The plastic cap that comes with most tanks. Does the same thing i guess but who wants some flimsy plastic cap that can crack and/or come off easily?
Marker Setup:
Alien '06 Interceptor
Lucky Un1tech
AKA Sidewinder
CP direct mount
rip'd Halo B2
Guerilla Ambush 68/45
Strengths:
Screws on & protects those threads!
Weaknesses:
O-ring doesn't actually fit on top.
Presses in on some tank valve pins.
Review:
Those yellow caps that come with most tanks do the job ok, i suppose, but i don't trust the threads of my tank to some cheap plastic thing.
Picked up one of these the day i bought my Crossfire. Can't say much about it other than it was worth the few dollars it cost. I can't imagine being stuck with a useless tank after the threads get messed up. Something like that could really ruin more than just my day.
I don't know if it's just mine or what but a tank o-ring doesn't stay put on my PMI thread cap. Falls off every time - it's too small.
The cap fits fine on my Crossfire but on my cousin's PMI tank, the thread cap pressed in on his tank's valve pin when screwed on completely. Same thing happens when i try to screw it on my Guerrilla Ambush tank reg. My cousin solved his problem by sticking an o-ring up in there as a spacer. Hey, that's where you could put a spare o-ring :P.
Conclusion:
It's cheap, protects your threads. If ever your yellow plastic cap fails, get one of these thread savers.