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.

[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.

[mowerman]

Besides the fact your going to have plenty of stick action with a duplex if you choose to split gears with no weight.. that transmission you have is a lot easier than a quad box

[Vladislav]

Ok, I have a few points to mention.

I looked over the photo's you posted. The engine in the truck is the most probably not original but later series Maxidyne 237 HP (ETAZ-675 or so, other guys will correct me). My guess is based on presence of the water/oil heat excheanger (cooler) at the left side. B's had turbo engines too but without it. Air intake pipe from the air filter to the turbo looks custom (home)-made. Also someone converted original air operated wiper drive with electric motor. Both facts may be even seen positive. At least the engine is more powerful but very similar to the original ENDT by the look so I wouldn't sorry about the fact.

If the truck had 9-speed tranny with two sticks I'm pretty sure you will have quite enough "fun" shifting it. At least in the beginning. The 2nd stick is a splitter. So you start in 2nd low than shift to 2nd high (with that 2nd stick) than you have to get 3rd low so... you shift both main from 2nd to 3rd and the compound from hi to lo. At the same time and without clutch! I may be a bit extra emotional on this since the most American guys who drive a vintage truck count that action as simple as apples. But an avarage European person who's skilled to drive a car with synchronized mechanical box finds himself in a total nightmare when tries shifting that unit. I don't want to spread too extensive description but just mention one more fact. 5th main (the top gear) is right front from you. Not right rear as in a car. So you shift the main stick left front-left rear- right REAR and than right front. That's just an addition to the no-clutch (or double clutch) shifting using two sticks.

Ok, further on. I would like to see the housing of your transmission from under the truck. And the front axle beam too. Since I have an interesting guess. At least the axle beam looks familiar to me by what I can see on the front truck's picture. And that's not an axle typical for a B-model.

Of one more interesting point is the rear axle ratio. It determines how fast you can go with the truck (using particular transmission and the drive wheels size for sure). If you look at the differential housing of each rear axle at the front area to the right of the propeller shaft flange you could see two data tags (plates) attached with rivets. Clean out the lover ones (but sure better both of them). There's a script "RATIO" on the lower plates and than 3 digits (6,34 or 7,33 or so) are stamped in. My bet you will find 9.02 there. But I may be wrong.

Also your truck has cool looking heavy wheel hubs for 20" tube type tyres. The hubs are not typical for North American market, my guess they were a special order for a certain East customer. The rims are split rims made as 3 portions. They must be reliable enough to drive safe if mounted right. Many people may say they're bad thing and nobody wants to work with them for tyre installation. But on my mind they're suitable for self service and are definitely a nice sample of the older days engeneering. So I would keet them if that was my truck. Sure it's yours so you're up to the choice.

The tyres on the FR axle look like made in Russia 12.00R20 or 11.00R20. 

[mattb73lt]

Getting back to the original question, will it fit? Yes and no. If it is the larger 72 series bell housing on both than yes as they are the same standard SAE size. BUT, there are two different bell housings and there is one for a single and one for a double disc clutch arrangement. A double disc arrangement was fairly common on the Quadraplex as it was the heaviest transmission offered. It also had a rear crossmember to support the weight of the transmission. An easy telltale between the two is the bolt arrangement around the bell housing, there's a 1" difference in thickness in the bolt bosses and length of the bolts. This also pushes the entire transmission back 1" to allow for this clutch. This leads to another issue in mounting the transmission to the frame mounts. You'll need offset mounts on the transmission to match up the mounts on the frame or you can re-drill the frame to move everything back.

