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..

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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?

[Mack guy Ata]

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

[Freightrain]

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.

[davehummell]

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

[mowerman]

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

[Freightrain]

The "20" is not really the number of useable gears.  Lo-lo can only be used in first gear,so 16 real gears in a single over box down to 13 in a double over box.

[Geoff Weeks]

10 hours ago, Mack guy Ata said:

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

I have a truck with 30 forward gear possibilities. 13 at best are useful. Nothing cool about ratios that for all purposes are the same. No "show-off" potential because it will just sound like you stepped on the clutch and off with no change in engine speed if the gears are overlapping. 

Map out what the ratios would be in each position and how the rpm would change. 

In my case with a plumbing change I could add 5 more useless selections!  With less than 1/2 the available selections being of any use, adding 5 more wouldn't change that one bit.