Category Archives: Th400 Transmission - Page 2

Cutting a case for an aftermarket bell housing

390168847Well, you could say i have prepped a few 400 cases for the jw performance ultrabell. it is pretty straight forward, and the kit comes with instructions and hardware. this is pretty much how i have found to be the quickest way to get the job done.

The installation to me is done in two parts: 1- remove the bulk of the housing 2-grind the edges for an flush true fit. To remove the bulk portion of the housing, i like to use my 4 1/2″ milwaukee grinder with a cut off disc rather than a sawzall. 390168894 On a lot of cases like the one shown, there is usually some kind of impression left from years of oil pan gasket contact. use this as a guide and make your first two cuts above this mark….cutting through to the thin part of the bell. I then basically make a circle cut around the housing above or even with the top of the oil pump flange. keep in mind we want to remove a large portion of the bell to get it out of the way.

390168856390168859Here in the two pictures to the left, you can see the housing with the bulk of the bell removed. Also noticed i am pointing to two of the 8 bolt bosses needed to attach the case to the bell. Not all housings can accept the ultrabell. if your case does not have both of these bosses, don’t even bother cutting it. you need all 8 bolt bosses for the case to stay attached properly. the pump is no big deal however, just flip the pump over and drill out two holes to turn a 6 bolt pump to an 8 bolt pump. A lot of the ultrabell failures are due to two things happening, either not enough clearance when it was cut to fit the housing or not all 8 bolts were used because the case was cheap shitted. i have tore down ultrabell trannys from other builders where the housing ripped off……and find that it only had 6 bolts holding it on rather than 8, and they simply put a nut on the back side to make it look like it had 8 bolts in the case!!!! 390168849 At this point it is time to switch wheels and finish fitting the housing. i use a flap wheel style grinding wheel to knock the material off the housing. It doesn’t plug with aluminum like a metal griding wheel would, and removes material rapidly. wear your safety gear with this too cause it throws a lot of material all over. pull the wife’s car out of the garage first and save an arguement!!!

390168896390168868First thing you do is knock the 3 casting ears off so the bell sits flush. you then set your pump in the case….or what i do is use a tore up piece a shit as a dummy plug…..then set the bell housing down on the pump using two bolts as alignment dowels through the works. Now if you have a working tranny with a busted bell and just want to fit the housing, they say removal is necessary to install the bell housing. Although i have never done it, i think if you are careful you can do it without removal. i always cut for fitting during a rebuild. Basically at this point you take your flap wheel and keep knocking down the edges until the bell housing sits completely flush to the face of the pump. Basically trial and error fitting. You have the case clearanced correctly to the bell housing when you can hold it flush and still wiggle it against the bolts slightly. the only point you have to worry about taking down too far is getting into the oil pan gasket.390168903 You don’t want any part of the case touching the bell housing- just the face of the pump. Remember to refer to your instructions or call JW performance with questions. the picture to the right is a bottom view of a properly clearanced bell housing where it meets the oil pan.

after clean up and final installation, you then install the 8- grade 8 bolts provided. the o-rings go between the bell housing and the pump face to prevent leaks. i torque them to 18-20 ft/lbs. DO NOT OVER TORQUE these things. it is just aluminum. the bolts that come with the bell housing are actually a special length as well and thread into the case beyond a stock fastener. these bell housings may seam like they aren’t worth a shit, but they do work! i just wish you could run something bigger than an 11″ torque converter!!

I.D. your tranny according to oil pan shape

1 Aluminum Powerglide
2 TH200 (Metric), TH200C (Metric)
3 TH250, TH250C, TH350, TH350C, TH375B
4 TH375C, TH400, TH475, 3L80, 3L80HD
5 TH200-4R
6 TH700-R4, 4L60, 4L60E
7 4L80E

J.W. Bell Housing

374721910Well, when the bell housing breaks or you want to go with a sturdier upgrade to your transmission the ultrabell from jw performance or coan engineering is an excellent option. Installation is pretty straight forward you saw off the bell housing and remove enough material to bolt the ultrabell flush to the face of the front pump of the transmission. i use a sawzall to rough cut most of the bell housing off, then a cut off wheel on the angle grinder and a flap-wheel style grinding disc to polish edges of the aluminum to final fit the ultrabell to the pump/case.

