crimp rings for pex to polybutylene coupling

[RustyShackleford]

Why do pex to polybutylene adapter couplings come with two crimp rings - one black and the other copper ? The outside diameter of the two kinds of pipe is the same (I understand the ID of pex is smaller, so it's important which end of the fitting is which). And my understanding is that even the pex crimp rings (the black one) is actually copper; so why can't you use those on both ends of the fitting ?

 

[Matt30]

Poly B does have a slightly bigger OD. They used to be two different sized rings, one had a PB marking on it for the poly and was slightly wider. Though in the last few years I have noticed the two rings were pretty much identical except for the colour. To save any confusion I just toss them both away and use my stock pex rings on both. They fit more snug on the poly and it works. The poly B ring on the pex would leak.

 

[RustyShackleford]

Ok, thanks, but according to these two sites, OD of PB and pex are identical:

https://sizes.com/materials/pipe_polybutylene.htm

http://www.pexuniverse.com/pex-tubing-technical-specs

Sounds like you know what you're talking about though, so I won't stress if I don't have the official PB ring.