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?

[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.

[Mack guy Ata]

Thank you all for the much needed information im pretty sure you guys have figured out that im as uneducated as it get in turking im only 26 and still learning tons of new stuff about these trucks and everyone in BMT is super nice to me. I did not know that was the case with 20 speeds but even tho i know now im still itching to put a 20 speed in there since i always wanted to dirve one with 20 speed (there is no logical reson for me to want the 20 speed but here im still wanting it. Its a part of the dream)
 

How about the 10 speed i have in my truck has low  high and 5 gears (technically 9 since it says no 5th gear on low) do i use the all 9 of the gears on that. My truck is not in driving condition right now so couldn’t drive it. Also why does it say no 5th gear in low. 

[steve s.]

The reason you don't use 5th gear with the compound in low is because that gear ratio is the same or very close to the ratio in 4th gear with the compound in high. There would be no reason to shift to that gear (5th lo), it would be about the same ratio. There are several gear ratios like that in many of the Triplex and Quadruplex transmissions also. There are gears there, and there might be a time when you would use them, but it would not be part of your normal shifting sequence. 

I find the Quadruplex is nicer to drive than the Duplex, because of how you shift; you use the compound more, and the shift lever is closer to the driver.  

There is nothing wrong with wanting to have a 20 speed transmission. It's your truck, have fun with it. you might not use Lo Split in the compound much, but it is very handy in tight spots or backing up. 

[67RModel]

Not sure which model of quad you are after but here are the ratios for a direct triplex and overdrive quadruplex. If you look study the ratios and percent drops in the sequence you can see why it is recommended some should not be used. You can use all 20 forward gears with no ill effects, however, in a practical sense it makes no sense to do so. 

[67RModel]

Honestly with an antique vehicle just bobtailing around with no load you can shift a quad or a tri as just a 5 speed with the auxiliary in lo or direct and then once in 5th shift the aux to hi and forget about all that shifting.

[Brocky]

21 minutes ago, 67RModel said:

Honestly with an antique vehicle just bobtailing around with no load you can shift a quad or a tri as just a 5 speed with the auxiliary in lo or direct and then once in 5th shift the aux to hi and forget about all that shifting.

The Mack engineers designed the complete power train to accept the heaviest (and overweight) loads that a customer could put on the truck. As a hobby truck, bobtailing or with a light load of toys, this usage of the entire available power train is not necessary. I would first determine what YOU are going to use the truck for?? If NO heavy work is planned, you can play with the present duplex and enjoy yourself, with out going to the work and expense of swapping transmissions.

[Freightrain]

I would follow Brockys advice.  Get it running and then see how they drive.  If you have never driven trucks like this it won't be a quick thing to just hop in a drive.  26 yrs ago when I bought mine it took a summer of driving to get proficient and comfortable.