I'm playing with regulator shim stacks so I'm doing a lot of pumping on my Mk4 and I have plenty of opportunities to try new piston o-rings.
The last Viton V90 didn't last very long and the x-ring ebay seller never responded to my question about durometer so I decided to just try a FKM 75 durometer X-ring from my theoringstore stash. The X-ring was a no go for me. It doesn't seal very well at all and I could even hear the bass piston scraping on the bore of the tube. There were signs of extrusion when I removed it.
Next I tried an 005 Disogen 9251 92 A durometer urethane o-ring from theoringstore. I've been reluctant to try them, because of their somewhat modest 250 F maximum temperature rating, but they are about as good as o-rings get when it comes to ruggedness in reciprocating seals, compression set resistance, and resistance to extrusion in situations where backup rings would otherwise be required. The transfer port within the piston's o-ring groove ("gland") makes the gland very wide and I've often thought about trying to custom fit the width of a Teflon backup ring on the side of the gland opposite the transfer port (which needs to seal against the o-ring rather than a backup ring during the upward stroke. The backup ring might be useful with non-OEM green o-ring choices but it must not interfere with the transfer port.
The Disogen 9251 urethane o-ring has endured a few hundred cycles now and it's still holding. I think the 250 F maximum is still practical and maybe Hill is using a urethane o-ring, disguised in green, instead of the more typical natural cloudy white color. It would also explain Hill's very conservative recommendations and cool-off periods with respect to pump heat accumulation. Even if its longevity turns out to be inferior to the OEM greenie, I'd be inclined to order some Mil. Spec. urethane duro 90s from Tim at Mac1 and try them. Mac1 is the only source I know for small quantities of mil spec urethane.
I'll keep everyone posted.
BTW, I've had excellent results after baking the Hill Dry-Pac desiccant (almost surely molecular sieve). I set my convection oven for 482 F (250 C), spread a thin layer of desiccant on a baking pan, and set the timer for 3-1/2 hours. I don't bother with a bell jar vacuum, dry gasses, or other complexities after removing it from the oven for cooling and storage. I simply pour the hot desiccant into a Bell canning jar and cap it until it cools.
Near milligram-accurate scale measurements before and after baking revealed that the desiccant holds 24% of its weight in water but, I believe it's not terribly effective in the pump after it reaches about 50% saturation. My dried desiccant lasts just as long as new Hill desiccant and I have no idea how many oven drying cycles it can endure before it loses effectiveness.
Unless you live in a very dry climate, don't shoot very much, and don't shoot a very high energy big bore PCP, I see no way that the Hill desiccant can last two or three months, as claimed. Regardless, I find that it works well when it's fresh, despite criticisms that drying should be done on the compressed air side of the pump (yup--better but extra complexity and pump-sapping dead air space). I change out my desiccant when I hear water spitting within the pump base when I quickly dump pressure to disconnect the pump.
After I've saved enough spent desiccant to fill my shallow baking pan, I crank up the convection oven. I save my Hill Dry-Pac refill bags and refill them, taping them shut with tape (metal tape preferred for improved humidity rejection). I only pay about nine or ten cents per kilowatt hour so it probably costs me somewhere around 35 cents to dry three or four refill bags full. Hill says "There is currently no method of regenerating the medium in domestic circumstances" but I read about many other methods of regenerating molecular sieve reported to be effective in online science and engineering forums and documents. Maybe my domestic circumstances are different from those in England. Maybe "most domestic circumstances" means your wife won't let you use the oven for industrial or "science fair" projects, as I call them.
That's why I bought my own counter top electric convection oven that I keep in my shop for such things (also heat treating lead cast bullets, drying 3D-printer filament, etc.).
