TheDieselStop.Com Forums: World class 6 spd Allison....7.3PSD

Bilbo,
Sorry to hear about the stroke. They are scary. And it doesn't help you being confused with all the information us experts on the internet, and you're local experts are giving you. But here goes some more for you to digest. Remember, were're talking on highway truck trannies, only.

From the late 50's to the early 70's allison built what everybody associated with allison calls the "six speeds." The family consisted of the MT25, MT30, MT32, MT40, MT42, and the HT70. They can usually be identified by the heavy cast iron main housing with pto access covers on both sides of the housing. The tag might read Torqmatic, or Fordamatic as alot were built for ford with their numbers on them. I suspect your old 8V71 had and HT 70 behind it. They had many different front and rear housings depending on what they went behind, and the parking brake assy on the rear. They could also be identified by the two levers coming out of the case - one for the shifter, and one for the tv linkage. None were true six speeds, but were set up like Birken said.

The next family was the AT-MT-HT families. Most were 4 speeds, some with a deep low granny gear bolted on, but there were two true 5 speeds with lockup - the MT654, and HT754, both big trannies.

The curent trannies are called the allison world transmissions. The 1000/2000/3000/4000 series. They are available/can be programmed/built to cover 4,5,6, or 7 speeds. They are the only allisons with fully automatic six speed gears available.

Now back to you. After reading your post, I'm guessing that you're thinking about putting one of these in your F350. I've regeared a couple with the AT540's/AT542's and it was tight and required a 2 inch body lift. I just can't imagine trying to squeeze a 640, or an old MT six speed in that chassis. I do believe your exhaust will have to be run outside the frame rails, under the trans, or through the hood! And neither one would do the job of the 4R100. Besides, all the hydraulic allisons are 1:1 in high range, so you'll it'll be like running your 4R100 around in 3rd all the time - the engine will be screaming and that expensive diesel will be wasted. And to get it right, you'll still have to do some wiring to get the electronic tps signals from the 7.3 converted over to the hydraulic input (modulator) the allison would require. If you're going to drive your pickup on the interstate, and expect any mileage whatsoever, Your best bet would be a 1000 series, and wire that bad boy in. I'm sure all the interface modules are available from Navistar, and you'd just have to wire up some loose odds and ends. If you had to pay somebody, why I think it would pay for itself over 12 months us compared to the fuel lost with the old ones. If you're still heck bent for an old hydraulic tranny, I would look for an AT542 with lockup torque convertor. They're rare, but they're out there. But I wouldn't put too big of a chip on it as it was designed for a 200 hp family of engines. All in all, in my opinion, a built 4R100 would do the job better all around. It just wouldn't have the gossip factor of saying "look mines got an allison in it!" though.

Cliff notes - sounds like the mack had an HT70, Allison makes no fully automatic hydraulic six speeds, no easy way out if you insist on a big allison in your superduty.

Hope this helps.