he 4L80E is a close relative of the TH400. It shares many components. Some of the components look similar but have slight differences so be aware of this when swapping parts between different year 4L80's and TH400's.
Parts that will interchange are the rear band, center support, direct drum (as an assembly), forward and direct clutches and the intermediates are similar. The 4L80E uses 4 intermediate clutch plates instead of 3 like a TH400. It has a thinner intermediate pressure plate, frictions, and steels. I use TH400 and 4L80 intermediate steel plates interchangeably to attain the desired clearance.
The 34 element sprag that is commonly used on a TH400 will fit the 4L80E and should be used on all but a stock rebuild.
I will add some pics and text here specific to a 4L80 rebuild. Other than some minor differences I will show, the rear rebuild is the same as a TH400.
Disassembly is relatively straightforward. Remove the dipstick, shifter lever, input and output speed sensors, tailhousing, pump bolts, and then the pan. Then disconnect the wiring (gently) from the various sensors and solenoids on the VB. Almost all the fasteners in the 4L80E are metric. The center support bolt is an exception and is a 3/8" 12 point, same as a TH400.
Next remove the valve body, then remove the 3/8" 12 point center support bolt and the Torx bolt for the overdrive section that is closer to the front.
Also remove the rear servo assembly, front servo piston, spring, and pin, parking pawl guide and spring.
You must remove the rubber o-ring from the input shaft so the pump can slide off, then you can gently pry the pump forward from the case near the valve body and shifter linkage area.
Remove the pump exposing the OD section of the trans.
You can then pull the input shaft and OD carrier assembly out. Then the OD clutch assembly comes out complete. The forward and direct drums will also slide out, and the intermediate band can be removed also.
The intermediate clutches and all the rest of the rear drive components can be removed exactly as a TH400.
Once the case is bare, you must replace the case bushing and seal. This is slightly different than a TH400. The 4L80E uses a seal to the rear of the case bushing to prevent lube oil from bleeding off. Since it does not use a mechanical speedometer gear setup, there are no gears needing this bleedoff lube. The lack of gears also has the side effect of no splash oiling to the output bushing. This bushing is fed pressurized lube. When replaced the output bushing must be aligned properly so that the lube can feed it from the side of the tailhousing.
Note the large orificed cup plug with rubber seal in the rear of the case. This is where the pressurized lube comes from to pass through the tailhousing passage to the output bushing.
Once the case is prepared for re-assembly, the rear portion re-assembles the same as a TH400. All the way up to the direct drum is the same. The forward drum is similar except that it doesn't have the input shaft attached. It is driven by the OD ring gear. Some will notice this is very similar to the design and function of the 200-4R, but more robust.
The center support contain the apply piston for the intermediate clutches. The seals need to be replaced and once assembled in the case with the intermediate clutch assembly in place, it must be air checked for proper operation. This is accomplished by applying compressed air to the 3/8 12 point bolt in the center rear VB area.
Forward drum rebuild:
Rebuilding of this drum is the same as the TH400, 3 lip seals, two on the piston, one on the drum. The direct drum is also basically the same.
Some use the bonded pistons which is a piston that has rubber bonded to it for seals. The whole piston must be replaced or in the case of the direct drum, can be replaced with a TH400 piston.
Assembled up to the center support, int band installed, ready for the direct drum.