Short answer: you need calipers, about €10 at any hardware shop, and your phone. Measure first the places where the part meets other parts —the opening it goes into, the holes, the clips— and then the overall size. Take photos from the front, the side and the top with a ruler next to the part, and send us the measurements exactly as you read them, without adding clearance yourself.

What you need

Digital or analogue calipers. They measure to 0.1 mm, which is what a fit needs. A ruler is fine for overall size, not for holes or clips.
A phone with a camera.
Pen and paper, or a photo of the part to write the measurements on.

The golden rule: fits first, size second

A part can have the perfect outside size and still not fit. The most common mistake we see in the workshop is measuring the part from the outside and forgetting the place it has to go into. For example, for a lid we get the outside diameter, but not the inside diameter of the seat or the depth of the rim: the model looks right, but the lid does not go on.

So measure in this order:

The place the part goes into: diameter or size of the opening, depth of the seat, height of the rims.
The distance between hole centres. Not the gap between their edges: measure outer edge to outer edge and subtract one diameter, or measure the gap and add one diameter.
Wall thickness, especially on clips and tabs: it decides how much they flex.
The depth of slots and holes. A photo from above does not show it: use the depth rod of the calipers.
The size of the latch hook: the small lip that holds the part in place. It is usually what breaks and the hardest thing to guess.
Finally, the overall size: length, width and height.

Do not add clearance yourself. Send us the real measurements, without “I’ll add half a millimetre just in case”. The assembly clearance depends on the material and the type of fit, and we set it when designing. If you add some and then we add some, the part will be loose.

How to take the photos

Three views: front, side and top, with the phone parallel to the part (not at an angle).
A ruler or the calipers in the photo, right next to the part, for scale.
A plain, light background, good light, no harsh shadows.
The broken area close up: it tells us where it failed and where to reinforce.
The place where the part goes: the opening, the seat or the part it fits with.

Common mistakes

Mistake	What happens	How to avoid it
Measuring only the outside of the part	It looks right but does not fit	Measure the opening or seat first
Measuring the gap between holes	The holes end up one diameter off	Measure between centres
Not measuring a clip’s thickness	Too stiff and breaks, or too soft and does not hold	Calipers at the base of the clip
Not measuring depths	Slots and seats come out too shallow	Depth rod of the calipers
Adding clearance yourself	The part ends up loose	Send real measurements; we add the clearance
Measuring only with a ruler	0.5–1 mm errors on holes and clips	Calipers for anything small
Photos at an angle	Perspective distorts proportions	Phone parallel to the part

What if the part is too complex?

If it has many curves or fits, the quickest option is to send us the part (or bring it to Avilés by appointment): we measure it and return it with the new one. For very organic shapes, such as handles or curved enclosures, we are preparing a 3D scanning service, which we already offered at our previous workshop; it will come to Hilonorte in the coming months.

Send your measurements

Send the photos and measurements on WhatsApp and we will reply with price and lead time in writing. If you already have a 3D file, upload it straight to the calculator. More about the process in how to get a spare part that is no longer made and on custom CAD design.