Rock Shox Mag 21 style air needle valve

[FSXStumpy]

Hey folks,

I ride a '95 Stumpjumper M2 with the old Specialized FSX carbon/ti air-oil fork. After nearly 20 years of service, one leg's air needle valves has failed. I can deal with it, but it makes accurate pressure setting a real chore.







Last night, I tore open one of the top caps (to replace a suspect O-ring, but took the opportunity to see how the needle valve was constructed), and took a few photos:

disassembled cap:



Needle Valve rubber element:




Now, from what I can see, that rubber element is "unobtanium". I intend to try to make a replacement by moulding an RTV sealant cylinder, with a small pilot hole cast to about 80-90% of its depth (this mirrors the stock piece's design), and I'll use a razor blade to cut the valve flaps in the closed end. Jam it up into the adjuster cap, and see if it works.

Anyone else here attempt this? Did it work? What material did you use? I'd appreciate any thoughts or advice on offer. This FSX WILL ride on!

Thanks,

J

 

[My_Teenage_Self]

I've just gone through the same with some RS Mag 21 SLs.

After much reading, I decided to have a go at making some.

So far they've been inflated/deflated about 10 times, and have since held pressure for well over a month in one hit. I was actually considering running a 'batch' and offering them here at cost (about £1 plus postage!)

but essentially;

find a metal/plastic sheet 8mm thick.

drill an 8.5mm hole through (ensure the bore ends up smooth) and smear the thinest layer of vaseline in it as possible

Fill with Automotive RTV silicone (ensuring no bubbles during the fill)

leave for AT LEAST a week, (that thickness takes a while to go off) then push out with an 8mm rod.

admire your plug, grease it, then fit into top cap.

sharpen a spoke, and make sure it's smooooooooth.

Grease said spoke, and push it through your fill hole.

Done.

Hope that helps.

 

[FSXStumpy]

That does, I'll give it a go. Not far off my original plan.

Thanks so much for responding! Love my old short travel air-oil forks...

J

 

[syncrosfan]

Depending on how original you want to keep your fork, you could retrofit it to proper Schrader valves. I have a '98 SID the first edition that had the stupid ball needle inflation valve, which of course was leaking air. A friend of mine got me two Schrader valves used on Oehlins air shocks. I drilled open the aluminum top caps and cut the specialty pipe thread in them. Put some Teflon tape around the valve threads, screw them in and voila: a fork that can be inflated with any shock pump on the market (hold air really well, haven't touched it in ca. 6 months, no air loss). The looks, well it's not original but with anodized aluminum valve stems it looks OK. I know I could have just bought aftermarket SID top caps with the schrader valves already built in, but I was too cheap to spend ca. 30$ per cap. (speaking of which I wonder if those said SID top caps have the same threading as the old Mag 21? In that case you could always switch them out).Here are some pics of how the FrankenValve would look like.

 

[FST4RD]

Anyone done that to Mag21s?

 

[FSXStumpy]

I've seen Mag 21's done that way, but on my bike at least, the Schraeders would strike the downtube should the bars be turned past 90. That'd probably destroy something... And like you say, I'd like to keep'em as original as possible.

I believe the Mag fork legs are rather smaller than the Judy family legs. Different threading. IIRC, 25.5mm vs 32mm?

Good ideas, though, and will be kept in my back pocket should RTV fail me. At least I don't need perfect valves, just something that leaks less than 5psi per minute, so I can get the screws in before the air's all out.

Will advise!

J

 

[syncrosfan]

http://www.ebay.com/itm/Rare-Rock-Shox- ... 2581a30c3a

Right now on e-bay! Get those and you'll be good for a while. Don't know what your budget is but hose will be the easiest fix for your problem.

 

[FSXStumpy]

Indeed, tempting... Thanks for the heads-up!

 

[My_Teenage_Self]

There's no guarantee that those valves don't leak too.

Honestly, the RTV works perfectly. If I find that they wear out after a while (and I can't see why they would any more than the originals) They've cost practically nothing, and take about 2 mins to swap.

Heck, send me your address and I'll knock you some up.

For the record, the screw doesn't really hold the air in, the rubber plug does all the work.

 

[FSXStumpy]

No, I intend to make a set for myself, along the lines of what you describe. Buying a set off the evil bay might get me sorted for a year or 2, but being able to make my own replacements will keep me set for as long as the forks exist. I like the self-sufficiency of that. I picked up a fresh tube of RTV last night, and will set up a 4 unit mould in the next week or so.

On the FSX, at least, with the screw in place, the rubber needle valve can leak, but it has nowhere to go. There is an O-ring that completes the seal.

I bought myself some time last night while the new valve cores get made, with a temporary solution: I made a spring-ring from a small spring, just a little smaller in diameter than the original rubber needle valve from the fork. I slipped this around the lower end of the valve, causing the needle passage to be constricted by the spring, then reinstalled the air cap. Where before the shock would loose all air pressure in about 5-10 seconds after withdrawing the needle, it now takes about 10 minutes to do so. In the time from needle removal to screw installation, it looses only a fraction of a pound for now, so I can keep riding while the RTV cures in the moulds.

This is kinda fun!

J