It is a 10 pack of o.e.m primer bulbs that replaces the Walbro 188-512. These bulbs are replacements for Homelite 01183, Ryobi part number 683974, MTD 791-683974, Tanaka 442-25143-800, Stihl 4130-350-6200, 41303506200, Echo 123-181-39130, 12318139130, 123181-39130Homelite 01123, McCulloch 224242-01, and others. The bottom of the bulb is .83 inch (21 mm) and the space between the narrowest point within the snaps is .60 inch. There are two fuel legs for an out and in line. These fuel legs are .125 (1/8″) in diameter. Some companies have changed vendors for this part through the years and might use a Zama A056013 bulb, which has a moderately larger diameter on the base and a larger bulb, but they both are interchangeable.