Section 14: Watch Screws — The Smallest Components That Matter Most

Screws are among the most widely used fasteners and connectors in watch assembly. Every movement contains a large number of screws, each serving to secure plates and control components in their precise positions. Every screw has its own specific purpose. Even the simplest functional movement contains more than twenty screws of various sizes; complex movements may have thirty to forty or more.

Watch screws come in a wide variety of forms, but all share the same basic anatomy: a shank, a head, and threads. In terms of dimensional standards, early watch and pocket watch movement screws were predominantly made to Imperial (inch) standards, with some metric — a somewhat inconsistent mix. Early screws also tended to be relatively long, with small head diameters, thick head heights, and a wide variety of styles.

Watch screws are generally manufactured from high-carbon steel T10A and must undergo heat treatment and electroplating. The defining characteristic of watch screws is their extremely small size — their dimensional precision and surface finish requirements are far higher than those of screws used in ordinary mechanical components.

At Aorawa Time, the quality of a movement's screws — their finishing, their bluing, their slot condition — is one of the details we look for in every timepiece.

Table 1-3-4: Common Watch Screws and Their Functions

Left-Hand (Reverse) Thread Screws

It is important to note that movement screws include both right-hand (standard) and left-hand (reverse) thread variants. Reverse-thread screws are identified by "three grooves" on the screw head surface as a marking. When tightening or loosening, the direction of rotation must be carefully observed. Small steel wheel screws are generally reverse-threaded. Some early watches did not use the three-groove marking system; reverse-thread screws in those movements are typically found on small steel wheels, while large steel wheel screws are almost never reverse-threaded.

If the tightening direction is confused during disassembly, the screw can be stripped very easily.

Plate Screws and Their Finishing

Fine watch movements place specific demands on plate screws. Their surfaces must be finely finished and polished, with chamfered edges and a slightly convex arc on the top surface — visually very beautiful. This is an important part of movement finishing. In first-tier watch brands, screw manufacture is taken very seriously (refer to the Geneva Seal's 12 rules for the relevant requirements).

The Screw Slot: A Window Into Service History

The slot on a screw head is the most easily damaged element during servicing — commonly referred to as a "chewed slot" or "buggered slot," particularly when the slot has been poorly re-cut or the technique was improper. A skilled watchmaker is especially careful when removing and installing screws during movement disassembly and assembly. The external appearance of the movement is an important aspect of a watch's overall condition — visible service marks must not be left behind.

A chewed slot is extremely unsightly. If a blued steel screw has a damaged slot, it is as if the movement's appearance has been ruined. Therefore, the condition of the screw heads alone can be used to assess how many times a watch has been serviced — and to evaluate the technical skill of the watchmakers who worked on it.

Screw Length: Getting It Wrong Has Consequences

During servicing, screws can occasionally be installed in the wrong position. Even experienced watchmakers are not entirely immune to this, because in complex movements it is not always easy to keep track of every screw's position — and some screws are very similar in size and appearance. Screw length, head shape, and the tip of the shank all require careful observation and identification; small differences are easily overlooked.

If the thread pitch does not match, the screw simply will not engage. But if the thread pitch matches while the length differs, the screw can be installed — and this is where problems arise. Using a long screw where a short one is required can cause serious damage or even destroy the movement: a screw that is too long will protrude after tightening, potentially damaging the dial, obstructing dial-side components, or causing them to jam. Using a short screw where a long one is required will result in the plate or component not being held securely, and the screw is also prone to backing out.

Beyond length, two other details require attention: the tip of the shank and the shape of the head. Shanks may be flat-tipped or pointed. Heads may be flat or chamfered. The shoulder of the shank is generally used for dial-side assembly or on the movement's intermediate plate — the pointed tip serves as a locating pin to facilitate installation. Screws with chamfered heads can reduce head height; the mating plate position must have a pre-cut recess (commonly called a "nest") to accommodate them. These screws are mostly installed on mainspring cover plates. Screws on bar plates, upper plates, and ratchet wheel plate surfaces — including those on the ratchet fork plate — are flush with the plate surface after tightening; these are called countersunk screws.

Some screw heads also have a shoulder step. The component being secured pivots on the second shoulder of the screw, allowing it to move freely. Examples include ratchet clamps and detent levers.

Rotor Screws vs. Large Steel Wheel Screws

The screws used on the automatic rotor are easily confused with large steel wheel screws, because their lengths differ. Using the wrong screw can potentially damage the automatic plate or drill through the seconds wheel jewel.

Blued Screws, Satin Screws, and What They Tell You

By color, screws can be divided into blued and non-blued variants. Blued screws are generally used only in positions visible on the plate surface. Some ETA movements blue only the screws on the automatic plate to a deep blue. Screw heads that have not been polished and present a "satin" (matte) appearance are mostly used on dial-side miscellaneous components or in more concealed positions — such as mainspring cover plates, date components, and dial foot screws. However, the screws used in lower-grade watch movements are universally in an unpolished, satin-matte state.

Screw Loosening: A Common Cause of Watch Failure

Different screws have different applications. When identifying the correct installation position of a specific screw, the rules and experience described above should be used as reference.

The screws on movement plates share the same style, but heads may be flat or chamfered. Modern watch movement screws are generally very short and small — overtightening can easily snap them or shear off half the head, a consequence of improper heat treatment. But undertightening risks insufficient clamping force and the screw backing out (a problem currently observed in some ETA movements).

Based on years of practical repair experience, the movement screws most prone to loosening are: large steel wheel screws, automatic plate and rotor screws, setting disc screws, and winding ratchet claw screws.

Screw loosening is one of the most common causes of watch failure. Many experienced watchmakers have developed the habit of applying a small drop of "green oil" (thread-locking compound) when tightening screws.

The screws inside a fine watch are not an afterthought — they are precision components, finished and blued with the same care as the bridges and plates around them. A watch whose screws are perfectly slotted, properly blued, and correctly torqued is a watch that has been assembled with integrity. Explore timepieces built to that standard at Aorawa Time.