Worst-case radial squeeze for piston, rod, and face glands — SAE AS4716 Appendix A stretch reduction, full tolerance stacking, eccentric shift, runout, and thermal contraction.

Inputs

O-ring seated in a piston groove, sealing against the cylinder bore.

O-Ring
AS568 Size — Optional (sets O-Ring ID & CS)
Dimension Nominal ± Tol Min / Max
O-Ring inner ØID —
O-Ring cross sectionW —
Dimensions
Dimension source

Aerospace gland per SAE AS5857 Design Table 4-1 (published limit dimensions).

Standard gland size
Dimension Nominal ± Tol Min / Max
Piston groove root ØG —
Cylinder bore ØB —
Piston outer ØP —
Gland (groove) ØG —
Rod ØR —
Groove depth (axial)D —
Groove width — enables gland fillGw —
i By default, “eccentric worst” squeeze includes decentering from the running clearance between the bore and the piston (or rod) — i.e. half the difference between their nominal diameters — on top of runout. That clearance lets the seal shift off-center even at zero dimensional tolerance, because it's a real design gap, not a manufacturing variation.

Check this if some other feature (a pilot, register, or separate close-tolerance guide) is what actually keeps the piston/rod centered, so this particular bore/piston or bore/rod fit never turns into real decentering. With it checked, eccentric squeeze reflects only the Runout TIR above — set Runout to 0 as well to see pure concentric-equivalent squeeze.

This does not change the direct squeeze calculation (cross section vs. groove/gland fit) — only the extra allowance for the seal shifting off-center.
Thermal Analysis — optional

Leave temperatures blank to skip thermal analysis.

Piston / groove CTE (1/°F)
Cylinder / bore CTE (1/°F)
Results
i Switches the tolerance case drawn in the schematic below — it doesn't change the results tables, only which stack-up the picture shows.

Nominal — mid-tolerance geometry (average of the min/max endpoints).
Min‑squeeze — the worst-case combination that produces the least squeeze (largest gland, smallest O-ring).
Max‑squeeze — the worst-case combination that produces the most squeeze (tightest gland, largest O-ring).

The Min↔Max slider interpolates the drawing between those two worst cases so you can see the ring compress or relax as tolerances stack up.

Why Min and Max still differ at zero tolerance: the “eccentric worst” squeeze used here also accounts for the piston (or rod) shifting off-center inside its bore clearance — half the difference between the bore's and piston's/rod's nominal diameters, plus half the Runout TIR. That's a real design gap, not a manufacturing variation, so it doesn't go away when you zero out the tolerance fields. If your assembly is centered some other way (a pilot, register, or separate close-tolerance guide) and this particular clearance can't actually turn into decentering, check “Assembly is externally centered” below the Runout field to remove it from the eccentric calculation.

Click the schematic to enlarge.

Design Guidance Bands
CheckRecommendedWarningCritical
Min % squeeze (eccentric worst case)≥ 10%0% – 10%≤ 0% (contact lost)
Max % squeeze (eccentric worst case)≤ 30%30% – 40%> 40%
Installed ID stretch X0% – 5%5% – 10%> 10%
Gland fill (worst case)≤ 85%85% – 90%> 90%

Bands follow common industry practice (Parker ORD 5700, AS4716 gland tables) and are intentionally generic. Dynamic glands typically run lighter squeeze than static; small cross sections tolerate higher percentages. Adjust ORingSqueeze.GUIDANCE to your application standard.

Method Reference
SAE AS4716 Appendix A — Worst-Case Squeeze
§1

Stretch Reduction — AS4716 A.2.1

Concept

Stretching an O-ring over its seat (the piston groove root diameter for piston glands, or the rod diameter for rod glands) thins its cross section before any squeeze is applied. This reduces the effective sealing interference at assembly.

The minimum-squeeze case uses the minimum O-ring ID with the minimum seat diameter; the maximum-squeeze case uses the maximum O-ring ID with the maximum seat diameter.

Face seals: the O-ring rests flat in the groove without circumferential stretch, so \(X = 0\) and \(R = 0\). The full nominal cross section is used for squeeze.
Equations
Stretch ratio (A.2.1) \[ X = \frac{D_{seat} - ID_{oring}}{ID_{oring}} \]
Reduction factor R (A.2.1) \[ R = \begin{cases} 0 & X \le 0 \\[4pt] 0.0001 + 1.06\,X - 10\,X^{2} & 0 < X \le 0.03 \\[4pt] 0.0056 + 0.59\,X - 0.046\,X^{2} & X > 0.03 \end{cases} \]
Installed (reduced) cross section \[ W' = W\,(1 - R) \]
Stretch limit: installed ID stretch above ~5% is generally avoided — beyond thinning the cross section, it accelerates elastomer aging and fatigue.
§2

Worst-Case Squeeze with Eccentric Shift & Runout — AS4716 A.2.3

Piston Gland (A.2.3.1 / A.2.3.2)

A piston floats on its diametral clearance within the bore. In the worst position the piston shifts fully to one side, opening the gap at the O-ring by half the diametral clearance plus half the allowed runout between the groove root and the guide bore.

