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TH-350 and TH-400 Similarities/Differences

Posts: 7841
Joined: 12/09
Posted: 12/16/13 11:06 AM

I brought this here, from another forum.,. where I had posted it a long time ago.


TH-350 and TH-400 Automatics

tuffnuff (
Moderator | Posts: 6978 | Joined: 12/09 Posted: 12/20/09
10:52 PM

Similarities and differences

TH400 was first implemented in 1965 as GM's first 3 speed trans and saw production until 1991.
It has been installed behind Chevy's, Caddys, Olds, Buick, Pontiac, GMC, Jaguar, Rolls Royce, and Ferrari powerplants.
IMO it IS the finest production 3 speed auto ever designed bar none.
It works great behind stock powerplants all the way up to 2000+ HP applications.
The TH350 was used from 1969-1986.
It had lots of variations and spinoffs.
It is a very underestimated transmission.
Though not as strong as a TH400 it takes plenty of abuse and works well in near stock form.
Used extensivley in GM cars and trucks and shares mounting location, spline count, and length with most other GM 3 and 4 speed manuals, as well as later Powerglides.
Similarities between a TH350 and 400 include input spline count, basic function of the holding elements and planetaries.
The similarities also include the weakness's of each.
They both use a somewhat weak intermediate roller clutch design. More on this later.
They both have a somewhat weak direct clutch apply.
The gear ratios although not the same are similar.
The TH350 being 2.52-2 first ratio, 1.52 2nd, 1-1 3rd.
TH400 being 2.48, 1.48, 1.00.
Major differences are the TH350's lack of a rigidly mounted center support, use of a large diameter sun shell (drive shell), as opposed to the TH400's use of a sun shaft, and a TH350 uses a smaller diameter low/reverse roller clutch.
The TH350 also uses a low/reverse clutch pack as opposed to the TH400's use of a band to serve the same function.
A few quick short notes.
A clutch pack in an auto trans function like a clutch with a manual trans.
It uses a friction lined plate with splines that will drive a hub or drum.
These friction lined plates are contained in a drum, usually, or within the trans case.
The frictions are sandwiched between bare steel plates.
So they act like a clutch disc sandwiched between a pressure plate and flywheel.
The clutch pack is applied using a piston that forces the clutches tightly together causing them to bind and lock.
The piston is applied using transmission fluid forced behind the piston under pressure.
Simple hydraulics.
The piston is sealed to a drum or case using lip seals or o-rings.
When the clutch is released, fluid pressure is released and springs force the clutch back to prevent the clutch plates from getting burnt.
That is one type of holding device.
Another is a band.
A band is simply a friction lined metal band, that tightens around a drum or plantary element causing it to stop rotating.  
These are what the whole transmission wraps around to operate.
Since both units operate on the same basic priciples many of the parts have the same function.
There is a forward clutch, this clutch is always engaged when you are in a forward gear range. It will be disengaged in reverse, neutral, or park.
Neutral and park are when the trans has no holding elements applied, so it is just the converter turning the input shaft slipping through the forward clutches, and at the same time the converter hub is driving the pump to produce pressure which is exhausted.
Park has a parking pawl that grabs the output planetary housing to prevent it from turning.
There is also a low/reverse clutch in the Th350 or band in the 400.
These serve to provide engine braking in 1st gear manual.
They prevent the reaction carrier from over-running.
They lock down the reaction carrier to provide reverse.
Both trans have an intermediate clutch pack. This clutch stops the direct drum from turning (through the intermediate sprag and race), which in turn stops the sun gear.

