Differnt Thermo-Bobs are dependent on which one you need:

The primary difference between all the Thermo-Bobs is just geometry to fit a specific application.  It's not like a '4' is better than a '3'.


  • The Thermo-Bob Original was just that - an older design with 6 bolts holding the housings together that could be used in multiple applications.  It used a 1.5" automotive thermostat.  We stopped making them back in 2014.


  • The Thermo-Bob 1 is a larger version of the Original, with 1 inch hose barbs machined into the housing.   It is popular with the Rotax crowd.  We still sell it today, as it uses the 1.5" automotive thermostat, and has 6 bolts holding the housings together.


  • The Thermo-Bob 2 is a 3-bolt design for the Kawasaki KLR and KLX models that bolts directly to the cylinder head.  It uses the smaller 1.25 inch thermostat.


  • The Thermo-Bob 3 also uses the 1.25 inch thermostat, but is an inline design to be inserted in the coolant hoses.  We make them in 5/8", 3/4" and 7/8" sizes.


  • The Thermo-Bob 4 also uses the 1.25 inch thermostat, but is primarily used on KTM motorcycles to fit their unique hose layouts.  All Thermo-Bob 4's have a side inlet, and have either a side exit or a straight exit.