I don't know if these will work on a tube as an extender. http://www.treefortbikes.com/product/33 ... 1908___696
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But, I use this with the tubulars as a replacement for the vittoria extenders on my 303 FC tubulars. I know you have clinchers and want an extender, but just a thought.
Beauty of these, inexpensive, different lengths, and the valve core is removable...critical for tubular latex repair/pit stop, valve needs to come out. Also, if you use latex sealant, like Tofu when you install new tires to help prevent flats.
Not sure why it wouldn't work for a clincher as an adapter, I believe they are the same size??
I run the 48mm and there is enough still sticking out, but with a tube and a valve that isn't removable, it would be way more than enough.