I-ring gasket for watch glass – how to choose the right size?

In many watches, the glass is not mounted directly in the metal case. Between the glass and the case, there is a plastic I-ring gasket, which is responsible for the proper seating of the glass and participates in sealing the connection.

Selecting the I-ring therefore requires matching three elements:

• glass,
• I-ring gasket,
• glass seat in the case.

It is not enough to know only the diameter of the visible part of the glass.

What is an I-ring gasket for glass?

An I-ring is a thin gasket with a simple cross-section, placed around the glass. Unlike a rubber O-ring, it does not have a circular cross-section.

The I-ring has several important dimensions:

• inner diameter – corresponding to the diameter of the glass,
• outer diameter – resulting from the inner diameter and wall thickness,
• wall thickness,
• height of the gasket.

See I-ring gaskets for glasses at Kurant Warehouse

How to read I-ring gasket markings?

An example gasket may be marked as:

30.8 × 30.0 × 40 × 125

In this marking:

• 30.8 mm – outer diameter of the gasket,
• 30.0 mm – inner diameter, i.e., nominal diameter of the glass,
• 40 – wall thickness 0.40 mm,
• 125 – height of the gasket 1.25 mm.

This can be easily verified mathematically:

30.0 mm + 0.40 mm + 0.40 mm = 30.8 mm

The wall thickness appears on both sides of the glass, so we add it twice.

Second example: I-ring 37.7 × 37.0 × 35 × 80

Marking:

37.7 × 37.0 × 35 × 80

means:

• outer diameter of the gasket – 37.7 mm,
• glass diameter / inner diameter – 37.0 mm,
• wall thickness – 0.35 mm,
• height of the I-ring – 0.80 mm.

Here too:

37.0 + 0.35 + 0.35 = 37.7 mm

How to select an I-ring when I have old glass and an old gasket?

This is the best situation, as you can compare all elements.

You should remove the glass and gasket from the case, and then measure:

• the outer diameter of the glass,
• the inner diameter of the old I-ring,
• the outer diameter of the I-ring,
• the thickness of its wall,
• the height of the gasket.

However, the old I-ring may be permanently deformed after many years of use, so its dimension should be treated together with the measurement of the case.

How to properly measure the case for an I-ring?

If we are selecting a new set, the internal dimension of the case seat, where the outer edge of the gasket is mounted is very important.

You should not measure the visible hole at the top of the bezel if it is not the surface where the I-ring is actually located.

After removing the glass and gasket, you should find the exact surface where the gasket is seated and measure it with the internal jaws of the caliper.

Why measure the case in several places?

The glass seat is not always perfectly round. An older or damaged case may be slightly deformed.

Therefore, it is worth taking measurements in at least a few directions and comparing the results.

If the results differ significantly, the problem may be the case itself, not the size of the glass or gasket.

Why is accuracy of 0.01 mm necessary?

In the case of watch glasses and I-rings, we work with very small dimensional differences. Measuring with a ruler does not allow us to determine whether an element is, for example, 30.35 mm, 30.40 mm, or 30.45 mm.

Therefore, to make an accurate measurement, you should use calipers that allow reading with an accuracy of 0.01 mm.

Then, based on the actual measurement, you select the available catalog size.

Can you measure glass without removing it from the watch?

You cannot reliably determine the actual diameter of the glass this way.

When mounted, part of the glass is inside the gasket and under the case elements. Therefore, calipers applied from above usually measure only the visible surface, not the full diameter of the glass.

With differences of tenths, and sometimes hundredths of a millimeter, such a measurement is not sufficient for proper selection.

If you want to know the dimension of the glass, you must first remove it from the case.

What to do if the old glass is broken?

If the glass cannot be measured, the basis becomes measuring the seat in the case and identifying the mounting method.

If the old I-ring is preserved, you can additionally measure its dimensions and check what size glass was mounted in it.

If both the glass and the gasket are missing, you need to select the entire system based on the precise dimensions of the case seat.

Glass diameter vs. outer diameter of the I-ring

These are two different dimensions.

For example, glass:

30.0 mm

can work with an I-ring:

30.8 × 30.0 × 40 × 125

Then the glass is 30.0 mm, while the nominal outer diameter of the gasket is 30.8 mm.

Therefore, a person who measures the case seat and gets about 30.8 mm should not automatically order glass 30.8 mm.

The most common mistake when selecting glass on an I-ring

It is very easy to confuse:

• the diameter of the glass,
• the outer diameter of the gasket,
• the diameter of the seat in the case.

For example, a customer measures the mounting area in the case and gets about 30.8 mm, and then looks for glass 30.8 mm.

Meanwhile, in that solution, there may be 30.0 mm glass along with an I-ring with an outer diameter of about 30.8 mm.

Is every I-ring for 30.0 mm glass the same?

No. The same diameter of glass does not mean the same gasket.

I-rings can differ in:

• wall thickness,
• outer diameter,
• height.

Therefore, you need to read the entire marking of the gasket, not just the dimension of the glass.

Why is the height of the I-ring important?

The I-ring must have the appropriate height for the case construction and the glass used.

For example, the offer includes gaskets with heights of:

• 0.80 mm,
• 1.25 mm,
• 1.75 mm,
• and other variants.

Using a gasket of incorrect height may change the position of the glass relative to the case.

I-ring vs. O-ring – they are not the same

I-ring for glass and rubber O-ring are two different types of gaskets.

The O-ring has a circular cross-section and is often used, among other things, in backs and other sealing connections.

The I-ring used with glass has a design adapted for seating the glass between the gasket and the case.

Is the watch waterproof after replacing the glass and I-ring?

Simply replacing the glass and gasket does not guarantee maintaining a specified level of water resistance.

The watch's water resistance is also affected by the gaskets of the back, crown, buttons, and the condition of the case itself.

If water resistance is important, the watch should be tested with an appropriate water resistance tester after the repair.

How to select glass and I-ring step by step?

1. Remove the glass and old gasket from the case.
2. Measure the outer diameter of the glass.
3. Measure the diameter of the gasket seat in the case.
4. If the old I-ring is available, measure its inner diameter, outer diameter, thickness, and height.
5. Check the profile and thickness of the glass.
6. Select the appropriate size of glass and corresponding I-ring.
7. After installation, check the proper seating of the glass.
8. If the watch is to be waterproof, perform a water resistance test.

I-ring gaskets for glass at Kurant Warehouse

Kurant watchmaker's warehouse offers a wide selection of plastic I-ring gaskets for watch glasses in many diameters, wall thicknesses, and heights.

See I-ring gaskets for glasses

If you are also selecting new glass, see our guide on diameter, thickness, and types of glasses:

How to choose glass for a watch – complete guide

If you are unsure whether you need flat, spherical, or lens-shaped glass:

Flat, spherical, and lens-shaped glass – what are the differences?