TB1 Thermo-Bob Kit


Popular with the ROTAX 912 and 914 crowd.


This kit has one-inch fittings.  It was designed for the four-cylinder Kawasaki motorcycles, yet other groups have found it useful for their applications. 

  • One of the ‘other’ applications is the ROTAX 912 and 914.  We sell this as “not for aircraft use” as the installation and operation liability lies with the owner / installer.
  • The installation diagram that some ROTAX users utilize is shown here.

The Thermo-Bob 1 is a machined aluminum external housing with an automotive thermostat inside.  It is designed with a coolant bypass, which allows coolant to bypass the radiator when cool, speeding warm-up and maintaining engine coolant operating temperature at 180°F.  The bypass fittings have 1/2 inch OD barbs, and allow the buyer to fabricate their own bypass hose (1/2 inch ID recommended) to route bypass flow from the Thermo-Bob 1 to the provided bypass tee.  This hose is not provided with the kit as some buyers like to route bypass flow through a heater core, so every installation is different.  Do not place a valve in the bypass line to stop the flow, as you always want full bypass flow for the thermostat to work correctly.

Thermostat temperature choice: you can choose from a 180°F (82°C) or a 195°F (90°C) thermostat to be installed in your Thermo-Bob 1 prior to shipment.  For most street-driven vehicles, we recommend the 195°F thermostat but you’ll notice we recommend the 180°F choice for the Rotax application.  Here’s why: although the boiling point of a 50/50 mix of Ethylene Glycol and water with a 16 psi radiator cap is 267°F at sea level, this number drops to 207°F at 10,000 feet elevation in the rare occurrence where your radiator cap fails to pressurize your cooling system.  This might not be enough margin on boiling if you are using a 195°F thermostat.

If you are located outside the US, we recommend you also purchase a spare thermostat for the future, as it is not easy to find Internationally… and when you do need one, you might not enjoy waiting 1 to 2 weeks for International delivery.