XS650: Viton® Valve Seals   Leave a comment

Had in mind to tart up one of the XS1 heads I have lying around. Picked up a set of Viton® valve seals for it. Reputed to outperform the originals. Got sidetracked and they sat on the shelf a while.

Viton® valve seal … metal jacketed


These are available with or without a metal jacket.

unjacketed valve seal


I am now sorting out a head for an extra SR500 motor. To go in the sidecar. Emphasis torque. Early 78/79 crankshaft and cases (larger crankshaft counter-weights), specially ground cam, larger valves, lighter valve springs, thinner spring plates, modified ports, carillo rod, O/S piston, modified/heavier flywheel etc. The XS650 and the SR500 use the same valve seals. No point leaving these sitting. Collecting dust.

Wonder how they will work.

Viton®. Flouroelastomer (FKM/FPM). A fluorocarbon based synthetic rubber. These contain 5% Teflon for reduced friction and wear.


  • have a wide and stable Temperature Range -30°C to 225°C (-10°F to 400°F)
  • resist hydrocarbons (fuel/oil)
  • excellent oxygen, ozone and weather resistance
  • incombustible
  • have excellent dynamic properties (coeff. of friction, tensile strength, range of hardness and resistance to compression set etc) – good abrasion resistance
  • have low permeation rates (esp. relevant to alcohols, hydrocarbons and biofuels (biodiesel, ethanol)
  • be solvent, mineral acid resistant (don’t really like ketones, amines or low mw esters,)
  • show low compression set


  • low alkali resistance
  • relatively poor mechanical properties
  • limited elasticity at low temperatures
  • the tensile strength decreases substantially at elevated temperatures
  • high price

The fluorocarbon rubbers are used for special applications that require good heat, oxygen or corrosion resistance and hot solvent and oil resistance.


  • car and airplane seals and hoses
  • fire-resistant coverings
  • heat-resistant insulators
  • o-rings, shaft seals
  • gaskets, fuel hoses, valve-stem seals

Note: Viton® is a registered trademark of DuPont. It is often misleadingly used as a generic term for FKM/FPM flouroelastomers.

For the chemically-minded FKM’s thermal and fluid resistance are a function of 3 main features

  • Extremely strong bonding between the polymer backbone carbon atoms and the pendant (attached) fluorine atoms
  • High ratio of fluorine to hydrogen atoms bonding to the carbons (high fluorination levels tend to create stable compounds)
  • Full saturation of the carbon backbone .. no covalent double bonds between carbons minimizes attackable sites

Structures of fluorocarbon rubbers

Monomers Structural unit Type designation Commercial types
VF 2 + HFP


Viton A, AHV, A-35, E-60,
Fluorel 2140, 2141, 2143, 2146


Tecnoflon SL, SH
VF 2 + HFP + TFE


Viton B, B-50


Tecnoflon T


KEL-F 3700, 5500, SKF-32


ECD 006


Typical FKM structure … flourocarbon rubber


An example of a structure of the fluorocarbon rubbers, VF2 / HPTFP / TFE -copolymer.


Posted March 23, 2014 by xscafe in Motor - Head, SR500

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