What stretch means
When an O-ring is installed on a rod, piston or male gland, its inner diameter is fit over a groove diameter larger than the ring’s free ID. The ring stretches around the circumference to fit. Some stretch is desirable — it keeps the ring seated and preloaded — but too much thins the cross-section, reduces squeeze and shortens life. On the tool this appears as circumferential strain.
Stretch % = (groove diameter − free inner diameter) ÷ free inner diameter × 100Why a small stretch helps, a large one hurts
A light stretch of a few percent keeps the ring firmly in its groove during assembly and gives a stable preload. As stretch increases, two problems appear:
- Cross-section thins. Rubber is nearly incompressible, so stretching the ID pulls material out of the cross-section. A ring stretched a lot has a smaller effective cord diameter than its free size, which directly reduces the squeeze you expected.
- Recovery and fit degrade. Highly stretched rings can neck down unevenly, take a set faster and are more sensitive to groove tolerance.
Because stretch reduces the cord, and squeeze depends on the cord, the two are linked: over-stretch quietly steals squeeze and can turn a seemingly correct design into a leaker.
Typical limits
For a permanently installed radial seal, keep installed stretch to roughly 5% or less. Parker’s ideal band for radial static and dynamic seals is 0–5% stretch. Momentary stretch during assembly — pulling a ring over a larger thread or shoulder before it drops into a smaller groove — can be much higher for a few seconds, but the installed stretch that the ring lives with should stay in the low single digits.
| Situation | Guideline |
|---|---|
| Installed radial seal (final state) | 0–5% ID stretch |
| Temporary assembly over threads/shoulder | Higher briefly; not a steady-state value |
| Face seal | Usually little to no ID stretch by design |
Cross-section reduction
The reduction of cord diameter with stretch is not linear, and good design tools apply a stretch-versus-cord correction rather than a simple ratio. The practical takeaway: once you exceed a few percent stretch, you must recompute squeeze using the installed cross-section, not the free cord size, or you will overestimate the seal.
Negative stretch and bunching
Rod (female-gland) seals face the opposite risk. If the ring’s free ID is larger than the groove it sits in, the ID is compressed rather than stretched, and the ring can bunch or spiral-twist, especially on long dynamic strokes. Sizing the ring so its free ID matches the groove — a slight stretch, never a compression — avoids this.
Check it with the calculator
The radial static, radial dynamic and boss seal calculators report circumferential strain (stretch) alongside squeeze and fill, across nominal, minimum and maximum conditions, and flag when stretch leaves the ideal band. Because stretch and squeeze interact, solving them together is the only reliable way to keep both in range.
Frequently asked questions
How much can you stretch an O-ring to install it? Momentarily, a ring can be pulled over a diameter much larger than its ID during assembly. But the installed stretch it lives with should stay around 5% or less for a radial seal.
Does stretching an O-ring reduce squeeze? Yes. Stretching the inner diameter thins the cross-section, so the installed cord is smaller than the free size and the actual squeeze is lower than a free-size calculation suggests.
What is circumferential strain on the calculator? It is the installed inner-diameter stretch expressed as a percentage — the same quantity as stretch — checked against the 0–5% ideal band.