Nitrile or Viton O ring?

[Green1]

Yesterday my Aprilia spat its front sprocket off and locking the rear wheel lucky for me at low speed not to far from home.
Aprilia being Aprilia have discontinued the O ring that sits behind the sprocket being a vital part to keep the sprocket and Circlip in place I'm now confused as to use Nitrile or Viton as I don't want it to perish and and spit the sprocket off again.
Part 24 on the attached pic the O ring is 18x3,5
https://www.fowlersparts.co.uk/main/img/assemblies/large/849fe02f9f5d4e87a20bea68b32ae5a2.png?6

[Laverda Dave]

I've just read this on a nitrile vs Viton site:

Nitrile Rubber

    Temperature: Nitrile is effective within a temperature range of -30°C to 100°C.
    General Suitability: Used for general purpose applications requiring resistance to aliphatic hydrocarbons, petroleum oils, water and hydraulic fluids.
    Environmental Properties: Nitrile vs Viton® is not resistant to degradation from weather and ozone exposure. Designed to resist most oils and lubricants, more importantly petroleum based lubricants, these seals have other benefits, such as superior abrasion and tear resistance, making them suitable for heavy duty industrial applications.
    Applications: A cost effective product, that works well with constrained temperatures and chemical resistance.

 
Viton® Rubber

    Temperature: Viton® seals can withstand temperatures up between -20°C to 210°C. Viton® seals are prone to failure below -15°C in dynamic applications, as they become inflexible and hard.
    General Suitability: Viton® is best suited for diverse sealing jobs that require high temperature and chemical resistance. Applications involving contact with petroleum oils, acids, silicone fluids and gases, halogenated hydrocarbons (trichloroethylene and carbon tetrachloride), aromatic hydrocarbons usually have seals and o rings made of Viton®.
    Environmental Properties: Viton® O-rings and seals have a wide range of chemical resistance (involving mineral acids, fuels, and oils) and this material is also highly resistant to oxidation, UV exposure, fungus, weather, ozone, and mould. This makes Viton® far superior to Nitrile in these regards. They are also designed to resist most oils and lubricants, more importantly petroleum-based lubricants.
    Applications: Viton® functions well in different automotive, chemical and appliance-processing industries.
    Cost: As an approximate guide, Viton® compound is ten to fifteen times more expensive than NBR. However, we stock both NBR and Viton O-rings and seals AS/BS, metric, French, German and other metric series at Polymax.

Given the above I would be tempted to go Viton for it's resistance to oil/water/acids and high tempertaure etc.

[Johnwebley]

My preference is viton, I think it is tougher,

See if you can contact a supplier, and ask the benefit of each.

I am lucky  ,I have a friendly engineering store locally, great for replacement O rings for tappet covers.

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[Green1]

I did think Viton but I always like someone else to confirm things when I'm not sure
The one on there was only a year old it had gone hard and there were only little bits of it left in the sprocket cover. I can't remember where I got it from and never had a problem in the 19 years of ownership and a handfull of new chains and sprockets in that time.

I'v never really trusted it. It seems like a daft way of fitting it

[Moorey]

 Go with Viton its the go to seal in the chemical industry. A little dearer but nothing to bother about.

[Moorey]

Speaking of Viton O rings, any links to the best places to buy from?

Iv'e been using these lately. Quick and cheap viton is viton.
   https://www.ebay.co.uk/str/sealsrus?_trksid=p2047675.l2563

[Green1]

I got a quote from ultimate parts earlier as there normally my go to supplier for Aprilia parts. They recon they have the O ring and circlip in stock about £4 in total plus £15 postage and you all thought DSS took the piss charging a fiver.

Scrap that I just refreshed there page and it came back as £12.54 all in. How did that change that fast? Anyway to late now as parts are on order now

[Johnwebley]

I fitted the slightly larger O ring,

They compressed ok,

No leaks,

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[AshimotoK0]

One thing to remember is that Viton 'O' rings are usually a little harder than standard Nitrile .. Nitrile typically 70 Shore A and Viton are typically 75 Shore A