There could be another issue if the transmission in the B85 is the smaller 67 series, but that doesn't sound like what you have currently installed. My suggestion as you get into this, is dig into what you have and get all the archive info you can from the Museum that you can. Next, get all the info you can directly of both gear boxes you have. What's in the truck may not be what was installed at the factory. There's a stamped pad on the left side of the main box of the transmission, aft of the cast Mack emblem that has "Made with Tetrapoid Gears"(or something close). Clean that off and get all the numbers off it. Also, don't go just by the shift plate installed in the cab as to what is installed. The duplex would start with something like TRD-72  or TRD-720, the Quadraplex will be TRQ-72 or TRQ-720. That extra zero means it's an over gear(overdrive) transmission, 9 vs 10 speed. In the case of the Quadraplex, double overdrive, 18 vs 20 speed. There are other versions of both transmissions, like adding an X to the TRD, which makes it a TRDX deep reduction transmission or an L, which means it's a lightweight version with aluminum boxes. There's a lot of little things that without asking questions and educating yourself can cause a lot of headaches and wasted time trying to fit things together that won't go or mount up.

Is this really difficult, no, it's actually pretty simple. Once you know what you have and what you're looking at. Picking the right components and making them work together is fairly straight forward. Asking questions here can be very helpful as many of us have been messing around with these trucks for a long time. Post some pictures of what you have and ask away. It'll save you a lot of time and aggravation.

 

[mowerman]

Oh by the way…. I get the cool part …. We were all 26 at one time… bob

[Mack guy Ata]

Thank you everyone for all these answers I’m getting. They are incredibly helpful and can’t stress enough how much I appreciate these information and you guyses patience with me.

- Thank you Steve s. I was afraid doing 5 lo would damage it but happy that’s not the case.

- 67RModel thanks for the ratio chart, it’s very helpful and yes I can see why some gears don’t make practical sense

- Brocky yeah wont tow anything too heavy maybe some ATCs and ATVs a load that is meaningless for truck like this the main use of this truck will be good times you know cruising down the highway going 40-50 mph and listening to blue grass and country music so i think you and other are right this 9 speed will be plenty of fun for me 

- Freightrain That is an absolutely beautiful truck. I’m sure I’m going to have several months of grinding gears until I can actually start driving it daily. I have driven a 1928 Cadillac that required double clutching but I’m sure that Cadillac was a child’s play compared to these trucks 

- Vladislav Thanks for the information here and your other replies to my other post you are always very helpful. Yeah sadly I don’t carry that American gene that allows them to effortlessly do that. I’m sure I will be grinding the those gears like crazy until I learn how to do it. I’m on a business trip but when I’m back I will provide you with those photos and numbers you asked for. Also of course I’m not going to change those wheels or anything on it. I will even get the vipers to work with air again if I can find the part. It will be restored as originally as it could possibly be done no modern parts to make it easy to use or so called more reliable, whatever Mack did in 1956 I will do it the same. 

- Mattb73lt Thank you this was the prefect answer to the question I have alot to learn and research. The shift pattern shown on the plate matches the way sticks move but i dont know if its factory original. Also yes this forum is a life saver just full of amazing very knowledgeable people like yourself. I have asked 2 questions on the forum and can’t stress enough how educational reading the answers were to me.

- Mowerman After reading all the answers I got I also think the 9 speed will provide me with plenty of shifting. Also thanks for understanding my point of view. 

 

Once again thank you all for your very helpful answers you all are life savers. I will most definitely be back with more questions and more information on my truck and its parts. If I missed any of your point or questions please let me know.

[Geoff Weeks]

Close ratio shifts between gears on a heavy truck (non-syncro) can be easier to master than wide ratio shifts on an older car (non syncro) you don't  have to wait for the engine to drop as much rpm between gears.  The key is not to try and shift as fast as a modern vehicle.

Air wipers are simple devices and can often be repaired easily. Some of the wiper motors can use O rings while others will need a formed seal, which can be made from O ring cord from an industrial supplier. In some cases they only need a good cleaning to work again.

[Mack guy Ata]

That makes me feel much better about learning how to shift it correctly.

I havent taken mine out yet but knowing that it will likely be a easy fix is a grate.

Thank you.

[mowerman]

Well, best of luck with that thing… it really doesn’t look like it needs a lot of attention. Some of us start off with stuff a lot worse shape than that one have a ball… Bob

[Mack guy Ata]

Thank you very much. I feel very lucky to have my truck engine seems to be in good shape just need to find a new water manifold. Found a 2023 list of part sellers trying to see if anyone on the list has one in decent shape