Mack guy Ata Posted yesterday at 07:04 AM Share Posted yesterday at 07:04 AM 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 yesterday at 12:42 PM Share Posted yesterday at 12:42 PM 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. 1 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 yesterday at 03:04 PM Share Posted yesterday at 03:04 PM 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.. 1 Quote Brocky Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590604 Share on other sites More sharing options...
Freightrain Posted 19 hours ago Share Posted 19 hours ago 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? 1 Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590617 Share on other sites More sharing options...
Mack guy Ata Posted 9 hours ago Author Share Posted 9 hours ago Thank you all - Steve s thanks for the pdf and the answer very helpful -Brocky thats a good point need to check that -freightrain yes im sure i will need new shaft for the 20 speed. The reason for me wanting a 20 speed is i always wanted a 20 speed truck you know shifting for 10 minutes to go from 0mph to 20mph no other reason i just think they are super cool 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590637 Share on other sites More sharing options...
Freightrain Posted 3 hours ago Share Posted 3 hours ago Most quad boxes are really just a Triplex with a deep under which is only good in 1st gear. So you don't really shift it that many times. A double OD quad has even less gears as you don't split 5th at all. Just a heads up if you aren't aware of how they shift. 1 1 Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590640 Share on other sites More sharing options...
davehummell Posted 1 hour ago Share Posted 1 hour ago Larry I found a triplex and installed it in my B it is double over and you don't split 5th either so in reality it is just a 13 speed 2 Quote Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590648 Share on other sites More sharing options...
mowerman Posted 20 minutes ago Share Posted 20 minutes ago I think that’s the way mine is. It’s got four positions on the auxiliary, but you can’t really use the low low in a full up shift it’s been so long since I drove it anywhere… I don’t remember if it’s a 15 or 20 speed lol .. bob 1 Quote Link to comment https://www.bigmacktrucks.com/topic/76375-question-about-thermodyne-transmissions/#findComment-590650 Share on other sites More sharing options...
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