TH400 basics on the internals you don’t see.

Basic assembly and some of the internal differences through the years:


it any automatic transmission there is always one holding member and one driving member to achieve gear reduction. the th400, like most aut338804040omatic transmissions, use a planetary gearset for gear reduction. the transmission is basically built in 2 halves, there is the rear section which contains the planetary gearset, your low1/reverse holding band, and the low roller sprag(hold 1st gear). here is a planetary w center support as an assy. as it looks when installed.

338804024the low roller sprag on the left is a common roller sprag on the right is a rare dog bone style sprag only found in 1964 vintage trannys only. this was due to gm not wanting to pay borg warner out the ass for their design so they changed it.

 the planetary set is held in place with a center support piece. the center support is exactly what it says, it helps support the case @ the center, provides many of the oilS3700008 passages to both lube and apply clutches, and seves as the inner race to hold the low roller clutch in place.  confused yet? it’s actually a lot simpler than it sounds so stay with me.


located between the front pump and the center support in the front section of the tranny are all of your shifting/holding clutches. there are 3. TH400_Aluminum_Drumstarting behind the pump(at the front of the housing we all see with the converter is not installed) is the forward clutch(seen above is the drum/input shaft). the small splined shaft that sticks out the front of the pump that turns is part of this clutch drum. this clutch applies in all forward gears to drive the transmission. in normal operation it applies once and considered a non shifting clutch. all it does is apply to drive the transmission in forward. that is it.

right behind the forward drum is the second/third gear drum, called the direct clutch drum. here in is the source of a lot of problems as it relates the351788749 the th400. the drum rests against the center support and meshes clutch discs to the forward drum. surrounding the direct drum is you low 2 band. this provides extra holding tension to the direct drum for 2nd gear. internally there is a clutch which engages to create 3rd gear….or 1:1 ratio.

reverse is created by dissengaging the forward clutch and engaging the direct clutch. this drives the gearset in reverse direction in a 1st gear ratio.


externally to the rear of the direct drum is the intermediate clutch. pictured 21here is not a th400 but is very similar.this is a holding clutch that is anchored to the housing and rides against the center support. this clutch meshes with a roller sprag on the direct drum. this clutch engages to create second gear by stopping the direct drum in it’s tracks the clutch mesh’s with a race and sprag to the rear of the drum.

 the purpose of the direct drum  sprag is to allow for 3rd gear function without having to spin the clutches and create excessive heat and wear. hold 2nd gear but rip loose quickly for 1:1 3rd gear function.  there were two series of direct drums and sprags. from 64-71 there were 16 element dog bone style sprags and 72-91 used a cheaper roller style sprag. a 16 element dog bone stule sprag is pictured above in the direct drum photo w the race pulled back to show the sprag. roller sprags have a tendency to explode…easily. the older style is preferred for heavy duty and racing applications AND is the only style you can use an aftermarket 34 element sprag in place of the 16 element stock sprag. this spreads out the load force more evenly to prevent failure at the 1-2 shift.

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