Urethane vs. Buna

The blowout in my AA Vigilante regulator yesterday was a result of a compromised polyurethane (urethane, no relation to urethra) o-ring around the large brass piston inside.

So I started looking for replacements.  The o-ring is very close to a tank o-ring, and I have a large pack of those that I keep in my parts pod.  I pull them out.  Not quite the right size, but worth a shot.

I stretch one out to fit the o-ring.  It snaps.

I try another one; that breaks as well.

I'm just a simple minded animal, so I try another one.  That breaks.

I've read elsewhere that the aging of polyurethane is still not a completely understood process.  It continues to "cure" and become brittle over time or something, which is what I have definitely experienced with many o-rings.

When I look for replacement o-rings of any type, Buna always comes up, and it's always freakin' cheap.

So what's the difference?

Apparently, polyurethane is more durable, so it's good for moving seals -- stuff like bolts, regulator pistons, etc.

Buna is good for static seals -- stuff that doesn't move.

For my vigilante, I probably want a urethane o-ring.

The more interesting question is what should I replace my now-dead tank o-rings with?  Urethane or Buna?

For my HPA tanks, I think buna works just fine.  I tend to leave my tanks on the gun and never remove them (having an on/off with bleed helps), so in that case it's more like a static seal.

If you're using CO2 where you MUST remove the tank to get it filled, you'd think you'd want urethane.  However, CO2 seems to seriously fuck up urethane o-rings anyways.  So you're still losing.

On the other hand, I think buna o-rings last much longer in storage than urethane ones do.  They aren't as prone to becoming brittle like I've seen with urethane ones.

So given all this on top of the enormous price differential between buna and urethane (a urethane tank o-ring is like 50 times more expensive than buna), I think buna is probably the way to go for tank o-rings.

Also, side note:  the reg seat that came with my Vigilante was worthless.  Was some sort of hard acrylic plastic that didn't seal properly, leading to extremely hot shots if you let it sit for any amount of time.  Replaced with an Automag reg seat (see, there was a reason I bought these things); worked fine after that.