Pick the wrong gland and the O-ring either does not seal (gap too big), or it gets crushed and tears (gap too small). In this second post of the O-Ring Design Reference series we translate the three gland types from Part 1 into actual numbers: how to read an O-ring's dimensions, what H and W mean on the hardware, and the three formulas that size an axial or radial groove for an AS568 O-ring.
Every rectangular O-ring groove looks the same at first glance: a slot with two opposite walls and a bottom. But the two pairs of walls do very different jobs:
| Surface Pair | Job | Relationship to O-ring CS |
|---|---|---|
| Sealing surfaces (the two opposite walls that press axially or radially against the O-ring) | Create the sealing force by being closer together than the O-ring's cross-section. | Distance between them (H) is less than CS. |
| Containing surfaces (the remaining two walls) | Hold the O-ring in place laterally so it does not walk, spin, or extrude sideways. | Distance between them (W) is larger than CS. |
If you ever see a gland that violates either of those inequalities, that is almost always the leak.
An O-ring is described by exactly three numbers. Every drawing, every catalog entry, every groove calculation starts from these:
(OD – ID) / 2.In an AS568 designation, the dash number (e.g. -214, -326) maps to a specific nominal ID and CS. The rest of the groove geometry is derived from those two numbers plus the hardware.
The formulas below are written out in long form so you can paste them directly into a spreadsheet or Python script.
The O-ring grooves into the piston, the piston slides into the bore.
| Quantity | Formula | Notes |
|---|---|---|
| Gland height | H = (Bore – Gland) / 2 | Gland here is the piston outer diameter (often called the gland diameter). |
| Gland width | W = W | Taken directly from the groove drawing on the piston. |
The O-ring grooves into the bore (gland housing), the rod passes through.
| Quantity | Formula | Notes |
|---|---|---|
| Gland height | H = (Gland – Rod) / 2 | Gland here is the bore diameter of the housing that holds the ring. |
| Gland width | W = W | Taken directly from the groove drawing in the housing. |
The O-ring sits in a groove on a flat face and is compressed axially between two flat surfaces — common in end caps and covers.
| Quantity | Formula | Notes |
|---|---|---|
| Gland height | H = (Out – In) / 2 | Out is the outer depth reference, In is the inner depth reference. |
| Gland width | W = W | The radial width of the flat groove. |
Tip. The H formula is structurally identical for all three types: subtract the inner hardware dimension from the outer one, divide by two. This is a useful sanity check. If your radial gland comes out higher than CS or lower than roughly 0.7×CS, recheck the dimension being subtracted.
Suppose you are designing a piston that mates with a bore, and you have chosen an AS568 -214 O-ring (nominal ID = 0.984 in, CS = 0.139 in).
1.500 in.0.111 in. That means Gland ≈ Bore – 2H = 1.500 – 0.222 = 1.278 in.-2XX series a width of 0.188 in to 0.250 in is common. We will revisit width in Part 4 when we cover gland fill.This is the exact same arithmetic a CNC programmer does when they machine the piston groove. The whole design chain starts with those two formulas.
The formulas above give nominal dimensions. In production:
Stacked the wrong way, this gives the largest O-ring in the smallest groove — the worst case for compression. You must run this tolerance stack to make sure nominal-case squeeze still satisfies the minimum recommended squeeze (>0.005 in / 0.13 mm) we will cover in Part 4.
Now that we know the hardware geometry and have a valid H and W, the next post zooms out to the O-ring itself: choosing the ID or OD relative to the gland so the ring is slightly stretched (piston) or slightly interfered (rod/face), without going so far that installation becomes a fight.
Continue with Part 3 — ID Stretch and OD Interference.