On the th350 there isn’t much more to it then that, but the th400 did not always have 8 pump bolts cast into the transmission housing. most of your older 70’s vintage case had all 8 bosses under the front pump even if only 6 bolts are showing. quite often you can drill the extra 2 bolt holes in the pump itself through the backside of the pump. however your 80 & newer chevy th400’s only were cast with 6 pump bosses. you must have all 8 bolt bosses to make it work. you MUST use the bolts provided with the bell housing kit. they are hardened grade 8 and a special length.374721907

The advantage to the ultrabell is that it will not snap from torsional stresses of forward/reverse shifting. This same action also breaks the bell housing in snow plow trucks for the same reason. In normal installation, it WILL NOT make the tranny stronger in reguards to vertical stress from a distributor protector or anything else as the car bends upward. The housing can simply rip off the front pump if it gets pinched in a lateral direction. I am currently working on some improvements to keep this from happening, but it’s top secret right now…
the bell housing is meant for drag racing, and is intended to run an 11″ torque converter. you can make a 12″ converter fit with some machining. for derby i prefer to have a custom built low stall 11″ built so the engine starts to pull around 1100-1300 rpm.

Th400 fail points

ok th400 fail points. after you get some kind of a clue what is in a tranny from above you can now recognize some of the common fail points.

 first is an easy one. if you guys know anything about hydraulics, accumulators are used to help cushion the effects of hydraulic shock. well, they are used in trannys too for this reason to give you that “cadillac” style. the 1-2 accumulator is aluminum and is not a big concern.

 however the 2-3 shift accumulator is not always aluminum. somewhere in the mid 70’s they started using plastic…and you guessed it, it cracks. when 316600931this happens it not only bleeds psi off the 3rd gear circuit, but the valve body backfeeds 3rd gear to help apply the direct clutch(we are going to call this the reverse clutch for sake of conversation)….yup you lose psi in reverse. this is often the first thing to go and create a weak reverse gear. and here’s where only the rear band holding the planetary in reverse is a problem. you lose psi anywhere in the circuit you lose everything….it has a lot to do with pascal’s law but you don’t need to concern yourself with it too much. since there is no mechanical sprag to fall back on reverse is often the first to take a sh!t in your tranny.

358343491the forward clutch is prone to burning out prematurely simply because it has a restricting orifice to cushion the shift into gear from park/neutral. when you slam shift it back and forth it never has time to apply fully and will slip & eventually destroy the clutch. reverse can go out for the same reason. on a used tranny it is really a crap shoot depending on how the tranny lived it’s life.

there are two different styles of clutch pistons aluminum and cheaper stamped steel. stamp steels started showing up in the mid 70’s aluminum is preferred as it is more resistant to warping from extreme use. however rebuild shops usually took them out and replaced them with stam316600827ped steel to make a buck. so there is no definite way to tell what your tranny has without tearing it apart. the forward clutch is usually just fine with a stamp steel piston but during a rebuild i susally replace the direct drum piston with an aluminum piston.

 the intermediate clutch can come apart, especially if a shift kit has been installed. the shift kit bypasses a lot of the cushioning effect of the accumulators and can crack the clutch backing plate(pictured right), or worse yet blow the groove’s right out of the case.  not  factor in a 1st/reverse only tranny as you don’t use any of this.

358343488reverse /direct drum quite often can shatter the backing plate that holds the clutch in. here’s an assy to the left as it looks upon removal when it goes to hell. this is a common failure in snow plow trucks and derby trannys. it’s more common than you think it can break like this which creates a weak gear..a stamp steel piston will crown and act like a wedge to make this happen quicker, but it will also happen with aluminum pistons358343493

here to the right is pictured just the direct backing plate. that let loose all at once and locked up the tranny the drum was also destroyed. on the leftis a good piece.

contrary to popular belief, a torque converter can explode internally and will do it more frequently than you think. a stock style or cheap aftermarket converters are not designed for the shock of forward/reverse shifting with you foot on the floor and not hitting the brake. if you drive like a jack ass you pay the price. more often than not it will give you the same symptoms as burned up clutches/seals. so if you have a tranny weak in forward and reverse you may have a bad converter. BUT if the converter went to hell chances are it sent sh!t through the pump and the rest of the tranny so you really need a new build reguardless.i have had real good results with guys using 6 bolt heavy duty gm truck converters. stall converters are a good alternative but then you will be creating A LOT of extra heat fast.

from what i have seen shift kits are a bad idea for demo derby. this is due to the fact that to create a hard 3rd gear shift most kits restrict if not block off completely the fluid flow to the reverse circuit comming out of the valve body.  some cheap made kits also boost psi to an ungodly level in reverse that encourages seal failure on hot trannys, especially with loose fitting stamp steel pistons. this is fine in a normal tranny , but there is already a boost valve internally on the th400 to aid in reverse band application. together with a higher line psi pressures well over 350psi  are not uncommon with the modulator unhooked.  in my book i like to see line pressure between 175 and 230psi. with shift kits, on a th400 you can make your own kit’s with a bit of research 😉 i think they are a waste of money on a th400.

also i recommend hooking up your modulator valve to vacuum on a stock transmission. this aids in spiking psi in the reverse circuit. as long as you shift low 1-reverse the psi will remain where you want it. my research thus far suggests the psi will remain in the 150-200 range and work fine. however if you decide to use regular drive range the psi drops down to less than 100psi. running without the modulator hooked up or using a mod plug should never be done without further internal mod’s to prevent psi spiking in reverse to dangerous levels.

