2. GM TH400

GM TH400 Transmissions

   The TH400 transmission is largely considered as one of the greatest transmissions ever manufactured. Its name in itself is synonymous with strength and durability. It was used in a multitude of different vehicles and has a well respected reputation that spreads many spectrums including: Muscle Cars, Full size Trucks, Jeeps, and even luxury vehicles like Ferrari and Jaguar. Its service for over 30 years earns the TH400 the well deserved title as one of the most legendary transmissions of all time.
       The TH400 Transmission was developed by GM’s Hydromantic division as a replacement for the original Hydromantic transmissions such as the Power Glide, The Super Turbine 300 and Jetaway transmissions. Although GM was hesitant in its initial development, it finally came to fruition after GM was able to obtain the licensing rights to the patented “Simpson Gear set”.   It was this major breakthrough that would set forth the foundation of the TH400. The importance of Mr. Simpson’s invention should not be underestimated as it would also lead to the development of other popular transmissions such as the Torquefite 727, Mercedes 3 speeds and Ford’s C6. The Simpson gear set is largely considered as one of the most major advances in the development of the modern automatic transmission and he earned well deserved millions of dollars through licensing fees. 
   Once GM had the ideal platform to work off of, Engineers went to work to design the optimum transmission. The TH400 was the first three speed, Simpson-geared automatic to use what is called “overrunning clutches”. This design would allow for both first and second gear interaction; this feature basically eliminated the need for simultaneous release and engagement of the clutch to make the 2-3 gear change. Further additions to this design include a large multi-plate clutch to provide smoother reactions. It is this key attribute that allows the TH400 the ability to withstand higher than normal input torque and numerous shift cycles of the transmission. In a nut shell, The TH400 was designed specifically to handle frequent stop-and-go driving. Commercial type vehicles were quick to see this advantage and hence the multitude of delivery and industrial vehicles were supplied with this transmission 
    The transmission was further strengthened by some innovative design changes. Significant added strength was achieved by the use of a cast iron center support for the transmission's concentric shafts that mate the clutch assemblies to the gear train. This improved center support was further enhanced by adding an enlarged contact point for first gear. The gear train's reaction carrier was restrained from counter-rotating the engine in first gear by a “roller clutch”. In other words, the first gear reactive force is evenly distributed around the whole case itself. This added design change significantly increased the transmissions ability to disperse and handle extreme engine torque loads via even displacement; therefore, drastically decreasing the likely hood of mechanical failures.
     The first TH400 transmissions were introduced in 1964 for passenger vehicles including Cadillac and Buicks, Followed next with Oldsmobile, Pontiac and some full-sized Chevrolets. Many of the Buick, Cadillac, and Oldsmobile TH400s produced during1965-67 were equipped with a torque converter that incorporated a Switch-Pitch variable-pitch stator.   The switch-pitch offered two stall speeds, applying the proper electrical signal selected a low or high stall. The high stall offered advantages of torque multiplication to “get off the line”, while the low stall decreased slippage at high speeds. The variable stator clearly offered big advantages for the Muscle car collectors and drag racers.
    By the late 70’s, Big GM passenger cars grew out of favor. The Oil embargo of 1973 coupled with growing tensions in the Middle East forced up gas prices. The lighter and more fuel efficient TH350 had taken over most passenger vehicles and the 700R4 overdrive transmission was ready to make its debut. By the 1982, The TH400 was rare to see in any passenger vehicles. The Presidential Limo (a modified 1984 Cadillac Fleetwood Brougham) used during the Reagan Administration was the last known GM passenger vehicle to use the THM400. Many one ton Chevy trucks continued to use the TH400 until the early 90’s when they too would change to an overdrive transmission. The U.S. Army HUMVEE was the last mass produced vehicle using the TH400.
Identification of the TH400
The TH400 transmission has a main case of cast aluminum with an over length of 24-3/8" long. The transmission has a smooth non-removable bellhousing. The case is also smooth with reinforcement ribs running forward to back near the rear bolt pattern. The rear mounting face of the transmission has a 6 bolt hexagon pattern with a 32 spline output shaft. The length of the output shafts varies depending on application. The shortest outputs shafts were only 5.9" OAL with a 1.4" Stick out. The shaft was commonly mated to a NP203 GM Transfer Case. This shaft is obviously desirable for Atlas Transfer Cases and Advance Adapters offers the shaft under part #52-7002.  The next shortest shafts were 1’ longer and commonly found behind the NP208 or NP241 Transfer Cases. 
 Another easy way to identify a th400 is by the Transmission pan. The transmission pan has a unique shape that can be best described as the state of Texas(see photo). An additional quick i.d. method is to look at the kick down system; The TH400 utilizes an electrical slide switch throttle kick down versus the standard cable kick down.

th400 pan
th400 tail houisng

The Jeep TH400
 The JEEP TH400 was used in several jeep applications beginning in 1965 and extending throughout 1979. The first Jeep application to use the TH400 was The Jeep Gladiator supplied with the 230 Tornado I6 engine. Jeep would continue to use the transmission on several other engines including: Vigilante V8, Dauntless V6, and eventually the AMC Engines. The early applications were typically Jeep j-trucks and wagons. These vehicles used the Dana 20 transfer case as the factory application. Jeep used an oddly engineered adapter to mate the TH400 to the Dana 20. Jeep apparently acquired 2WD TH400 output shafts from Cadillac with a large robust 32 spline output shaft; however, the actual adapter that mated the transmission and the transfer case was a flimsy cast adapter which was heavily prone to failure. Advance Adapters offers a replacement adapter kit to correct this faulty design under Kit PN50-0500. Many jeep owners have chosen to upgrade to other types of transfer cases like the Dana 300 instead of investing in a new adapter.   Advance Adapters offers an easy Th400 to Dana 300 adapter kit that utilizes the the existing th400 output shaft avoiding transmission dismantling. The kit is sold under Kit PN50-6404
The Jeep TH400 was also used in the late 70’s mated up to the chain driven Quadratrac transfer case. This version of TH400 was used exclusively behind AMC Engines. The output shaft of the TH400 was a long coarse 10 spline shaft that was unique to this application. Although Advance Adapters offers adapters to change the Quadratrac, A new output shaft will be required for pretty much any conversion. The Qudratrac was designed for vehicles with an offset rear end which complicates transfer case conversions unless an axle swap is performed.
 Another oddity of the JEEP TH400 was the rear bolt pattern and the engine ring adapter. The rear of the AMC TH400 has a 4% clocking of the bolt pattern. The clocked TH400 was probably used for front driveshaft clearance issues and apparently used exclusively during the AMC years. Advance Adapters offers an extra clocking pattern on many of our adapters to counter this rotation. We recommend contacting Advance Adapters directly when dealing with this version to verify your adapter needs.
 Another unique transmission feature during the AMC years was: The engine ring. Jeep initially ran a thick 4” adapter ring to mate the transmission to the engine block. In 1974, the adapter ring was omitted and a new TH400 case was developed with an AMC bolt pattern on the case itself. The ring itself is not necessarily an issue of quality or design; however, the Chevy adapter that Advance Adapters supplies for adapting a V8 to the AMC TH400 will only work with the TH400 without the ring. By the beginning of the 80’s the th400 was retired by jeep and the introduction of the torqueflite transmissions was under way.

The TH400 has a case length of 24.25" long and the gear ratio's are 1st 2.48:1, 2nd 1.48:1, 3rd 1:1.

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