Both tubes had 1/4" inch tears around the base of the stem. The theory is that I caused the tear by jerking around the stem when using the hand pump. Also, when mounting my new tires, I had trouble getting the bead to set at the stem - the tube was stuck between the bead and the rim. I "solved " this by pushing the stem up into the rim / tire to force the tube into the tire, thus setting the bead. This could have weakened the tube at the stem.