On the diagram it says:
When you use a saw blade with aperture 25, mount a 25 washer on shaft ...

I don't have a miter saw but it is sad that the one you bought did not come with any washer.

I believe it is like circular saw too, sometimes saw blade has bigger hole than fitting post on the saw, thus you need to put washer (well it is called bush, like metal ring). It is to reduce the hole size to fit in snugly into the blade post.

I believe you can buy easily but not sure if some blades come with them when you buy one. The one I bought, none but the blade I bought has same hole size with the post. So, you don't need to put one.

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