I bought these extensions to enable me to mount some older Vitoria tubulars on some aero rims. Since they work by removing the core from your existing valve, installing the extension, and then reinstalling the core into the extension, you could add sealant to the tires without removing them from the rims if you needed to. Also, they are beefy enough to hold up to pumping by hand.
Work perfectly on my Vittoria Corsa Elite tubulars. Unscrew the Vittoria valve with the provided tool, screw in the Conti extender, screw the Vittoria valve into the Conti extender. Done. No leaks.
Unlike over the top extenders, the in line extenders enable you to accurately inflate your tires to the desired pressure. Additionally, if you ever need to use a foam sealant, the in line extender will yield better results. Make sure you tighten them properly to the tire stem using the provided tool and check for any leakage prior to mountinggluing your tubular tires.
I love these extenders. They are easy to install and they don't leak. I have used them over and over with different tires and I have even used tire sealent with them. You have to be careful putting them on that you don't turn too hard and ruin your tube or tire by twisting off the stem on the tube. Other than that they are great.
These valve extenders are great quality and much more functional than the old style that fit over the top of the valve core. I love the way they work. My only issue is that I got the 30mm extenders, which are just slightly short for my Zipp 404 wheels.