This special Spacer measures 9.525mm (id) x 13.525mm (od) x 16.5mm (long) & is known as a Bearing Support Spacer. This Bearing Support Spacer fits between the Bearings, along side the 8mm O-Ring when fitting Bearings into a TRAMPA hub when mounting the wheel onto 9.525mm Axles.
Over time, during the usage of your board, the Bearings & Spacers are put under a lot of force. Ultimately this force has to and does go somewhere... The Bearings use a Steel ring in their assembly where as the Bearing Support Spacers are made from Aluminum. Steel is much stronger than Aluminum & when forced against each the aluminum will loose out every time! This is what happens to your spacers when positioned between the bearings... The Bearings put all the force into your spacers & as a consequence over time the spacers shrink...! & then with enough time its possible for an annoying rattle to start.... That's when you fit these 16.5mm spacers assited with the little 8mm O-Rings to prevent the rattles!! :-)
The Rubber O-Ring is 2mm thick & needs to be stretched to fit onto the 9.525mm Axle. The O-Ring can be squashed slightly therefore spring back to its original shape. This tiny movement eliminates rattles