Changed the seals on my shock (product code CSN7) a few weeks ago. A few family rides on land rover tracks were fine, then got out to glentress on my own this afternoon. Halfway up i noticed the wiper seal had partly blown out from its seat, so i locked the shock out and bailed before doing any further damage. Did a bit of googling then stripped the air can this evening. There was no sign of the shock locking down when i released the positive air, and the main seals all looked fine even tho i had found the seals distorted a little during fitting – once the white split rings were in place everything looked fine. No sign of damage or distortion to the air can or the wiper seal seat either. I did notice its a real struggle to get the air can pushed back up to its housing to screw it back on – even with compression and rebound dials set to what the various guides say on how to do an air can service. It was the same when i replaced the seals a few weeks ago.

So, a) should there be that pressure when re-assembling – is that causing the wiper seal to blow off, and / or b) any other ideas?

Afraid your air can has started to collapse, if you pick seals out again you will see the damage. Time for a new can, they aren’t super cheap but do come with the seals fitted.

It’s usually caused by a build up of float fluid in the negative chamber meaning the pressure ramps up too fast.

What Pigyn says. If there’s oil (float fluid) on the negative side this can prevent the shock reaching full extension and the result is that the concentric machined rings in the air can can become deformed and allow the seal to pop out. Easy to check by removing the can and the seals and having a look at the inside of the can.

I had this on an old rp23. Went for service, first rude after wiper seal started coming out as the seat the seals in deformed. Had a new air can unfortunately

Is it the wiper seal seat that deforms, or the seat for the black rubber seal with the white retainers either side? Have stripped it again and taken the wiper out, no sign of any deformation. There is a slight linear score on the inside of the air sleeve tho…
Cleaned the internals and tried a dummy fit without any float fluid – still quite hard work to get the air can back up to engage the threads of the top bit.

It’s the top retaining ridge for the main air seal with the white glide rings. If you pull out the dust wiper again and look inside it will be more obvious

Right, i’ve pulled the dust wiper and had another look. First off, the shock as it was when the wiper blew:

View post on imgur.com

Still can’t see any obvious signs of deformation:

View post on imgur.com

View post on imgur.com

looking inside:

View post on imgur.com

I have a 15yr old float R kicking aorund, i’ll take that apart tomorrow night to have a look at its seal seats and maybe provide a comparison.

I reassembled with no fluid in the negative side, just a wipe of grease on the seals. Still had the same struggle to get the air sleeve up to the eyelet assembly – the internals are having to compress to get it that last little bit, so once assembled with 0psi in the +ve air chamber the shock is measuring 185mm with 45mm of travel showing. Pumping the shock up to 110psi (20% sag for my weight) extends the shock to overall 197mm with 56mm travel (7.756/2.20inches). The extension starts once about 10psi is in the body. Interestingly (confusingly) the fox product code gives 7.875/2.0in, whereas the spec drawing on the website gives 7.875/2.25, so i presume the headline details on the product code checker web page are wrong.
So it seems to me like the air trapped in the negative side during reassembly is unable to escape during the reassembly and thus remains there – is this right? or is there a one way valve that should allow that air to escape? Could that be causing the wiper seal to pop out if the pressure is ramping too quickly – because there is already some residual pressure in the -ve side? :-/