Mack guy Ata Posted 9 hours ago Share Posted 9 hours ago So i own a 1956 b85 mack and there is a 1961 mack b61. My truck the b85 has a mack 9 speed and the b61 has a 20 speed quadruplex. both trucks have a I6 thermodyne. Dont know exact engine model waiting on answers from the mack museum would the 20 speed fit in to my b85 or will i need a different bell housing for the 20 speed Quote Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/ Share on other sites More sharing options...
steve s. Posted 3 hours ago Share Posted 3 hours ago Flywheel housing sizes are standardized. If your flywheel housings are both the same size, the Quadruplex should bolt up where the 9 speed was. Here is a link to help explain the SAE sizes: Sae PDF | PDF | Technology & Engineering Keep in mind that the Quadruplex is longer, and may not clear the crossmember at the back of the cab. Quote Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590599 Share on other sites More sharing options...
Brocky Posted 1 hour ago Share Posted 1 hour ago Be sure to check input shaft lengths and how it fits your bushing in the flywheel.. You may have to change the length of your drive shaft?? Then check U-joint angles.. Quote Brocky Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590604 Share on other sites More sharing options...
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