Kids Ride Shotgun Pro Trek Knock Block Adaptor Knock Knock Trek bikes with a Knock Block headset need this adaptor to fit the Shotgun Pro seat. It replaces one of your spacers so the seat mounts securely around Trek's steering stop system. Features Trek Knock Block specific, required to fit the Shotgun Pro on Knock Block headsets Simple spacer swap, replaces one of your existing 10mm spacers Works with the included Pro spacer, which replaces a second 10mm spacer Minimum 30mm stem clearance, with one of your own 10mm spacers kept directly under the stem | Kids Ride Shotgun | Pro Trek Knock Block Adaptor Black