Bypass Hoses
Bypass Hoses are the small rubber hoses that allow coolant to exit the cylinder heads and flow to the radiator via the upper coolant pipe. Their name is something of a misnomer, as they don't actually bypass anything.
Replacement
Because of their relatively difficult-to-access location, these hoses should be replaced during the Timing Belt Service. For nearly the same price, silicone bypass hoses are also available which are much more durable (not to mention easier to install/remove) than stock hoses.
Due to frequent corrosion, it's a good idea to replace their sister part, the Coolant Bypass Tubes at the same time.
