In the Cessna 172's nosewheel strut are several O-rings, but the
one that seals against the chrome piston sometimes twists in its groove
and a spiral failure (tear) occurs. All the outer-cavity oil escapes,
then the nitrogen leaves and the strut goes flat. The failure appears
to happen rather suddenly.
These are MS28775-329 rings. The rubber is an ancient type,
probably around for 40 years or more, and while there are much better
O-rings on the market now we can't use them since Cessna does not
approve them and no one, as far as I know, has done an STC on a better
ring. Some folks recommend Granville Strut seal, which I have tried and
haven't noticed any better life. The stuff is supposed to swell and
soften the seal.
The failure I see seems to be due to the ring sticking to the shaft
and getting rolled over. An anti-friction additive might be a better
bet. Does anyone who has worked on these things know if there are any
approved additives like that?

Dan