Seals and O-rings

AS568 O-ring sizes and FKM vs FFKM selection

AS568 is the standard size list for inch O-rings. Each dash number fixes an inside diameter and a cross section, so “AS568-214” means the same size from every manufacturer; the material and hardness are specified separately.

For semiconductor equipment the material choice usually matters more than the size: FKM covers many general seals, while FFKM is used where heat, plasma or aggressive chemicals shorten FKM life.

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AS568 dash-number series

Cross section by dash-number series (Parker O-Ring Handbook ORD 5700)
Dash series	Nominal CS	Actual CS	Tolerance
-004 to -050	1/16 in	0.070 in (1.78 mm)	±0.003 in
-102 to -178	3/32 in	0.103 in (2.62 mm)	±0.003 in
-201 to -284	1/8 in	0.139 in (3.53 mm)	±0.004 in
-309 to -395	3/16 in	0.210 in (5.33 mm)	±0.005 in
-425 to -475	1/4 in	0.275 in (6.99 mm)	±0.006 in

Within a series the cross section stays the same and the inside diameter grows with the dash number.

How to pick the size

Measure or read the groove: gland depth and groove width set the cross section; the groove diameter sets the inside diameter.
If you have an old part number such as Parker 2-214, the last three digits are the AS568 dash number (AS568-214).
Do not change cross section without changing the groove.
Perfluoroelastomers may need more squeeze than FKM; Parker notes that groove recommendations may need adjusting for FFKM and for semiconductor applications.

FKM vs FFKM at a glance

Upper service temperature in air (DuPont Kalrez technical literature)
Material	Upper service temperature	Typical semiconductor use
FKM (fluoroelastomer)	204 °C (400 °F)	General vacuum, gas-line and fitting seals at moderate temperature
FFKM, Kalrez 9100 / 9300 (plasma grades)	300 °C (572 °F)	Plasma etch and deposition chambers
FFKM, Kalrez 9400 / 9500	310 °C (590 °F)	Plasma processes
FFKM, Kalrez 9600	315 °C (599 °F)	Plasma processes
FFKM, Kalrez 7075	327 °C (620 °F)	High-temperature applications such as metal CVD

Limits are based on air oxidative stability; limits in specific chemicals or plasmas are often lower. Use the compound data sheet for your process.

When FFKM is worth the cost

Seal temperatures above the FKM limit.
Exposure to plasma, ozone, or aggressive wet chemistry where FKM swells or erodes.
Low outgassing or low particle requirements inside the process chamber.
Where seal change-outs cost more in tool downtime than the seal itself.

What to send with an O-ring RFQ

AS568 dash number (or ID and cross section) and quantity.
Material: FKM with hardness, or FFKM with the compound or the process conditions.
Process: temperature, chemistry or plasma gases, vacuum or pressure, static or dynamic.
The OEM part number if you are replacing an equipment seal.

Frequently asked questions

What does an AS568 dash number tell me?

It fixes the O-ring inside diameter and cross section. For example, every size from -201 to -284 has a 0.139 in (3.53 mm) cross section. It does not specify material or hardness.

What is the temperature limit of FKM vs FFKM?

DuPont lists FKM at about 204 °C (400 °F) upper service temperature in air. Kalrez FFKM grades range from about 220 °C to 327 °C depending on the compound; plasma grades such as 9100 and 9300 are listed at 300 °C.

Can I drop an FFKM O-ring into a groove designed for FKM?

Often yes for size, but Parker notes perfluoroelastomers may need more squeeze than FKM for best sealing, so check the groove fill and squeeze before switching.

Which AS568 sizes and materials does MST Store list?

MST Store lists AS568 sizes in FKM 75 Shore A and in FFKM, with the FFKM compound and hardness confirmed in the quotation. Other sizes and compounds can be sourced on request.

Quote AS568 O-rings

Send the dash numbers, material and process conditions. MST confirms the compound with the manufacturer and replies with price and lead time.