As a postscript to this, I recommend getting a gear puller ready before starting
because the pin at the base of the gas shock does not move in normal gear operation and is thus almost certain to be corroded in place. One can hammer it out, and people do, but there is a risk doing that because the gas shock body is aluminium. If you crack it, it is scrap…
As a further postscript, I think the upper seal should have a bit of Loctite thread locker on it. Otherwise it can quite easily pop out. The CMM does not mention this but the original seals were so hard to remove (even destroying them) that Socata probably did use something like that.
If the seal pops out, the upper o-ring also becomes dislodged, but the lowest o-ring maintains the pressure just fine. In fact, as I wrote above, Socata assembled one of the two units with just one o-ring and it lasted 20 years 
Fascinating write up Peter. I recently did the mains on my US based P28R (it was listing rather to the right, and as long as we were doing one, the overhaul kit for those was pennies, as measured in aviation dollars). While it started getting low and I was out on a trip I popped into hangers at a few fields and offered some beer money for a wiff of nitrogen to keep me going for a bit — although I suspect this level of access to Nitrogen at random airports is a US phenomena. I can confirm that the lifting apparatus was “go out to the end of the wing and lift it up a bit more”.
One rather colorful gentlemen explained to me a different field repair for a completely blown strut — namely a length of radiator hose wrapped around the strut, secured with a couple of hose clamps and ginger landings. I make no comment on how exactly that would fare in a ramp check, but funnily enough when I mentioned it to my A&P he said “oh yeah, that works”, so I guess it’s a known method.
I did ultimately buy a normal sized bottle of nitrogen to keep in the hanger for those spring and fall temperature changes requiring a bit of a top up. I got a used welding tank on Facebook marketplace and swapped it out at the local gas dealer (the same people who swap out my oxygen tanks).
Thanks
The bottleneck on this job is the need to jack up the wing (only one jack is needed) so you have to borrow that somewhere, because it has to be fully extended to be gassed up.
Why exactly not gas it up for decent stand height ??
Calculate the required pressure, based on the rod diameter and the weight. It is 2000-3000psi, which is nontrivial especially as 3000psi is what comes out from a brand new gas cylinder.
However, it is pretty obvious that the 666 psi stamped on the unit is not enough; it wants to be about 700 psi for the TB20. A small difference makes a big difference to where it ends up sitting.
Why insist on nitrogen in shocks at all, air got 80 percent of nitrogen and 20 percent oxygen will not matter at all
It can be air but needs to be dry to avoid corrosion, and nitrogen is widely used because – like most gases – it is produced by cryogenic distillation and is thus automatically dry.
The strut top seal on the Socata popping out just confirms my less enthusiastic respect for Socata designers
Which aspect of the top seal would you improve on and what with?
One thing I need to sort out for next time is a tool which ensures the top seal goes in perfectly square. One also needs – if using a hydraulic press – one with a pressure gauge so one can see when the seal has bottomed-out.
This is what happened
and you may be right, but
from a small air leak at the lower oring has built up between rings the upper o-ring will press out the wiper , so better leave out the upper o-ring.
I do not believe the oil filling to reach up to the o-rings so then no oil will leak past them. It is only air fill that pushes on o-rings.
The lifted wiper seal may be useful as a tell-tale of small air leak inside but then you just look at stand height of LG and that should do. Fill up
as required while aircraft on wheels, not worthwhile for jacking up the lot.
That sounds plausible, but it is necessary to lift up that 2nd o-ring only maybe 1-2mm and the pressure under it will escape into the base of the wiper seal, from where it will easily escape to the outside.
Nothing surprises me re Socata lack of honesty (seen way too much, like my new plane having been built with a lot of used avionics, resulting in a warranty bill of 5 digits) but this would be subterfuge by the gas shock supplier – prob90 some obscure French company catering for the homebuilt market, now long ago bankrupt – rather than Socata who would have not known anything about it.
Quite likely any known “seal popping” issue would have been quietly fixed with Loctite on the wiper seal, which I did this time. Those seals were a total bastard to remove and had to be cut into bits to get them out, suggesting they were a bit more than a “press fit” 
It will be interesting if the other one pops out… I should probably try to push some Loctite onto the wiper seal on that.
