stem sizing
 

I have been buying 26mm and 25.4 mm bar size stems.I can't be sure I have been sent the right size. Is there a correct way to measure..they don't put the bar size on the stem? Is it possible to just change the outer bracket, the stem side seems the same on both? thank you

Both the clamp and the stem itself will be shaped to fit a specifc size of handlebars. About the only way to confirm the size would be to attempt to put different size bars in and see what fits. The clamp for 25.4mm bars should not fit onto 26mm bars. Likewise, the clamp for 26mm bars should fit evenly on the circumference of 26mm bars without any gaps or play.

Unless you had a method of precisely locating three points on the radius of the clamp/stem (optical comparator or similar device), you will not be able to measure the stem size at a difference as small as 0.6mm.

stem sizing
 

I just seem to have a larger gap using the 25.4 on the 26 bar.

Exactly. That larger gap is why you should never try to use 25.4mm bars in a 26mm stem. The clamp and stem will only be contacting the bars at 2 locations instead of making 360 degree contact.

stem sizing
 

thank you.