Re:
HI Paul I suspect most Easton stems, would be for 26mm diameter bars which seemed to have become the standard for drop bars before the 31.8 was adopted.i have an Easton with flat bars in use on a Thorn bike and suspect they are also 26mm but will measure and post a correction if a different size.
I Recently had to change stems on other bikes and had to resort to using shims which to be fair work well.