Thank you for selecting Aerospace Aviation360 as your go-to distributor of part number 21-173 found under Piston Internal Combustion Engin by Hydroid Inc. The National Stock Number assigned to this part is 2815016243834, and the Federal Supply Class is 2815 Diesel Engines And Components. Our website is owned and operated by ASAP Semiconductor, a leading distributor of aviation and aerospace part numbers like 21-173.