That's a bit of a problem if you're trying to attach a PEX pipe to certain fittings and the pipe doesn't fit. It may be flexible, but it's not so flexible that you can expand the opening with your ...

allowing a fitting to be inserted before the pipe’s elasticity closes it again. This tightening down of the PEX over fittings creates a waterproof connection. ProPEX expanding tools can be hand ...

There are several different manufacturers of PEX. It is very important that you know which brand of pipes you're working with ...