RS 32mm Solo Air Servicing - Which o-ring in negative air piston?

notb4dinner

Likes Dirt
Did an air spring service on my Recon Gold last week and lost track of which o-ring I removed from the bottom of the negative air piston.

There are two o-rings in the service kit of the same diameter but different thickness that would fit but if I install the thinner of the two it seems like the piston doesn't seal - very high pressures and the forks are still soft, although randomly they'll stiffen up especially if I cycle them quickly a few times in succession.

If I install the thicker of the two I can't seem to get the piston to fit back into the uppers. The other piston definitely takes a thicker o-ring but also has a wider o-ring groove so isn't as difficult to insert.

Picture for reference - A is the thinner o-ring, B is the thicker, the third marker shows the location on the piston.
Apologies for the lack of dimensions - I'll run the calipers over everything later on if needed.

Considering cross-posting to the fuckwits thread...


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Nambra

Postmeridian
I reckon I came across that exact same problem a couple of years ago with the same fork - the o-ring in the service kit was too fat and I couldn’t get that air shaft back in the stanchion no matter how hard I tried. I can’t remember whether I re-used the old o-ring or went for the smaller one in your pic.

Definitely not worthy of the fuckwits thread - it’s the “universal” service kits that are supposed to cater for all year models that are issue.
 

link1896

Is not a gynaecologist but will look at your fork
Notb4dinner, do you have a means for accurately measuring the diameter of the two surfaces the O-Ring needs to seal against? I can calculate for you the ideal O-Ring and you can go and buy the correct one.
 

notb4dinner

Likes Dirt
Thanks guys, glad I'm not the only one to have run into this.

The relevant dimensions are:
Piston OD: 27.5mm
Piston OD inside the o-ring groove: 22.5mm
O-ring groove width: 3.3mm
Upper bore: 27.6mm (may be a bit off since I had to take OD less wall thickness because there's a step on the inside before the wall the piston seals against)


Bit of a shame to have to go back to first principles given I've got the over-priced service kit sitting in front of me.
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link1896

Is not a gynaecologist but will look at your fork
21.5 x 3 mm in n70 buna is a good fit based on those numbers. gives a 4.65% stretch over the piston, fills the groove to 85%, is crushed 15%.

RS stanchions can be a bitch to get the piston into. Do they have a chamfer on the transition to the smaller OD or is it a 90 degree lip?
 

notb4dinner

Likes Dirt
Thanks, really appreciate that! I did have a go at using a couple of online calculators myself but didn't manage to find one that would directly recommend an o-ring based on the data I had.

There is a small chamfer on the lip (and yes both surfaces are well lubricated).
 
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notb4dinner

Likes Dirt
21.5 x 3 mm in n70 buna is a good fit based on those numbers.
On the basis of that I went back and measured all the old o-ring's I'd removed and there's a ~3mm one there that's not in the service kit (the thinner one is 2.something and the thicker one is 3.5mm), so I either I've lost one or Nambra's right and the forks came with a ring that's not in service kit. Will be off to the bearing shop tomorrow.
 
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