I have just changed the standard air filter on my 2008 883 Sportster for a K&N unit that fits in the standard "can".
When I removed the old filter, I noticed some pieces of rubber in the filter which turned out to be pieces of the "o" rings which seal the filter backplate to the breather. I have not replaced these yet as I could not see what useful purpose they might serve, and my bike is running well.
Does anyone know why O rings are fitted in this very hot environment?

 

Gonna take a guess and say that they provide a seal to keep unfiltered air out of the inside of the air filter which could then go into the other parts. Plus without them you're gonna have two metal parts vibrating against eact other.

 

If I was to take a guess, I would have to agree with trmac

 

No O-rings on mine and it runs just fine! 99 883c with the K&N

 

They are there to prevent the blow-by oil, from running down the engine behind the back plate. They are a common size o-ring, available at most auto parts stores.

 

+1 - The correct answer........

 

Thanks to all, particularly Wraith, who I think has the correct answer. I think I will leave things ( like Woodworking1) until any leakage becomes a problem.

 

I had only had my 2007 XL883C a few weeks. Long enough to ride to Sturgis and back (3,000 miles). By the time I got home there was a little oil on my head and blowing back on the oil tank. The o-rings were missing. Apparently when the previous owner replaced the air filter with a full flow filter he didn't install the o-rings. I just used a little high temperature grease to help seal it and tightened down the tubes good. No more problems. If the leak comes back I'll go buy some o-rings.

 

This is what I thought.

 

they seal the breather ports so they don't leak oil out on the engine, and they also seal the same area to keep dirty air from getting sucked in around the air filter (not much but some)