Fluorosilicone (FVMQ) vs FKM O-Rings

Fluorosilicone and FKM compared

The handbook describes fluorosilicone's mechanical and physical properties as very close to plain silicone's, with better fuel and mineral oil resistance and poorer hot-air resistance. The fluorosilicone O-ring page and the FKM O-ring page carry each family's grades, callouts and sizes.

Fuel service: where each one fits

Both families resist fuel, so the choice turns on temperature and motion. For a low-temperature seal in a fluid that attacks silicone, the handbook's answer is fluorosilicone: it keeps silicone's cold flexibility and adds the fuel and mineral oil resistance plain silicone lacks. FKM covers the same fuels with a longer chemical list and more heat, and it is the family for a seal that moves. Prints that name AMS-R-25988 or an M25988 part number are asking for fluorosilicone, and prints that name AMS7276 or an M83248 part number are asking for FKM.

One caution is written into the government's own fluorosilicone specification. The scope of MIL-DTL-25988, now inactive on its DLA ASSIST record, says the material is not suitable for phosphate-ester hydraulic fluids and generally not suitable for the gas turbine and helicopter transmission lubricants it lists, and it advises against silicone seals in engine oil systems for new designs. A print that puts fluorosilicone near those oils is worth a question to the drawing owner before the part is ordered.

Cold, heat and motion

Cold is where fluorosilicone earns its place: the handbook takes fluorosilicone down to about -73 C, while standard FKM stops at -26 C and special low-temperature FKM grades reach -46 C. Heat is the reverse, since fluorosilicone stops at 177 C and FKM is rated to 204 C, and higher for a shorter life. Chemours rates its Viton grades for time-limited service at 232 C (450 F), a limit counted in hours of exposure rather than a continuous rating.

Motion is the sharper divide. The handbook calls silicones as a group low in tensile strength, tear strength and wear resistance, gives fluorosilicone nearly the same mechanical properties, and limits both to static seals. It notes one moving use: a fluorosilicone O-ring acting as the spring behind a U-shaped shell of FKM or another wear-resistant material. FKM runs in moving seals too, though the handbook sets its low limit warmer in motion than at rest. The temperature range guide sets both families beside the others the handbook covers.

Where each one is used

These rows report published maker guidance and the documents prints name; Conquest quotes the material your print names. The FKM and silicone comparison covers the case where fuel is absent, and the brand-name fluoroelastomer post explains how an FKM callout differs from a brand callout.

Callouts, sizes and paperwork

The print governs. A drawing that names AMS-R-25988 or an M25988 part number is quoted in fluorosilicone to that document, and one that names AMS7276, AS83248/1 or an M83248 part number is quoted in FKM; the M25988 page, the AMS7276 guide and the M83248 guide carry the part numbers and the document statuses. A commercial print may give only the family and a hardness, and then the certificate names the compound that shipped. A swap from one family to the other is the drawing owner's change, never a substitution made at the quote.

Both families come in the AS568 dash sizes (M25988/3-028 is the AS568-028 size in fluorosilicone) and the ISO 3601 metric series, and fluorosilicone, like silicone, takes a tighter extrusion gap in the gland; the fluorosilicone O-ring page carries that gland note and the groove design guide the gland figures. A certificate of conformance ships with every order and names the part number, the specification, the manufacturer, the lot, the quantity and the manufacturer's cure date.

Parker is a trademark of Parker Hannifin Corporation; Viton is a trademark of The Chemours Company. No affiliation or endorsement is implied.

Sources

1. Parker O-Ring Handbook ORD 5700 (2021 printing), sections 2.2.7 (FKM), 2.2.8 (fluorosilicone) and 2.2.13 (silicone), read 2026-09-14 and 2026-09-27
2. Parker O-Ring Handbook ORD 5700 (2021 printing), section 3.10.2 (low temperature) and the hydraulic fluid compound list on page 3-16, read 2026-09-27
3. Chemours, Viton Fluoroelastomers Selection Guide C-11021 (9/19), page 1: time-limited service at 232 C (450 F), read 2026-09-27
4. DLA ASSIST record for MIL-DTL-25988 (scope and status), read 2026-09-24
5. SAE AMS-R-25988, AMS7276, AMS7259, AS83248/1 and AS3581 (titles only)
6. DLA ASSIST record for MIL-R-83248 (cancelled 21 June 2001), read 2026-09-25; the MIL-R-83248/1A cancellation notice (Notice 5, 2008-07-24), read 2026-09-29; SAE-AS83248/1 (ident 286630, adopted 2025-11-03), read 2026-09-25
7. ASTM D1418 and ISO 1629 (titles only)
8. SAE Mobilus current-revision record for AS83248/1A (issued December 2014), read 2026-09-27