A few times when we have bought mast extensions we have had a problem. The problem is that, for extra resistance, durability, or both, the mast extensions come with a pin that goes through the cylinder at the bottom of the tube. The trouble is that power-joint mast-foots (mast-feet?) that have a long pin will be too long to click in. Most of our mast foots have a long pin since these are more comfortable to fumble into the mast base than the short ones. What you need is a short pin mast foot. Just make sure that when you are buying your mast-foot and extension, they are compatible. It is one of those small details that you don’t think about and only realize when you actually put the two together.