First, remember that the main purpose of the contact tip is to transfer the welding current to the wire. To do that, the wire must make good electrical contact with the wire. The curve in the filler wire from the wire cast helps. It acts like a spring to keep the wire in contact with the contact tip, but if the hole in the tip is too large, you lose contact intermittently. If this happens, you’ll begin to see burnbacks and have to buy many contact tips. The same thing will happen if you use a wire straightener and remove all the cast from the wire. On the other hand, if the hole is too small, you’ll also have feeding difficulties. You can’t feed a 3/64-in. (0.0468-in.) wire through a 3/64-in. hold. The hole diameter must be somewhat bigger than the wire diameter for good feeding. How much bigger? In my experience, the right hole diameter for feeding aluminum wire is 0.008 in. to 0.009 in. larger than the actual wire diameter. A clearance of 0.015 in. is too large. You might want to take 10 contact tips or so and plug-gauge the hole diameter; you might be surprised at what you find. Most manufacturers do a good job of controlling the diameter, but if you see a lot of variation among tips, I recommend changing suppliers. A popular misconception is that you need a larger contact tip for feeding 4043 than for feeding 5356 because 4043 expands more when it is heated. Actually, the coefficient of thermal expansion of 5356 is higher than that of 4043, but they’re close enough that you generally can use the same contact tip for either.