When it comes to full-size 4x4 trucks and SUVs, the General Motors Turbo-Hydramatic 400 (TH400) automatic transmission is perhaps one of the most coveted automatic transmissions in existence. Thanks to a healthy production run from 1966 to the early 1990s, core units are both plentiful in wrecking yards and easy to upgrade, thanks to a steady supply of rebuild kits and replacement parts offered through the aftermarket. Truck builders favor the TH400 over the more abundant TH350 because the internals are larger and feature more robust support hardware. The TH400 is also desirable over the TH350 (both have cast aluminum cases) due to the former's input/output shaft strength, and the fact that a TH400 has a center shaft support. For those reasons, we decided it was time to highlight a few builder tips for making the Turbo 400 the best it can be. Our plan was to swap the one shown here in place of the NV4500 manual gearbox in our newly revived project SuperBurb. As such, we wanted this transmission to handle 450 lb-ft of torque and a 7,500-pound truck without issue. We enlisted the help of Quality Transmission in Seaside, California-a shop with a knack for ensuring repeat customers. Follow along as we show off a handful of improvements everyone should consider when rebuilding a TH400 automatic.
 The first tip to consider...  The first tip to consider when rebuilding the proven TH400 is the inclusion of an aftermarket shift kit such as this one from Transgo. The contents of this blue box will resolve nearly 75 percent of all common problems encountered with the Turbo 400. Items such as improved gaskets, seals, and springs are just a few examples of what one would find inside this kit. |  Upon disassembly, Quality...  Upon disassembly, Quality Transmission's technician found the likely cause of our donor's demise: this plastic filter tube had a small chunk missing, which would have caused air to leak into the vital lifeblood (ATF) of the valve body. It's funny how a $2 part can destroy a perfectly good transmission. |  This is how the filter tube...  This is how the filter tube is supposed to look. |
 After our donor unit was completely...  After our donor unit was completely disassembled, the case was placed in a parts washer to remove any metallic debris from the internal cracks and crevices. While our donor case was being cleaned, Quality's technician showed us the end result of improper operation and, essentially, neglect. Shown here are the Third gear apply steel discs-the metallic discs between the friction clutches. Notice the discoloration and warpage of these steel plates. This is caused by low pressure, which causes slippage between Second and Third gear shifts. The frictions found between these plates were in similar shape-worn out and definitely in need of replacement. |  Here you can see the new clutch...  Here you can see the new clutch frictions and Kolene steel discs that make up the Third gear apply cluster. These products come from Alto Products and are designed for high-torque applications. They use a special high-friction material to provide improved holding force while also increasing longevity and life span. |  Another place we found obvious...  Another place we found obvious signs of wear was in the Second gear drum apply band-essentially, it's what makes engine braking possible. Note the burnt color of the center area of this band. This discoloration is usually caused by excessive heat, and is typical for vehicles that see heavy downshifting. |
 Here you can see the new heavy-duty...  Here you can see the new heavy-duty Second gear apply band from Transtar. Loaded with proprietary friction elements to improve performance, this heavy-duty band will help grab the drum and hold it with minimal slippage. This band should enable our SuperBurb to get the torque to the ground and deal with the added heat of off-highway use. |  Another attribute of the Transtar...  Another attribute of the Transtar Second gear apply band is the way the steel backing terminates at the eyelet for the anchor pin. Note how the unit on the bottom is welded on the end in addition to the two small spot-welds; this improves strength and ensures that the forward band does not fail under load. |  One essential procedure that...  One essential procedure that every TH400 rebuilder should consider is block sanding the valvebody mating surface. This helps ensure a leak-proof seal between the valvebody and its mounting surface. If overlooked, this vital intersection may have slight warpage and would likely develop a pressure drop and/or fluid leak within the transmission-which contributes to direct clutch failure. |