Classic Hifi Care

These used to be thick as thieves back in the day. The ultimate Mack, one of the most popular models ever built, second only to the R model. These things could be outfitted with chrome and a sleeper that made a really nice truck. When Mack discontinued the L after the 1956 models, Mack owners raised such a fuss that Mack re-introduced the cab and sheet metal as the revised B-75. These trucks had a wider front end sheetmetal so you could order a non-Mack engine or you would have room for a turbocharged Mack engine. This one has a 400 Cummins, which was a popular swap. The first truck I ever drove was a '66 B-75 with the new Mack 250 Maxidyne turbo- diesel and twin-stick six speed gearbox. To this day, it was the prettiest truck I've ever seen.

https://www.youtube.com/watch?v=c-lhRyclbys

That was fun, but I still don't understand the shift sequence. and I liked the way he confessed to blowing his shifts. At the very end, that Bucyrus-Erie shovel, that was the competition, where I came from. Bay City was the home of Bay City Shovels. They didn't make it into the hydraulics age. I always keep an eye open for old devices, looking for the "Bay City" cast into the counterweight. As kids we'd see a lot of them.

Chris Campbell

Well, I never was a fan of the Quadraplex gearbox, or the Triplex, for that matter. I didn't get into running my arm thru the steering wheel to make a fast shift, particularly when downshifting. With a 400 Cummins, I would drive it like an eight speed. Shift thru the first five gears in Low Compound, the shift the compound up two more gears. With a high-torque 400 horsepower engine one could get by with that. When you were running the original 180 horsepower Thermodyne engine, you needed all 20 gears then about 20 more. I never was a Thermodyne fan. All they really accomplished was to put out lots of exhaust smoke. You got good at shifting a Quadraplex really fast when you ran the old Thermodyne. Mack wisely put those out to pasture except for cement mixer use when the Maxidyne came out. Mack was the first truck manufacturer to make turbocharging standard across its entire line. Even the 237 was a huge improvement over the Thermodyne. At 673 cubic inches, it was relatively small in displacement but made massive torque at 1200 rpm. You could lug it like a bulldozer. The Thermodyne was 711 cubic inches, but didn't make a lot of torque. You had to drive one like you drove a Detroit Diesel. Like you slammed your thumb in the door.......

The Maxidyne 250 was a souped-up version of the ENDT-673 237. It turned 2400 rpm rather than the 2200 of the 237, had a bigger turbo and had the fuel turned up some. 13 horses made a big difference. When they got the 673 block up to 325 horses, I thought that was as stout as a Mack could get. Wrong....

I think they used that goofy shifting pattern in the compound so you would not confuse the two sticks in the heat of battle. The twin-stick six speed gearbox gave you the same six gears of reverse as the Quadraplex, the traditional Mack twin-stick look, but with the practicality of the five speed single stick. My favorite Mack transmission. I liked the nine-speed and the twelve-speed as well. Basically the five-speed with a built in, air shifted compound box.

My '67 Kaiser-Jeep Corp M54A1 6X6 5-ton, factory with a Mack ENDT-673 210hp.

5-speed Spicer synchro & 2-speed Rockwell transfer with automatic front axle engagement.

22,000 lbs curb weight of right-of-way...
Dunno know why it posted vertically, just enlarge it to see correctly.

That is quite the grocery-getter, Dennis.......

Wonder why they didn't use the five-speed Mack transmission?

You need a rig like that when you are on the 101. It's not safe to be on that road with a small car....

That would be really fun back here in Michigan running around on the Silver Lake Sand Dunes. Dennis, you should make a road trip to Michigan and take me for a ride on the dunes.

Bill

Conelrad wrote: ↑Thu Nov 16, 2023 4:22 am
22,000 lbs curb weight of right-of-way...

I nautical navigation, it is known as the gross tonnage rule. My 26' sailboat may have technical right of way over the 1000 foot freighter, but I always honor the gross tonnage rule.

Chris Campbell

Dennis, what is the final drive ratio on that? Did it have Mack rear ends? Mack ratios were 4.56. 4.44, 4.33 and 3.88. Most of the five-speeds used a 4.44 ratio. The 12-speed Super Liner was on 4.33's. I had an Autocar with a 335 Cummins on 3.88's. Scary fast..... I got more speeding tickets in that thing than anything I ever drove. Getting a speeding ticket in a Mack was unheard of......

I hope this doesn't result in TL,DR...

The series of tacticals like mine was designed in the very early 50's, built by International, Diamond T, Mack, and Kaiser Jeep over the years, with completely interchangeable parts.

Spicer had the lock on the largest synchro 5-speed trans used in all of them for many years.

The rest of the drivetrain was of Timken design, later supplied by Rockwell, with a highly compliant 44,000 lb tandem set and 6.44:1 ratio. These were not speedy road trucks, but real off-road workhorses. Top speed in mine with the 14.00 rubber is 59 mph. Even has wet brakes with an Bendix air-over-hydraulic booster. It stops just fine, loaded or not.

The original fielding was powered by a Continental 6602 6cyl gasser @ 234 hp (100CI/cyl). Thirsty, but made a LOT of power, up there with the Mack 707 engine.

With a newer contract in '65, the Test & Evaluation Command went with the Mack ENDT, as it was an easy fit in the chassis, better fuel mileage and lifespan. Spicer redesigned the gearset with a 5th over to compensate for the lower RPM.

3 years later and a lot of $ spent on Mack Diesels for that reign of trucks (tractor, dump, cargo, LWB cargo, tanker, expansible van, bridge truck,wrecker & cab/chassis), they went with the slightly cheaper Hercules Hypercycle turbo multifuel engine, said to run on most combustible fluids, to include Kerosene, gasoline, jet fuel, & diesel. That it did.

It never did really good job at anything; it had temperamental head gaskets, an unsolved resonance item with the crank, and was underpowered with a displacement under 500CI.

A 1969 production run with an extended hood & fenders (the 800 series) to accommodate a Cummins 250 NA was much improved over the multi, even without a turbo.

In the mid 80's, a new generation (the 900-series) was let with either Cummins 8.3 or a Cat diesel, Allison or Clark powershift, computer controlled eng & trans, tilt front end, and straight wedge air brakes. Blessed with a host of electronic and mechanical problems, many upgrades were done in their short lifespan.

They were getting too soft on drivers, many new recruits couldn't even work a manual trans. No thanks.

The proof is some trucks like mine, built in the 50's, are still running some 70 years later in the world. "deleted", mine is pushing 60 now. Until the last couple years early series 5-tons were still in service in a few units.

Not so with the newer ones, the surplus market is full of them. I know of several original 50's gasser rigs still running in private collections.

Most of this data is correct: https://en.wikipedia.org/wiki/M39_serie ... %976_truck

D

Our little volunteer fire department in Missouri had three old 6X6's they used for water trucks. They were powered by the Hercules engine. The turbo screamed on them almost like a Mack. They had a straight five-speed transmission and could seriously have used more gears in the hills with 2000 gallons of water on board. You had to double-clutch them to downshift, or you'd miss a gear, have to stop, then start over. If you were on a hill, the water would slosh to the back of the tank, making it harder to get going again. The only good thing I can say about those old trucks was that they would start well below zero. They had an old Ford F-800 water truck with a 534 V-8 in it that ran a lot better, had more power and a two-speed rear end.