more clearance in a counterbore than a plain one, and it will mark a soft seating face if it is pulled down against it. In aluminium housings that mark is often the first sign that the screw was seated by hand and not by a key.

Stainless completes the picture. The thread galls in aluminium and in itself, and a stainless cap head screw that has been tightened and released a few times will start to feel rough in an aluminium thread. Assembly lubricant or a coated thread insert is the answer, not a different head.

Sizes, Head and Socket

The recorded range is M1 to M14, with lengths made to the drawing. Custom lengths up to 2,000 mm are subject to DFM review after drawing submission. For metric socket head cap screws the quotation carries head diameter and height, knurl length and pitch, socket size and depth, thread and overall length.

Socket depth against head height is the dimension to quote together. A deeper socket looks helpful and removes the material that keeps the head from splitting under a key, so the depth is a compromise that should be stated rather than left to the standard.

Knurl length is the other number. A knurl that runs too far up the head leaves a thin rim at the top, and one that stops too low does not give fingers enough to grip. On a visible part the band also reads as a design line, and customers often specify where it sits.

What the Knurled Cap Head Gives and Costs

Four features define these metric socket head cap screws, and each one has a consequence for the joint or the process:

The hand-running behaviour is the reason many buyers choose the knurled version of metric socket head cap screws rather than the plain cap head. On a cover with a dozen screws, running each one down by hand and then following with a key is noticeably quicker than doing both with the key, and it also reduces the chance of cross-threading, because fingers feel the thread start.

The cost appears on soft surfaces and in close counterbores. The knurl is a ring of ridges that will print into an aluminium seat, and a head that is a few tenths larger than a plain one may not clear a counterbore that was designed around the plain form.

Plain Stainless and the Socket

The recorded finish is passivation on A2 stainless with no coating. On a socket head screw that matters at the socket wall, where a coating would build and narrow the hexagon, and at the knurl, where it would round the ridges that provide the grip.

Material and Finish Options for Socket Head Cap Screws

Where a joint is assembled and released often, the limiting factor is galling rather than corrosion, and a silver-plated or lubricated stainless screw is the practical answer. Where the joint is set once and left, plain A2 is enough for indoor machinery and A4 covers the wet cases.

Where a coated steel screw is specified for strength, the socket is re-checked after plating. A hexagon that was in tolerance before plating can be tight enough to refuse the key, and on a cap head that is the difference between a usable joint and a scrapped screw.

Where Knurled Socket Head Cap Screws Are Used

Three assemblies where the hand-running behaviour pays for the knurl:

Why Shi Shi Tong for Metric Socket Head Cap Screws

Four reasons that matter on a knurled socket head, each tied to something the drawing has to prove:

How Metric Socket Head Cap Screws Are Made

Metric socket head cap screws are headed on single-die machines that form the cylindrical head, the straight knurl and the hexagon socket in one stroke, then rolled with the metric thread and passivated as a batch. The first part of every run is measured on head diameter and height, knurl position, socket depth and thread.

Forming the knurl and the socket in the same stroke is the constraint that shapes the process. The knurl die presses ridges into the outside of the head while the punch cuts the hexagon inside it, and the wall between the two is fixed by the head diameter. A head that is slightly under size produces a screw that gauges correctly and has one or two ridges standing proud at the top, where the metal had nowhere to go.

Stainless is harder to knurl than steel. The ridges form less crisply, the tool wears faster, and the band has to be checked on the first part rather than at the end of the run; a knurl that is shallow enough to look smooth will not give fingers anything to hold.

Inspection: Head, Knurl and Socket

What decides whether metric socket head cap screws pass is the head, the knurl, the socket and the thread. The inspection agreement covers those four, and the criteria come from the drawing.

Head, knurl and socket	Thread and start checked
Core hardness checked	Salt-spray testing of the grade

Head diameter and height are measured on a projector, and the socket is checked with the production key as well as by measurement, because a hexagon that is shallow or has a burr at one corner will accept one key and refuse another. The knurl band is checked for position, length and ridge condition on the first part of every run.

A sample from each batch is run down by hand and then tightened with a key at the joint torque. That check proves both behaviours at once, and it is the one that predicts whether a customer’s assembly line will be faster or slower with the knurled version.

Certificates and Material Documentation

Every batch is traceable to the wire it was made from and to the passivation batch that finished it. The grade and heat number appear on the material certificate supplied with the parts, and the inspection records follow the criteria agreed on the drawing.

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 and, where the grade is listed, the third-party report that covers it. The documents carry the drawing revision and the batch number, so a repeat order keeps the same socket depth and the same knurl.

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 metric socket head cap screws the first article is run down by hand and then tightened with a key, because that is how the part is used. Buyers visit during the process to review samples.

Buyers and quality engineers visit for sample review, process discussion and supplier audits; the visit is arranged around production so the headers and the passivation line 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

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Long Thin Screws: Drawing and Manufacturing Requirements What to put on the drawing for long, thin parts: datum, shank, thread position and handling. Read the guide

How Long Does It Take to Customize Extra-Long Screws? Where the time goes in a custom long-screw order, from drawing review to first article. Read the guide

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Metric Socket Head Cap Screw FAQ

Order Metric Socket Head Cap Screws

How a socket head cap 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.