Th400 basic operation

here’s a reader’s digest version of how in normal operation a th400 will shift:

 1st gear- forward clutch engages to drive the planetary gearset/all other clutches disengaged/ one way low roller sprag in the rear planetary holds the gearset to create a 2.48:1 ratio. in low 1 the rear band would also apply for extra holding power. at this point due to planetary action the forward drum rotates clockwise but the direct drum spind counter clockwise at this point.

 2nd gear- forward clutch engaged. the intermediate clutch applies to stop the direct drum dead in it’s tracks. this creates a 1.48:1 ratio. the low roller sprag that was holding then is renderred useless as the carrier switches direction. so you still have only 1 holding member.

 3rd gear- forward clutch engaged. the intermediate clutch dissengages and the internal clutch of the direct drum engages making a solid unit…..1:1 ratio. at this point the guts of the tranny spin as a unit. there is no gear reduction therefore no holding member in use

 reverse- forward clutch dissengaged. direct drum then engages and is driven off an outer hub of the forward clutch. now we have a problem we need a holding member but now if you are following this at all you guessed it, the low roller clutch won’t hold it either. this is where the rear band engages to hold the carrier all on it’s own. in normal operation this is fine but for derby this is a major source of problems.

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.

Th400 Housings

the GM turbo 400/TH400 was in production from 1964-1991. over the years there were many changes to the design to both improve and reduce cost’s in later years. this is a short guide to explain the TH400’s features, changes, and hopefully identify your transmission to see what you have in your shop so you can determine for yourself if it’s worth the money for a rebuild.


 first off let’s go through the different housings.

 -first we have the roundtop bell housing case. this was used in 64 cadillac, 64-66 buick nailhead engines, and with an adapter plate it was installed in jeeps in later years. this is an oddball bird and you usually find it still hooked to an engine. basically the bell housing looks like a half moon with a provision for the starter to clear on the driver’s side similar to a later model transmission.

-next we have the 65-67 cadillac th400 transmission which appears like it’s more octagonal and has no top ears to the tranny like the later model BOP cases we are familiar with. these were unique to the cadillac in only these years and were paired behind 429 power plants.

-next we have our all too familiar BOP transmissions. for the sake of this section i will divide this into two generations of bop castings. first we have the 68-7heavy casting BOP housing6 heavy casting trannys used in buick,olds,pontiac, and cadillac. also heavy cases appear in 77-79 cadillacs in a short shaft version as well. these were among the heaviest castings as far as bell housings ever cast for the th400. the easiest way to tell what you have  is to take a gander .notice the extra material under the top ears. the extra material is carried all through the casting with extra material cast into the valve body area and the side ribbing . i figure about 5-6lbs extra aluminum over a chevy case.light BOP case housing


 then we have the lighter BOP cases used in 77-80 in metric gm full size. it is basically a lighter BOP case notice there is no extra material under the top ears. these housings are about as strong as a chevy case.

-then we have the chevy th400 case. used 65-91, and 73-later checker cabs. although there were many different castings i have seen over the years i have not seen any of them not crack in a demo derby. i have had “HD” housings that actually mic out thinner than late 60’s chevy car tranny housings.

 starting in the early 80’s the chevy th400 were primarily only in heavy duty applications and chevy/gmc trucks.  some of these were cast “HD” in the bell housing which was in referrence to a heavier duty rear planetary set internally.

  later model 88-91 th400 all had a slightly heavier casting(sorta) and had heavy duty internals, but did not have the “HD ” cast in the bell.  i mean as far as bell housing it is less that 1/16th of an inch. these 88-91 trannys also had different plumbing at the valve body than all other years in the casting, so the gasket used on these housings is different than the rest. some also had 4L80 parts incorporated toward the end of the run of the TH400 as believe it or not quite a few parts interchange between the two trannys.

in any and all situations an option such as a full girdle or a JW ultrabell which bolts to your front pump in place of the housing is a much stronger option. if you run a slider shaft it is not a matter of  if but an issue of when you will break a transmission housing.  heavy re-enforced oil pans aid in strength of a housing as well but not enough to make it worth the cost usually. food for thought guys!!