F1 Clutch slave cylinder O-Ring

[hms86f1]

I have an F1 clutch slave cylinder w/ the o-ring inside the bore. I measured the original as OD=39MM, ID=31MM and cross-section=4MM. Is that correct? I found a source for an EPDM (brake fluid safe) O-ring of those metric dimensions but see that they come in different hardnesses: 40, 50, 60, 70, 75 and 90, the standard being 70. Does anyone know about the correct hardness?

Also, in the Pantah forum I saw reference to the O-ring size as "a-218" Is that a non-metric (inches) size equivalent?

Any clarification is appreciated.

Thanks

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[hedton]

Maybe the same as on the Mille?
https://forum.bevelheaven.com/viewtopic ... 471#p23471

[hedton]

Ai overview: The correct hardness for the clutch slave cylinder piston O-rings housed inside the bore of a vintage Ducati Mille (as well as related Bevel square-case models like the S2 and MHR) is 70 Shore A.When sourcing replacements for this specific dynamic application, ensure they meet the following critical specifications:Material: EPDM (Ethylene Propylene Diene Monomer). Do not use Nitrile (Buna-N) or standard Viton, as they are chemically incompatible with DOT 3, DOT 4, or DOT 5.1 brake fluids and will quickly swell and degrade.Hardness: 70 Durometer (Shore A). Avoid harder 90 Shore A rings, as they will cause an excessively stiff clutch lever pull and fail to conform properly to the aged cylinder bore.Dimensions (for the factory 2-groove Mille casing):Lower O-ring: 31.34 mm x 3.53 mmUpper O-ring: 31.42 mm x 2.62 mm.