On a echo 21 or 31 cc weed eater motor, are the threads metric?

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.

The easiet way to tell what size you have is to take the engine to the hardware store (a real one not a home center) and try different nuts until you find the right one. You can always buy a metric thread gauge (also at the hardware store), most of the time you can't use the thread gauge there because of the plastic wrap.

Peter