Min-squeeze case (piston) \[ S_{min} = W'_{min} - \left[\frac{Bore_{max} - Groove_{min}}{2} + \frac{Bore_{max} - Piston_{min}}{2} + \frac{runout}{2}\right] \]
Max-squeeze case (piston) \[ S_{max} = W'_{max} - \left[\frac{Bore_{min} - Groove_{max}}{2} - \frac{Bore_{min} - Piston_{min}}{2} - \frac{runout}{2}\right] \]
Rod & Face Glands

Rod glands mirror the piston logic with the housing groove diameter, rod diameter, and the rod-guide bore diameter. The guide bore sets the rod’s eccentric shift allowance.

Min-squeeze case (rod) \[ S_{min} = W'_{min} - \left[\frac{Gland_{max} - Rod_{min}}{2} + \frac{Bore_{max} - Rod_{min}}{2} + \frac{runout}{2}\right] \]

Face seals have no eccentric shift term — squeeze is purely axial cross section minus groove depth:

Face seal \[ S = W' - D_{groove} \]
Percent Squeeze
Always referenced to W′ \[ \%S = \frac{S}{W'} \]
§3

Thermal Contraction & Expansion

Concept

Elastomers shrink roughly an order of magnitude faster than metals. A nitrile O-ring at \(\alpha \approx 9{\times}10^{-5}\,/\!{}^\circ\text{F}\) loses about 1.3% of its cross section going from 75 °F to −65 °F, while steel barely moves.

Each dimension is scaled independently by its own material’s CTE. For a piston gland: the groove root and piston diameters scale by the piston CTE; the bore scales by the cylinder CTE. The reduced cross section \(W'\) scales by the seal CTE, then the full squeeze calculation is re-evaluated at the operating temperature.

Note: fluid swell (volume increase from chemical absorption) is not included. Add the volumetric swell fraction from compound data sheets to the fill and maximum-squeeze checks.
Equation
Linear thermal scaling \[ d_T = d\,\bigl(1 + \alpha\,\Delta T\bigr) \] \[ \Delta T = T_{final} - T_{initial} \]
CTE Defaults (mid-range, /°F)
Materialα (×106 /°F)
Nitrile (NBR)90
Viton / FKM83
Silicone (VMQ)100
Steel 304/3169.6 / 8.9
Aluminum 606113.0
Titanium Ti-6Al-4V4.8
§4

Gland Fill

Concept

The O-ring must have room to deform under compression and to grow with heat and fluid swell. Fill compares the seal’s cross-sectional area to the groove’s cross-sectional area. Common practice limits worst-case fill to 85–90%.

Worst-case fill uses the maximum cross section in the minimum groove space and groove width — the combination that leaves the least room.

Above 90%: no margin for thermal expansion or fluid swell; risk of extrusion damage and seal distortion on assembly.
Equation
Gland fill \[ Fill = \frac{\tfrac{\pi}{4}\,W^{2}}{depth \times width} \]

where depth is the radial gland space (or axial groove depth for face seals) and width is the groove width.

FillStatus
≤ 85%OK — adequate room
85–90%Warning — monitor swell
> 90%Critical — redesign
References & Important Notices
  • SAE AS4716, Gland Design, O-ring and Other Elastomeric Seals — Appendix A, sections A.2.1 (stretch/reduced cross section) and A.2.3 (squeeze cases). AS4716 is a copyrighted SAE standard; this page implements the calculation method and is not a substitute for the standard itself, which governs gland dimensions, surface finish, and application limits.
  • SAE AS568, Aerospace Size Standard for O-rings — source of standard O-ring inner diameters, cross sections, and tolerances (also published as Parker ORD-5700 Table 4-2). The size data was cross-checked against Trelleborg and Prepol AS568 catalogs. Two typographical errors were corrected: −015 cross section (0.070″, printed “0.007”) and −377 ID (9.975″, printed “9.925”). Sizes −468 to −470 are omitted pending verification. 9xx boss-gasket gland data is governed by SAE J1926-1, not this calculator.
  • Parker Hannifin O-Ring Handbook (ORD 5700) — general design guidance on squeeze, stretch, and gland fill. This calculator is not affiliated with or endorsed by SAE International or Parker Hannifin.
  • CTE values are representative mid-range figures from ASM Handbook Vol. 2, MMPDS-01, DuPont/Solvay data sheets, Parker ORD-5700 Appendix B, and NSWC-11 §3.2.3. Verify against the specific compound and lot data for the application.
  • Results are a design screening aid based on worst-case linear tolerance stacking. Verify final designs against the governing standard and by test. Thermal results assume linear CTE with no fluid swell.