Editing TH400 rebuild tech (section)
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===Center support assembly=== It is best to have the center support ready ahead of time. The center support houses the intermediate apply piston, and supports the direct drum. It has sealing rings that seal it to the direct drum, allowing it to transfer fluid to the drum to apply 3rd gear, and reverse. Disassemble, clean, and re-assemble using new lip seals. Pay attention to the orientation. Use the old lip seals to be sure you select the properly sized new seals. [[File:Center_support_rings.jpg]] Install the piston using a lip seal installer or feeler gauge, be sure the piston depressions align with the depressions in the center support, and it sometimes helps to use a tilting motion as you install it. You can also use thin plastic (cut from a large plastic soda bottle) as a seal protector/installer, or use the proper Kent-Moore J-tools. Once the intermediate apply piston and the return springs, guide, retainer plate and snap ring is installed, you must install the center support to direct drum sealing rings. *There are three choices here, shown below: #Solid teflon is on the left #Scarf cut teflon, center #Cast iron, right [[file:Seal_rings_t-400.jpg]] *Cast iron is what most kits will contain, and what was used stock in most TH400 transmissions. Cast iron is the easiest and will work fine for most applications but if using any type of valve body that will not require a modulator teflon is best as it will prevent wear on the direct drum. *Scarf cut teflon was used on later models of the T-400. *Solid teflon is used on the 4L80 series and is interchangeable. I prefer solid teflon, although it is the most time consuming type. If using teflon, be sure there are no burrs on the center support ring grooves. Installing the teflon rings requires a protector to prevent cutting or gouging them, and a sizing device. It helps to heat the rings in hot water before installing so that they will stretch easier. A piece of plastic (from a paint can cap) that is cut to size can be used to install and size the rings. Leave it on until you are ready to install the direct drum. One sealing ring has been left off, 2nd from top groove. This groove separates the direct (3rd gear) and reverse feeds from each other. When dual feeding the directs (as done in this build), this sealing ring is not necessary. Dual feeding the directs on a TH400 will be covered in more depth in another section. [[File:Center_support_rings.jpg]] Custom modification of the center support. The sealing ring is left off, the ring land has been machined off. This allows the oil to flow easier, quicker to the direct drum. Original unmodified center support is on the right in the photo. [[file:T-400_custom_center_support.jpg]] Photo below shows the thrust washer on the back side of the center support installed. It is held in place with some assembly grease. [[file:T-400_center_support_washr.jpg]] Pictured is a late style reaction carrier (left) and an early style. I believe the early style was only used on 1964-1966 models. Notice the actual sprag-type element used on the early style on the right. I prefer these types for high HP transbrake builds although not mandatory. Early style reaction carrier is also found in some HD versions of the THM400 used in medium duty Chevrolet/GMC trucks and the 1991-up 4L80E. [[file:planets.jpg]] My "compressor/sizer" for the sealing rings. Radiator hose with band-type clamp. You also use a soda pop aluminum can section. Be careful- the thin metal can cut the teflon rings and fingers. [[file:Center-support-compress.jpg]] Next, you will finish assembling the rear gear train portion for installation into the case. Install the center support into the reaction carrier. You will need to rotate the center support into the roller clutch assembly. Center support will then slide into the assembly. [[file:Rearassembly.jpg]] You will install the rear thrust bearing into the case or the stock selective spacer in the case with the stock thrust bearing on the rear planet (using assembly gel or petroleum jelly to hold in place). You will also install the low/reverse (rear) band. If required you install the fretting snap ring into the case. ATSG covers when this is necessary, most later model ('71-up?) transmissions have this. [[file:Casefretring.jpg]] I install the part of the rear servo to help hold the band in place. Not always necessary but it helps. [[file:Barecase.jpg]] Now you will need to lift the rear assembly and install. This rear assembly is relatively heavy. It requires you to lift by the center shaft, you cannot lift it by the center support as it will slide out of the reaction carrier. I use a paper towel to pad it slightly, and grip the shaft between my middle and index finger. I lift it using one hand under the output carrier until I'm ready to set it into the case. Then all the weight is on your fingers for a few seconds. If you have a helper they may be able to hold the output shaft once it passes through the rear case hole and hole in the bench. The center support must line up with the holes in the case, you can see two angle cut tangs on the center support in about the 8 and 10 o'clock positions as pictured, these can be used to line it up properly. [[file:Rearhalf_install.jpg]] Once the assembly is seated in the case, be sure the feed holes in the valve body area line up properly, and that the rear band apply is in the correct location. Install the center support bolt finger tight, then install the beveled case ring with the gap at the 9 o'clock position. Then using a screwdriver through one of the feed holes in the valve body area, force the center support in a counterclockwise direction and tighten the center support bolt. The center support snap ring has a taper on one side. The tapered side is installed ''up'' as you look into the case. Next is the intermediate clutch pack to install into the case with the proper snap ring. [[file:T-400_intermed_clutch.jpg]]
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