Sierra International 18-5752 Universal Spark Plug Boot The Sierra 18-5752 Universal Spark Plug Boot is interchangeable with the following: Mallory 9-28123. Each Sierra product meets or exceeds the original equipment part it replaces. Size: 8 mm. Sierra Universal Spark Plug Boot Meets or exceeds the original equipment quality Sierra part number: 18-5752 Interchangeable with the following: Mallory 9-28123 Boot size: 8 mm