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Need an O-Ring? Know the Size?

O-Ring Basics – Measuring them.

The O-Ring, a round mechanical seal that keeps gas or liquid from passing through an opening. An engineering marvel if you will. However they are not perfect and, over time they may wear out and require replacement, consequently getting the correct size is vital. We’ve put together some simple to follow instructions so you get just the right size for the job.

The Three D’s of the O-Ring

There are three Dimensions that you need in order to obtain the perfectly sized O-Ring for your application. These are the Outside Diameter (OD), the Inside Diameter (ID) and the all important Cross Section. Sounds simple and, to be fair, it really is. All measurements should be recorded in millimeters. Start by laying the ring on a flat clean surface.

  • OD – Use a ruler or tape to measure the across from one side of the outer edge to the other.
  • ID – Repeat this but, measure across from one inside edge to the other.
  • CS – Place the O Ring into a Vernier Calliper and lightly clamp the jaws. Be very gentle now, don’t squeeze it.

Record all three measurements, in millimeters don’t forget, and Bob’s your Uncle. “Ah, but I don’t have a Vernier Calliper” I hear you say, and this is not a problem. There is a very simple formula we can use to work out the CS f you have both the ID and OD. In fact as long as you have two of the three dimensions there is a handy formula to help you work out the other.

O-Rings – The magic Formulas

The cross section is equal to the Outside diameter, minus the Inside diameter divided by two. Or in equation terms CS=(OD-ID)÷2 

The Outside Diameter is equal to the Inside diameter plus twice the Cross Section or OD = ID+(CS x 2)

The Inside Diameter is equal to the Outside diameter minus twice the Cross Section or ID = OD-(CS x 2)

If your O-Ring is cut or has snapped we can still work it out. Measure the length and the Cross section of the ring. Then divide the length by π (3.142), this will give you the circumference. Take the Cross section from this to give you the Inside diameter.

Length÷ π = Circumference.

Circumference – CS = ID

Of course, if you don’t have a ruler, or really just don’t fancy attempting to measure it yourself then pop one into us and our knowledgeable Sales team will do the hard work for you.

O-Ring Dimensions

W here is equal to CS above

8 Responses to “Need an O-Ring? Know the Size?”

  1. Iain Cairney says:

    Hi, do you do small orders. It’s for my bike brakes, small prints and maybe 16 pieces. I can measure them next week but thought it worth asking the question first.

    • Hi there, thanks for getting in touch. I’m pleased to say that we do indeed do small orders, just drop us a line or give us a call when you’re ready and we’ll be happy to help.
      kind regards, Nicki

  2. Philip Lawson says:

    I need some fuel injector o rings I’m hoping you can help me.
    The size I need is 7.21 Id by 2.2cs , 12 item in total.
    I look forward to hearing from you
    Phil Lawson

  3. derrick walton says:

    need oring that handles gas od 12.35mm id 8mm thickness 2.05mm need to get close as possible to the od the id can range a little larger mabe 8.38mm the thickness can vary preferably a little larger phone 2523486502

  4. richard spencer says:

    hi was looking for just a couple of o rings 15×6 inside x 4mm cross section if you can help richard

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