Noticed this morning that my forks were only giving up 50mm of travel before bottoming out rather firmly. So I twiddled the compression and it did nothing didn't lock out didn't firm up, nothing. Lowered the coil u-turn to 95mm and then they only had about 10-15mm of very firm travel (I didnt try the lockout/compression at this point). I was hoping that it was simply an o ring in the compression circuit so when I got back I removed that, it was still happening. The coil was undone just at the top because I didn't have the time to mess around with oil today and it still happened.

Am I right in thinking that an o ring in the rebound circuit has popped off and the oil has dropped down into the lowers and the uppers are bottoming out on this oil?

Would a simple strip down and a refresh of the oil and said o ring sort this out?

Am I right in thinking that an o ring in the rebound circuit has popped off and the oil has dropped down into the lowers and the uppers are bottoming out on this oil?

Sounds like it, yes.
There's an inner and outer O ring on the rebound shaft 'plug' in the bottom of the stanchion. I dunno which one fails but I replaced both on my Pikes and that sorted it.

I got mine from a bearing/O ring supplier, I'll see if I can find the thread I started about it.

🙂

Have a read through this

http://www.singletrackworld.com/forum/topic/pikes-leaking-damping-oil

But the important part is this
The inner seal (on the base plug at the bottom of the damping leg, that the shaft slides through) is VERY close to 3.53mm cross section by 12.29mm internal diameter. That's a tiny bit bigger than the original, with a slightly fatter x-section. Once fitted it feels a bit tighter than the one I removed.

For the outer seal I ordered 3 sizes to try them out –

2.62 x 22.23
2.50 x 23.50
2.50 x 23.00

I can't remember which one is in there offhand, but they all seemed pretty much identical once fitted. I'll get back to you on the exact one, but IMO it makes little or no difference.

And I got my O rings from here
http://simplybearings.co.uk/shop/index.php?cPath=19_4501

Great stuff. I already have a couple of o rings that are quite close to those sizes so I will just strip them down and put in what I have available and see how that works out. If it isnt any good I will order some different sized ones.

Any tips/ideas on oil weight/volume? I'm 14st if that makes any difference.

You can get oil weights and volumes from the service manual.

I was being lazy. found out now and got a bottle of Yamahas finest fork oil.

Right so the O rings I had were the wrong size so I just cleaned up the original ones and put them back in. I went for a blast around the Monkey Trail this morning and they seem fine. So far.

I have a little theory about how it happens without any obvious signs of wear on the O rings. I think I may have had the lockout turned on and without thinking wound down the travel. This will have caused a build up of pressure in the damping leg and forced oil pas the seal. Thats what Im hoping anyway but I'm going to get some spare O rings incase it happens again.

Judging by the colour of the oil they needed a service anyway so its all good.