How to Size O Rings

30 April 2020
How to Size O Rings

O rings are used to create a secure seal between two surfaces in either static or dynamic applications. Because the purpose of an O ring is to create an impenetrable seal, getting the size exactly right is essential. If it is too large, fluids, dirt, grit and gasses can flow around it. If it is too small, overstretching it to get it into place will compromise the seal’s integrity and it will fail prematurely. Knowing how to size O rings is important to avoid seal failures and the waste of time, money and productivity.

Fortunately, learning how to size O rings is not overly complicated. It does, however, demand absolute precision. The old adage to measure twice and cut once definitely applies to O rings. Callipers and micrometres are the best tools to measure a small O ring. An O sizer or Pi-Tape will work better for larger ones. Sizing cones are designed so you can slip an O ring onto the cone and see at a glance what standard size it is, but this is not always the most precise measure because of the potential to stretch the O ring as you place it on the cone. You can size an O ring in metric or imperial and, if ever in doubt, you can always count on us and our o-ring size chart that include both measurements.


Understanding How to Size O Rings


Two separate measurements are key to accurately sizing O rings. The internal diameter is the distance across the diameter measured from the inside edge of the O ring. It is also known as the inside diameter and is often abbreviated as ID. The cross-sectional diameter is the measure of the O ring’s thickness or height/width. This is usually shortened to CS. The outside diameter of the O ring is a less significant measure. When you know how to take the ID and CS measurements, you know how to size O rings.

At Abbey Seals, we want to ensure that the O rings you select are perfect for your application. Our O ring size charts also include the measurements for the shaft or cylinder involved too in order to achieve an optimal fit for your specific application.

What if you size your O ring but don’t see precisely what you need on our size charts? Just let us know, and we can create custom O rings in the material, size and colour you need. If you know how to size O rings, we can create whatever size O ring you require quickly and usually deliver it to you anywhere in Ireland within 24 hours. Contact us and we will get you the O ring solution you need.