TH400 Direct Drum

351788749Now, one of the achilles heals of the th400 is the direct drum, which houses the 3rd gear/reverse drive clutch. the drum itself is a very strong piece. however, the intermediate sprag set up located to the rear of the drum is a weak link. some of my earlier blog post’s from way back address the different drums over the years so i am not going to rehash all of it. from the factory there were two styles- shown here is a pre 1971 drum with 16 element lug type sprag. 71 and later had 8 rollers…and they let loose and explode in heavy duty or race application. so, you want to start off with an early model drum. you then can get an aftermarket 34 element borg warner sprag that fits the older drum. the more lugs there are- the more it evenly spreads the load. when in 1st gear, the direct drum spins the opposite direction as engine rotation. to hit second gear, this drum comes to a dead stop to give you planetary action to create second gear. that shock load is immense in a racing application….and it’s not a matter if rather than a matter of when it will explode the roller clutch. in normal driving conditions….and to be honest anything up to 400 hp nbot using a trans brake or manually shifting….will usually handle a stock roller clutch. for full manual, trans brake, or heavy hauling, the 34 element sprag upgrade is a must. as a matter a fact on the 4L80 transmissions they went back to the 34 element lug sprag. i have been told the 4L80 drum and sprag assembly will interchange with an older 400 but never tried it as a total assy.

another weak link is the direct drum backing plate358343488. these things are prone to breakage primarily where system pressure has spiked or there was a clutch failure-it burned the clutch and overheated the backing plate causing it to crack. to keep this from happening, i ALWAYS make sure i use a rigid aluminum piston for the direct clutch . it is not quite as critical on the forward drum as it is not considered a shifting clutch. the stamp steel pistons will have a tendency to belleville/crown when installed in the direct drum from slam shifting and high pressure. this creates a wedge and in effect breaks the backing plate from the inside out as you can see by the picture. notice this one has an aluminum piston- the piston was cracked at the center  from a pressure spike due to a hung psi regulator-doing the same thing. so using an aluminum is not fool proof, but it drastically reduces fail rate. you can also use the backing plate out of a 4L80 which is made of a stronger material than the replacement 400 backing plates.


Another upgrade for a th400 that is a standard feature on the 4L80 is the spiro wound clip that retains the intermediate sprag. at high rpm, the stock retainer clip(laying on top in the pic) will have a tendency to release  due to centrifugal force. i have even seen some performance trannys go as far as welding in the stock clips to prevent this. although it is a bit of a pain to get it in, i suggest using the 4L80 spiro clip on all  performance trannys using the 34 element sprag. cheap insurance.

now last, but not least, is to apply both sides of the direct piston in 3rd gear application. primarily this is done by omitting the center seal of the piston and blocking oil port to reverse either using a set screw installed in the center support OR using a small freeze plug in the oil port of the case. you can also wedge a check ball in the port and stake it in place lARS017either. any one will work. you can also remove the second sealing ring down on the center support as well. this will allow the oil to flow more evenly into both passages of the drum to the piston. shown here to the right is an aftermarket direct drum and notice there is no provision at all for the center seal or seal ring for this reason. lightened billet drum are the cat’s ass, but they are not cheap and usually only last about 5 seasons for someone running a trans brake and a large bracket racing schedule. your wear rate may vary depending on manufacturer. you can also omit the center seal on the forward drum to achieve a similar effect for derby application only.

if you forget to plug the reverse oil port, it will lock itself up so don’t forget. what this does is give you full surface area of the piston to give apply force to the entire surface of the clutch. you will still have reverse as it will backfeed through the 3rd gear circuit. however, this mod will delay the reverse apply and for this reason should never be done on a derby tranny. this is only for street and forward race applications where reverse is not critical.


now, i am going to divulge a bit of a trade secret. shown here in the pic to the left is a center support and i am pointing to the 3rd gear apply port. this IS NOT the ony you plug for racing. the one you want for the modification above is the one to the right. the port at the center is the hollow bolt going to the intermediate clutch. now, i mentioned before that omitting the center seal creates even strong apply for 3rd gear. by omitting the center seal in a 1st/rev tranny you do the same in reverse. so you basically do the same racing modification ass396007831 backwards. ya plug the 3rd gear port and meter all your reverse apply through the reverse port. you control your reverse apply using a drill bit similar to the 3rd gear port.

another thing that i also do on 1st/reverse is to omit the intermediate sprag all together. like the check balls, it is unnecessary. you can then spot weld the lubrication ports to force extra oil to the planetary gearset

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