Both have a direct clutch that applies to give you 3rd gear, it is alos used to drive the planets in such a way to provide reverse.
Both use an intermediate band. It's function is to provide engine braking in 2nd gear manual.
It prevents the intemediate sprag from over-running.
Both use a internal/external gear type pump.
The pump creates the hydraulic pressure to operate all the circuits of the trans.
Back to the one way roller clutch or sprags.
The problem with these items on most transmissions is that they are not designed to take serious abuse and drag racing delivers serious abuse to them..
In a TH350 or TH400 there are two one way holding devices.
In most TH350's and 400's they are a "roller clutch design" and not a sprag design.
The sprag design is usually considered stronger but is more expensive to manufacture.
Early TH400's used a sprag design in both elements.
However the stock design was not much stonger than the roller clutch type used in later models because it used a low element count.
The TH's use the "sprags" as we'll call them from now on, for 1st gear, and then 2nd gear.
If you are driving normally with either unit with the shifter in the drive, or 3rd gear, position.
As you leave from a stop the transmission valve body or brain, senses that the road speed is low via the governor. So it is in 1st gear. 1st gear auto.
In 1st gear the low/reverse sprag is holding the reaction carrier from turning counterclockwise. The sun gear is being driven.
If you are shifted into manual 1st the sprag and the clutch are preventing it from turning either direction. If it is only locked from turning counterclockwise you have 1st gear on acceleration but you do not have engine braking.
When you positively lock it both directions you have low gear with engine braking.
When the governor senses higher road speed, the centrifugal force forces the gov weights out against hydraulic pressure and causes the governor to send a signal back to the valve body that forces the valve body to shift to 2nd gear. 1-2 shift valve.
In second gear auto (lever in 3rd but trans in 2nd), the intermdiate clutches apply.
The forward clutches are still engaged.
This causes the low reverse sprag to freewheel or over-run.
The intermediate sprag and clutches lock the direct drum to the case.
So it goes from 84% of engine rpm turning backwards (being driven by the planetaries), to zero rpm.
This action locks the sun gear and drives the ring gear (reaction carrier), changing the planetaries operation from compound to simple reduction.
The intermediate band is used for engine braking in 2nd by preventing the direct drum from over-running in the intmediate sprag when decellerating.
When the TH350 or 400 shifts to 3rd, the direct clutch applies, and the intermediate sprag begins to over-run.
The intermediate clutch stays engaged.
You still have the forward clutch in operation.
This locks two elements of the planets together (sun gear and input ring gear) causing direct drive and no gear reduction.
In reverse the low/reverse holding element is used in combination with the direct clutch to cause gear reduction and reverse rotation.
The TH350 and 400 depend on proper calibration to provide dependable operation and prevent unwanted failures.
Since they share some common weakenss due to their common design the fixes are also common between the two.
The intermediate roller clutch or sprag being the weakest and most common failure of the two.
In absolutely stock form failures are not that common due to properly calibrated accumulated shifts from 1st to 2nd gear.
However as soon as you begin to try to get harder shifts you compromise the longevity of these parts.
There is a fine line you want to walk here.
As you put more power into the transmission, you want less clutch slippage because slippage is wasted energy which creates heat.
The heat can cause friction material failure.
A cooler can help this but the temps seen at the clutch friction surface are extreme compared to the operating temp of the fluid.
So we obviously do not want slippage, even milliseconds can kill a clutch in extreme power applications.
So calibration to control shift timing is critical here.
Cannot shift too hard or it breaks the sprag/race, cannot let it slip or it kills the clutches.
What you are ideally looking for is a very quick shift with no overlap and no lag.
In auto mode overlap isn't a problem but manually shifting the low clutches or band must release quick to allow the intermediates to apply.
If both are applied at the same time they work against each other, effectively putting the trans in 2 gears at once.
This causes the clutches themselves to wear/slip/heat.
It also makes shifts "feel" very brutal.
There are upgrades to replace the weak sprag for both the TH400 and the TH350.
The TH400 can use a early (1965-1971) direct drum with a smooth inner race and an aftermarket 34 element sprag assembly.
The sprag ranges from $45-75 in the aftermarket, the drum can be found in core trannys, or through core suppliers, trans parts suppliers, GM counter, ranging in price from $15-200.
ATI offers a new inner race, sprag, outter race assembly that can be retrofitted to a later drum using a press. It is also about $120 as I recall.
A 4L80-E drum will retrofit to the 400 and already has a 34 element sprag.
Plenty of options and usually a cost effective upgrade.
