This solid carbon steel spanner bushing is popular when someone is wanting to reduce the inner diameter of the axle or bolt hole to 5/16" inner diameter. This can be used to double spanner a bushing which means you can insert this inside of a 3/8" spanner to further reduce it to 5/16". This spanner can also be used on any bore size of a 3/8" to further reduce the size to a 5/16" inner diameter.