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.

Advantages:

• 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

Disadvantages:

• 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.

Applications:

• 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		FKM	Viton A, AHV, A-35, E-60, Fluorel 2140, 2141, 2143, 2146 SFF-26
VF 2 + HPFP		FKM	Tecnoflon SL, SH
VF 2 + HFP + TFE		FKM	Viton B, B-50
VF 2 + HPFP + TFE		FKM	Tecnoflon T
VF 2 + TFCIE		CFM	KEL-F 3700, 5500, SKF-32
PFMVE + TFE + X		FKM	ECD 006

 

Typical FKM structure … flourocarbon rubber

 

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