Generally speaking, the “tightness” or “looseness” of a thread is dictated by the pitch diameter. Measuring the outside dimension of a thread is measuring the major diameter and has zero bearing on the pitch diameter. An example to wrap your head around the concept:
Lets say I have a ¼-20 nut and I want to turn a screw for it on a lathe. I could start with undersized round stock (.240”
and cut shallow threads into it and it would still be too tight to fit the nut, even though the outside measurement is only .240.
On the other end, let’s say the round stock was oversized, (.255”
and I cut really deep threads into it that came to sharp V’s. It might be a loose wobbly fit in that ¼-20 nut even though the major diameter measures .255”.
I just wanted to explain here that measuring the outside diameter of a screw thread usually doesn’t tell you the whole story. And, changing the minor diameter of the tapped hole usually won’t affect the fit of the screw (though it will affect the strength of the threads). When it comes to thread fits, your concern is the pitch diameter, and you’re probably ill equipped to even measure that.
Short answer: the pitch diameters of the aluminum screws probably grew out of specification when they were anodized. You can buy an oversized tap. However, sometimes just running the tap in and out of the hole while putting some weight on the tap will loosen the threaded hole a bit.
