Once the hardware's H and W are sized correctly (Part 2), the next design lever is the O-ring itself: picking an ID or OD so that the ring sits in the right amount of tension — or pre-compression — before pressure is even applied. Get this wrong and you either damage the ring during installation, or it wears itself out during service.
This post covers the four-percent numbers you should keep in your head, plus the practical reasoning behind them.
There are two competing risks you are trying to balance:
The right answer is to preload the ring so it always sits where pressure wants it to sit — and to keep that preload small enough that no one has to wrestle the assembly together.
For piston seals, the O-ring's ID should be smaller than the gland (piston) diameter, so the installed ring is always slightly stretched. That way, even at the largest possible O-ring ID and smallest possible gland diameter, there is still some hoop tension pulling the ring against the sealing surface.
| Quantity | Formula | Range |
|---|---|---|
| ID Stretch | Stretch = (Gland – ID) / ID | Min 0 %, Max 5 % |
Interpretation. If you ordered a piston with gland diameter = 1.000 in and an O-ring with ID = 0.975 in, the stretch is (1.000 – 0.975) / 0.975 = 2.56 %. That sits comfortably inside the 0–5 % window.Rod seals are mirrored: the OD of the O-ring should be larger than the gland (housing bore) diameter, so the ring is always squeezed radially against the rod.
| Quantity | Formula | Range |
|---|---|---|
| OD Interference | Interference = (OD – Gland) / OD | Min 0 %, Max 2 % |
Note the tighter tolerance. Rod seals see more axial run-out, and a heavy radial preload on a rod increases friction — so the allowable range is intentionally half of the piston range.
Face seals behave like either piston or rod depending on which side the pressure comes from. The rule is to place the O-ring where it will already be when pressure arrives.
| Pressure Direction | What to Preload | Formula | Range |
|---|---|---|---|
| External pressure (pressure pushes the O-ring inward toward the bore centerline) | Stretch O-ring ID against the inner hardware dimension In | Stretch = (In – ID) / ID | Min 0 %, Max 5 % |
| Internal pressure (pressure pushes the O-ring outward toward the bore wall) | Interference between O-ring OD and outer hardware dimension Out | Interference = (OD – Out) / OD | Min 0 %, Max 3 % |
The "where it would be as a result of the pressure" wording is doing real work: it means the preloaded position should match the pressurised position, so the ring has no distance to travel when the system is energised. That kills a whole class of nibbling and extrusion failures at the gland entrance.
A quick troubleshooting map, useful when a freshly built assembly leaks or wears prematurely:
| Symptom | Likely Stretch/Interference Mistake |
|---|---|
| Ring nicked or torn at assembly | Too much stretch on a piston seal — pick the next size up. |
| Ring walking axially in a rod-type groove | Too little OD interference — pick the next size down. |
| Ring wedging into the extrusion gap on pressure spike | Stretch/interference is fine, but extrusion gap (Part 5) is too generous. |
| Ring shows circumferential compression lines after service | Stretch is negative — O-ring ID is larger than the gland. Pick the next size down. |
Standard AS568 dash numbers jump in fixed steps. Often you cannot hit an exact stretch number; you pick the closest dash and confirm you are still inside the 0–5 % (or 0–2 %, 0–3 %) window. This is normal — the bands are wide enough to absorb small AS568 increments.
Once you have your final O-ring dash number, the next step is to recompute the effective CS (which decreases slightly because of the ID stretch) and use that for your squeeze and gland-fill calculations. Those cross-section reduction tables are coming up in Part 4.
With the O-ring picked and the gland sized, the design converges on three numbers that determine seal performance: compression squeeze, compression ratio, and gland fill. Part 4 walks through each, including the ID-stretch cross-section correction tables.
Continue with Part 4 — Compression Squeeze, Compression Ratio, and Gland Fill.