How To Measure The Thread And Length of a Screw

How To Measure The Thread And Length of a Screw

Today, to attach parts or accessories to a bicycle, the use of screws may still be necessary. However, there are currently a large number of different screws. To find the ones you really need, you have to know how to measure them. This is particularly true of the length and thread of the screw.

Screws are used to secure objects to a bracket. They can also be useful for putting different things together. On a bicycle, screws may be needed to securely attach parts or accessories. In any case, it should be remembered that these small mechanical parts all work the same way. When it comes to screws, it’s usually just the size, screw thread, and material of construction that vary. 

Indeed, all these elements differ according to the type of objects or materials to be screwed. Thus, you should know that the screws all have a specific identity card.

Identify the different types of screws

Currently, screws can consist of several parts. First, there is the head of the screw. This is the part that allows us to tighten it using our various tightening tools. A screw can also be fitted with a barrel (optional). This is the small smooth cylindrical part that can rest on the bore of the flanges. Then we find the screw thread (the threaded part). Optionally a screw can also be provided with a point. This is intended to guide or drill specific support.

Currently, screws can be identified by the shape of their tips. Several types can be found on the market. These include chamfered end screws (CH). There are also rolling end screws (BR). These first two types are the most common. For automatic tightening, the manufacturers also offer sectional pilot point screws (PN). In this area, there are also cylindrical pilot point screws (PY).

To identify a screw, we can refer to its type of head. Regarding screw heads, 3 main types are to be known. These are the externally driven screws, recessed screws and multi-drive head screws. Externally driven screws are those that require the use of wrenches. To tighten the socket head screws, use a screwdriver instead. And as for the so-called multi-drive screws, these are screws mainly used by professionals. They can be tightened using different tools.

How to measure a screw?

The screws are offered in different sizes. Therefore, to meet our needs, we must ensure that they are chosen in the appropriate dimensions. Here, their length and diameter are to be considered. All are measured in millimeters.

Measure the length of a screw

The length of a screw is thus measured from its point to the base of the widest part of its head. However, it should be remembered that this length “L” is measured differently depending on the type of screw.

As you can see in the diagram, the length of a countersunk screw (the leftmost) is taken as the total length of the screw. Here, this length “L” is the distance from its point to the top of its head. In the case of a round screw or a flange screw (in the middle), the length is more the distance from the point of the screw to the flat base of the underside of the head.

And finally, with regard to a drop of tallow screw (on the right of the diagram), the measurement is carried out from its point to the widest part of its head.

Measure the Diameter of a screw

For its part, the diameter of a screw is measured at the level of the screw thread. Note that on Cécatre screws, this measurement corresponds to the external diameter of the thread. The order of magnitude is symbolized by the letter M. It is the symbol of metric thread. “M10” will then mean that the screw has a diameter of 10mm. “M9” will mean that its diameter is 9mm, and so on.

Note that there is no absolute rule for choosing the length of a screw. This choice remains to be made depending on the use, the same for their diameter. It all depends on the diameter of the hole where the screws will have to be inserted. However, it is accepted that in terms of length, the screw should measure 3 times the thickness of the part to be fixed. For long screws, a length of 2 times the thickness of the object may be sufficient.

Some tips for measuring screw threads

There are currently little tips for measuring screw threads. To do this, we can for example use our Smartphone. Just download a free app from Google Play . Once installed and calibrated, this application then allows us to measure the thread of a screw or the length of a nut. To do this, you just have to put them on the phone screen. Yep, it’s that easy.

On sites such as RobertAxleProject, small tools are offered. They take the form of a PDF file to download and then print. Once this is done, we then obtain a sort of graduated plane on which it is sufficient to place the screw or nut to be measured. 

All the indications on how to proceed are indicated directly on the file. Be careful, however, because to be effective, the measurement tool must be printed as it is offered on the site.