Manual for servicing a Showa Shock here

[BruKen]

http://idriders.com/proflex/files/shock%20service.pdf

http://www.mx-tech.com/downloads/Showa_shock.pdf

[BruKen]

Dealer says to use Pro RSF 5 weight oil and pressurise with Nitrogen to 8.5 - 10 bar.

[BruKen]

Well gentlemen, having used the first manual link above as a guide I can assure you that if you follow it letter perfect the job is a doddle. I got some O-ring seals and the shaft seal  from a seals specialist for under a tenner, 5wt oil for 8 quid and saved myself 100 pounds on what turned out to be a very simple job. Oh, I needed a tyre valve extender to be able to pump gas into the bladder in the resevoir. In the end I used argon. Nitrogen is used in shocks to prevent the oil foaming, but as all gas is in the bladder I put my faith in Boyles Law. So saying, provided you use compressed air from a tank (not a bicycle pump) and you have a moisture filter on and a filter on the pump, I can see no reason why you couldn't use air SO LONG AS IT IS DRY. The amount of gas, even at 120 psi that goes in would not fill a thimble. Even though the bladder is the size of a egg cup it is emptied of gas and crushed when you put the end seal on the shock back in ( without which you would no get the seal back in because of hydraulic lock, you deflate the bladder to suck the seal down to get the retaining C ring in)

Pics to follow

[Mustang]

So saying, provided you use compressed air from a tank (not a bicycle pump) and you have a moisture filter on and a filter on the pump, I can see no reason why you couldn't use air SO LONG AS IT IS DRY.

from the manual you reference located between step 32 and step 33 is this bit of wisdom ..........

Caution:
Air should never be used in shocks since it will
expand with heat and could cause excessive
pressures in the shock resulting in a potential
serious problem.

[BruKen]

Indeed that is why I thought to mention it. The issue is air humidity and condensation leading to vapour and increased pressure when hot. If the pumped air is dry and filtered I cannot see a problem. After all air is used in the front forks quite commonly. I have used argon in mine. More often than you would give credence for, things are done a certain way because that's just the way it always has been done. Why nitrogen? Because it's the cheapest pure gas to produce and is stable. Why in shocks....to prevent foaming of the oil through impurities. The showa has a bladder, so the two never mix. Pure air without moisture has the exact same pressure temperature @ volume properties as every other gas. That is Boyles Law.

[Mustang]

ok
 a scientist I am not and I guess I just follow tradition .........
but even you used gas instead of air , .......correct??

[BruKen]

That is correct. I used what the equation refers to as an ideal gas
Simply because I have it readily available.

Relation to the ideal gas law
 http://en.wikipedia.org/wiki/Charles%27s_law

What I also did was express an opinion with some backup. I am not god nor did I design the shock. Like everything posted here it is the end users own responsibility if they follow advice gained from others experience and shared here. Fair? I am not a mad scientist btw lol.

[Mustang]

hell I pilot a sidecar rig .. you mean there is supposed to be something besides a spring back there .............

[BruKen]

Hah! I don't think there is a man here who doubts your prowess at all things tiger.

[rybes]

in case anyone has problems findin the seals for ya rear shock. i got mine from this place

http://www.k-tech.uk.com/ very helpful people to deal with.

mine was a kayaba shock and the part numbers are

dust seal  (KYB-120301600201)
o ring  is 37 x 5
rod seal  (KYB-120271600101)

they are year old part numbers so theyve possibly changed. so check before buyin