How to Measure Screw Size?
Screws are probably among the smallest and most overlooked parts of an RC car.
We use them everywhere: to hold suspension components together, secure the chassis, mount electronics, assemble shocks, fix body parts and much more.
So when a screw gets damaged, stripped, bent or simply disappears, replacing it sounds easy.
Until you realize that not every screw is the same size.
A screw can look almost identical to another one while having a different diameter, length or head shape — and using the wrong one can lead to an improper fit or even damage the part.
So how do you actually measure an RC screw?
That’s what we’re going to look at in this tutorial.
The tool you need
Before taking any measurements, you’ll need a tool that can measure small dimensions accurately.
For RC work, a digital caliper is particularly useful.
Unlike a regular ruler, a caliper allows you to take precise measurements on small parts and can be used to measure both external and internal dimensions.
You don’t need anything complicated. The important thing is having a measuring tool that gives you enough accuracy for the small dimensions commonly found on RC cars.
Getting familiar with the digital caliper
Before measuring a screw, it’s worth understanding the basic parts of the caliper.
1 — SAE / Metric conversion
Allows you to switch between measurement systems.
2 — Power button
Turns the caliper on or off.
3 — Zero button
Sets the current position as zero, allowing you to reset the measurement before taking a new one.
4 — Internal measuring jaws
Used to measure the inside dimensions of an opening or recess.
5 — External measuring jaws
Used to measure the outside dimensions of an object, such as the diameter of a screw.
For screw measurements, the external jaws will be the ones you use most often.
Let’s measure some screws
Not all screw heads are shaped the same way.
That matters because the correct point from which to measure the screw can change depending on its head design.
For this tutorial, we’ll look at four common types:
- Cup Head Screw
- Button Head Screw
- Flat Head Screw
- Set Screw
The red line shown in the reference image indicates the distance to be measured for each type.
1 — Cup Head Screw
A cup head screw has a conventional head that sits above the surface of the part.
When measuring its length, measure from the underside of the screw head to the tip of the screw.
The head itself is not included in the length measurement.
This is an important point to remember when replacing a screw: two screws can have the same overall appearance but different usable lengths depending on how they are measured.
2 — Button Head Screw
A button head screw has a rounded, low-profile head.
Just like a conventional head screw, its length is measured from the underside of the head to the tip of the screw.
The rounded head itself is therefore not included in the stated screw length.
3 — Flat Head Screw
A flat head, or countersunk screw, is different.
Its head is designed to sit inside a countersunk hole, allowing the top of the screw to become flush with the surrounding surface.
Because of this design, the length is measured from the top of the head all the way to the tip of the screw.
This is one of the most important differences to remember when measuring screws.
A countersunk screw and a button head screw of the same nominal length are not measured from the same starting point.
4 — Set Screw
A set screw is different again because it normally has no conventional head.
It is designed to sit inside the threaded hole, with its end pressing against another component.
For this type of screw, simply measure the entire length of the screw from one end to the other.
What exactly should you measure?
Once you know how to measure the different head types, identifying a screw becomes much easier.
There are several dimensions worth checking:
Diameter
Measure across the threaded section using the external jaws of the caliper.
This gives you the screw’s nominal diameter.
Length
Measure according to the type of screw head:
Conventional / button head → underside of the head to the tip
Flat / countersunk head → top of the head to the tip
Set screw → end to end
Head dimensions
When necessary, you can also measure the diameter and height of the screw head.
This can be useful when you need an exact replacement and clearance around the screw is limited.
Why measuring correctly matters
When working on an RC car, a screw that is only slightly different in size can make a difference.
A screw that is too long may interfere with another component.
A screw that is too short may not provide enough thread engagement.
A screw with the wrong head shape may not sit correctly in its mounting position.
And a screw with the wrong diameter or thread can damage the threaded hole.
That’s why identifying a screw shouldn’t be based only on how similar it looks.
Measure it.
A simple habit worth keeping
The next time you remove an unidentified screw from your RC car, don’t immediately put it in the parts box with a guess at its size.
Take a moment to measure it.
Check:
The screw diameter.
The screw length.
The head type.
And, when necessary, the head dimensions and thread specification.
It only takes a few seconds with a caliper, but it gives you a much more reliable reference when you need to replace the screw later.
Keep learning
Understanding small details like screw dimensions may seem simple, but these are exactly the little things that make working on an RC car easier.
The more familiar you become with your tools and measurements, the less you’ll have to rely on guesswork when maintaining, repairing or building your rig.
More RC lessons coming soon.