Question about thermodyne transmissions

[Mack guy Ata]

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

[steve s.]

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.

[Brocky]

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..

[Freightrain]

5 hours ago, Brocky said:

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..

This∆∆∆∆∆

Could have a clutch brake on the quad box, could have 2" input and 9 spd could be smaller and shorter.

Back box will be longer for quad over duplex which changes driveshaft.

What reason are you wanting to change them?  Is the quad a double overdrive?