It corresponds to part number 8 in the exploded view of the cooling system and is the thermostat bypass hose. At start up, when the engine is cool, it allows water to bypass the engine block and pass through the hole in the closed thermostat and circulate, thereby preventing the exhaust manifold from overheating.

OEM Equipment for the following engine models (and the boat that we are aware used these engines... There could be other combinations as well):
• M-18 (Catalina 27, Catalina 30)
• M-25 (Catalina 30)
• M-25XP (Catalina 28, Catalina 30, Catalina 34, Catalina 36)
• M-35 (Catalina 34, Catalina 36, Catalina 38)

• Inside diameter: 3/8"
• Length: 3-3/4" 300236