I got the rubber o-ring on the mast extension jammed up the mast the other day. Took a while to de-rig and eventually had to break it.
Question: is it really required?
I thought it might reduce shock between the extension and the brittle carbon mast during landings, but then there isn't an o-ring between the mast pieces. Also the whole thing is of course really tight while rigged. Is it really there just to keep sand out?
Worry?
(I just hate going to Bunnings)
Nah dont worry, Bunnings wont have one that size anyway.[}:)] (actual size is not important,they just wont have "that"size)
Dont know if you need one or not though.
Transeals are the go for all O rings and bloody cheap too.
EP,
The O ring is used to contain the collar if you turn the extension upside down or fling the extension horizontally. IMHO, the O ring is needed.
You need to go to a specialist hydraulics outlet, such as Enzed or Bearing Service etc. Either take the extension and collar or take the O ring from another extension, so that the sales guy can use it for a sample. You'll probably have to buy about half a dozen (but they are cheap), because they probably won't sell singles.
Hope this helps,
yes you can lose the collar if you turn the extension upside down. So just do like me and keep it upright or tight the rope around it when you stock it. 
The main purpose of that O-ring is actually to avoid the contact between the carbon of the mast and the alloy of the collar. Carbon and alloy in salt water are not a good mix and the collar get corroded faster.
I personally don't bother about that o-ring but I had to remake a new alloy collar as mine just broke because of the corosion (it was quite old though).
