Don't know about 21 or 31cc, but the 25.4cc Echo weedies I have are 8M1.25, which means it's a 8mm diameter shaft with a 1.25mm pitch.

Then again, I have other 25cc weedies that I think are also made by Kioritz, and they have 8M1.00 instead. Best way to check is to put a metric ruler on the thread. If the threads line up with the mm ticks, then it's an 8x1.00 thread, if the mm ticks line up every 4th thread, then it's 8x1.25.