Ok. So these worked for me. I'm almost through the set after months of carful, correct stretching. One thing to note is that they are cheap so people who are more sensitive to metals might have a hard time. In which case you might want to seek glass ones instead. The other thing is that its not uncommon to have or develop nitrile allergies during the stretching process. I did in my teen years so knew this ahead of time. It is also not uncommon for black o-rings to be made of nitrile. In fact, the cheap sets usually will be unless otherwise stated. I had to buy silicone replacment o-rings from another site and I recommend others to do the same. Overall they are serving their purpose and with proper care I have avoided pain and infection.