A TH350 can be upgraded using an aftermarket drum, 36 element sprag assemblies.
great upgrade but at near $300 a pop, may not be worth the cost when by the time you buy it, install it, freshen the tran, you could have a TH400 with the upgraded assembly for not much more and have room to grow.
The reason they break sprags is the severe loads placed on the sprag on a 1st to 2nd gear shift.
Remember that the direct drum weighs 14 lbs loaded on a TH400, it is spinning 84% of engine rpm in 1st gear, opposite of engine rotation.
When it shifts to 2nd the sprag must stop the drum from turning.
So on a quick 1-2 shift at 6500 rpm, that little sprag must bring a 14 lb drum from just under 5500 rpm to a complete stop as fast as the shift happens.
Then on a 3rd gear change it goes from the sprag keeping it locked to engine rpm again.
Keep in mind on a manual downshift from 3rd to 2nd it goes from engine rpm to 0 too.
Not a problem on an accelerated downshift like passing gear, but on a decelerated downshift it is harsh.
Next common weakness to both is the direct drum apply area.
Since the direct drum is used to apply 3rd and reverse the direct apply piston is split in half.
So each gear only gets half the piston area in stock form.
This weakness even shows up behind stock motors.
Reverse is not a problem because we are not usually giving the transmission full engine torque in reverse (LOL).. it also has the planetaries taking load off the direct clutch because of the gear reduction, torque multiplication, as well as it not being a shifted gear. You put it in reverse and all the clutches are applied. No shifting to cause a clutch to slip as it goes to the next gear.
So if you are one of the weird ones using reverse for full engine torque (WTF are ya doing?) you'll probably not burn the directs up.
However going from 2nd to 3rd at WOT behind a stout powerplant taxes the direct clutches.
The TH350 has a lighter direct drum to have to spin up from zero rpm to engine RPM. Slight advantage but not much.
We fix the direct clutch failures by using both halves of the piston to apply in 3rd gear.
Several ways to go about this.
On a TH350 almost all "shift kits", valve body reprogramming kits, etc.. accomplish this.
If you've installed one you probably remember the "fluid transfer plate" that sandwiches between the valve body seperator plate, and the support plate to the front of the valve body. Just a plate with a passage cut into it to let fluid get to both sides of the direct piston in 3rd.
The lip seal on the middle, dividing the piston can also be omitted, and then a passage plugged to acomplish the same thing.
On a TH400 the only valve body kit that I'm aware of that "dual feeds" the direct clutches is the Transgo Reprogramming kit.
It has a patented sandwiched seperator plate that does the same thing as all the TH350 kits.
It can also be modded internally same as the Th350 to accomplish the same effect.
I prefer to use the TransGo kit on a TH400 because in my opinion it preserves the integrity of the fluid circuits.
On a TH350 either way will do fine.
I personally don't care for the gasket that is required in the TH350 Valve Body kits.
Now for other strengths, weakness's, etc..
The TH400 has a very rigid center support placed in the center of the transmission.
it is actually bolted to the case.
It provides support for the reaction carrier, low sprag, and direct drum, as well as the sun shaft.
IMO if you did all the tricks to both trans as far as upgraded sprags, dual feeding directs, more clutch surface area, etc..
This is one of the key differences why a TH400 is stronger.
It simply holds the heavy internal rotating parts more stable, centered, and with more bushing surface area.
The TH350 does not have quite as much support internally and therefore is rougher on bushings and thrust materials.
the TH350 direct drum drives the sun shell.
The sun shell is about 6" outside diamter shaped somewhat like a bell with lugs that catch outer lugs on the direct drum.
The sun gear is attached to the sun shell by thin wire snap rings.
This shell gets loaded by the direct drum, the higher the loading, especially during shifts, the harder this shell gets "cocked".
Since it is 6" diameter is has more of a tendecy to get side loaded, causing greater bushing wear, and is weaker by design than the TH400's sun shaft.
The TH400 sun shaft is about 1.5" diameter with splines that are driven by the center of the direct drum. So very little side loading. And any side loading is better controlled by the TH400's generous center support bushing, probably 3" of length compared to not quite an inch on a TH350 in stock form or a little less than 2" in modded form.
TH400 intermediate clutch pressure plate is held into the case by a snap ring, locking into a groove in the aluminum splines in the case.
Under extreme pressure this snap ring can beat the lugs out of the case.
There are fixes for this but the best thing to do is keep line pressure down to prevent this failure.
Excessive line pressure is rough on pump gears and housing, lip seals, drum to sealing ring surfaces, converter, and potentially the engine thrust bearing.
This should pretty well cover the basic difference between the 2 units and their strengths/weakness.
Hope this explains some of the mystery about these automatics,,, away.

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

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