Measuring the case for watch glass – how to correctly choose the diameter?

The simplest situation when replacing glass is the ability to measure the original glass after removing it from the watch. The problem arises when the glass is broken, missing, or an incorrect size was previously installed.

In such cases, it is necessary to determine the method of seating the glass and accurately measure the place of its installation in the case.

Do not measure only the visible hole from above

One of the most common mistakes is placing the calipers against the visible hole from the dial side and assuming the obtained value is the diameter of the glass.

The visible hole does not have to correspond to the diameter of the glass.

The edge of the glass may be located:

• under the edge of the case,
• under the bezel,
• in the plastic I-ring seal,
• in a special socket located deeper in the case.

Therefore, it is necessary to find the place where the glass is actually seated.

What exactly do we measure in the case?

This depends on the method of mounting the glass.

We may be dealing with:

• glass seated directly in the case,
• glass mounted on an I-ring seal,
• glued glass,
• glass secured in a special ring or bezel.

Therefore, before taking measurements, it is necessary to first identify the construction.

How to measure the glass seat in the case?

After removing the glass, seal, and remnants of the previous installation, it is necessary to find the surface where the outer edge of the glass or seal actually seated.

The inner diameter is measured with the inner jaws of the calipers.

The measurement should be done gently, so as not to force the jaws and inflate the result.

Why is it worth measuring in several directions?

The case is not always perfectly round.

It may be:

• slightly deformed after a hit,
• worn out,
• uneven after a previous repair,
• factory-made with slight tolerances.

Therefore, the measurement should be taken in at least a few directions, e.g., vertically, horizontally, and at an angle.

If we get clearly different results, the cause should be determined before selecting the glass.

Why should the measurement have an accuracy of 0.01 mm?

Watch glasses come in very similar sizes. Two consecutive glasses may differ by only 0.1 mm.

To decide which catalog size is correct, it is worth knowing the actual dimension with an accuracy of 0.01 mm.

For example, a result of:

30.37 mm

provides much more information than a measurement simply stated as:

about 30.5 mm.

This does not mean that we order glass of 30.37 mm. An exact result helps correctly select the available catalog size and consider the method of installation.

Do not round the measurement prematurely

If the calipers show 30.37 mm, do not immediately write down "30.4 mm" and treat it as a certain diameter of the glass.

First, it is necessary to know:

• whether we measured the glass or the case,
• whether there is an I-ring,
• whether the glass is pressed directly,
• what the method of installation is.

Only then can the measurement be properly interpreted.

Measuring the case with glass mounted on an I-ring

If there is an I-ring seal between the glass and the case, the diameter of the seat in the case is not the diameter of the glass itself.

For example, the arrangement may look like this:

glass 30.0 mm + I-ring = outer diameter of the system about 30.8 mm.

So if we measure about 30.8 mm in the case, it does not automatically mean that we should buy glass of 30.8 mm.

See: how to select an I-ring seal for watch glass

What to measure if the old I-ring is preserved?

If the glass is broken but the old seal is in good condition, it is very helpful for identification.

It is worth measuring:

• the inner diameter of the I-ring,
• the outer diameter,
• the wall thickness,
• the height.

However, it should be remembered that the old plastic seal may have deformed over the years, so its measurement should be compared with the dimension of the case.

What to do if there is neither glass nor seal?

Then it is necessary to determine the entire system from the beginning:

1. identify the method of mounting the glass,
2. measure the proper seat in the case,
3. determine the depth and construction of the seating area,
4. establish whether a seal is needed,
5. select the appropriate diameter and thickness of the glass,
6. determine its profile.

In such a situation, the diameter of the case alone does not provide an answer as to what glass should be used.

Diameter is not everything – thickness must also be determined

Even after determining the diameter, it is necessary to select the correct thickness of the glass.

For example, glass 30.4 mm can occur as:

• 30.4 × 0.8 mm,
• 30.4 × 1.0 mm,
• 30.4 × 1.5 mm,
• 30.4 × 2.0 mm.

Each of these variants has the same diameter but may be intended for a different case construction.

The profile of the glass must also be determined

After measuring the case, it is necessary to determine whether the original glass was:

• flat,
• spherical – convex,
• lens-shaped,
• plexiglass with a specific profile,
• or another special variant.

Two glasses with the same diameter may have completely different heights and not be proper substitutes.

See: flat, spherical, and lens glass – differences

Measuring lens glass

In the case of lens glass, there are additionally two thickness values:

• thickness at the edge,
• thickness in the central, highest part of the glass.

For example, lens glass 3.5 × 1.5 mm has about 3.5 mm thickness in the center and about 1.5 mm at the edge.

The diameter alone does not allow for the proper selection of lens glass.

Can glass be selected only by the watch model?

If reliable documentation of parts for a specific model is available, it can greatly facilitate selection.

However, for many older or less popular watches, the exact designation of the glass is not available. In such cases, measurement is necessary.

Can glass be measured while still mounted?

This does not allow for a reliable determination of the actual diameter of the glass.

Part of the glass is inside the case or seal. Measuring from above gives the dimension of the visible surface, not the entire glass.

If the old glass is intact, it is best to remove it and measure directly.

The most common mistakes when measuring the case

• measuring the visible hole instead of the actual glass seat,
• measuring only in one direction,
• using a ruler instead of calipers,
• rounding the result prematurely before determining the method of installation,
• not considering the I-ring,
• selecting only by diameter without checking thickness,
• ignoring the profile of the glass.

Example: I measure 31.2 mm in the case – what glass should I buy?

Based on the information 31.2 mm in the case alone, it is not yet possible to provide the correct diameter of the glass.

First, it is necessary to determine:

• where exactly the measurement was taken,
• whether the glass was mounted on an I-ring,
• what the thickness of the seal was,
• whether the glass was pressed directly,
• what profile and thickness were used.

Only a complete set of this information allows for the correct selection of the element.

How to read glass diameter designations?

If after taking measurements you are looking for the appropriate size in the catalog, remember the watchmaking method of notation.

For example:

304 = 30.4 mm
312 = 31.2 mm
325 = 32.5 mm

See: how to read watch glass size designations

How to select glass step by step?

1. If the old glass exists – remove it.
2. If it is missing – identify the method of installation.
3. Measure the proper seat in the case in several directions.
4. Check if there is an I-ring seal.
5. Determine the diameter of the glass.
6. Select the thickness.
7. Determine the material: plexiglass, mineral, or sapphire.
8. Determine the profile: flat, spherical, or lens-shaped.
9. Before installation, compare all dimensions again.

Glasses for watches at Kurant Wholesale

The watchmaking wholesaler Kurant has in stock over 80,000 pieces of watch glasses in a very large number of diameters, thicknesses, materials, and profiles.

See the offer of watch glasses

If you have old glass and want to know which parameters to check:

How to select glass for a watch – complete guide