TH350 rebuild tech
The GM Turbo Hydra-Matic 350 transmission (aka "T350", THM350 "TH350", "T-350", "Turbo 350", etc.) was used between 1968-'86 in RWD and 4WD GM vehicles in the USA. The T-350 was still produced many years beyond 1986 for 'export sales' only. It is a relatively rugged, compact transmission that lends itself to a wide variety of applications and modifications.
Basic rebuild information
- This WIKI is not intended to be a replacement for an ATSG or factory tech manual. First time builders will likely need a tech manual. Recommended is Ron Sessions' book "TH350 Handbook" in the HP book series. It has good pictures, a lot of good info, but has some outdated info. Outstanding value for the money.
- This is not a step-by step pictorial. That said, the following should help give a better understanding of the T-350 and possibly help when a question arises.
The Sessions and ATSG manuals are the preferred texts.
Several locations were stamped with ID numbers. here are some example:
There was also a T-350C version that has a lockup torque converter (TCC). The case has a "C" cast into it, and there will be an electrical plug adjacent to the shifter shaft on the driver side of the tranny. Not to be confused with a TH350 non-lockup tranny with a screwed-in electrical connector on the passenger side of the case. This is a pressure switch that was used ~1972-'73 for the Transmission Controlled Spark (TCS) system. This disabled the distributor vacuum advance in all gears except high gear, and is a single wire switch threaded into a pressure port near where the cooler lines are located. In the case of this switch, you can either leave it in place, unused- or remove it and plug the port. There have been reports of some T-350 trannys having a temp sender in this area as well.
There aren't a lot of places making high stall torque converters for the T-350C.
The T-250 has a band adjuster stud and locknut on the passenger side of the case near the cooler lines.
Rebuilding the T-350
Bushings and thrust washers
A T-350 is known to be rough on bushings. Bushings are critical in that they allow the rotating parts to ride true on center, and for the lube to make it to the rear of the trans.
Be prepared to replace almost ALL the bushings in a T-350. Might as well order a bushing kit, as well as a thrust washer kit. I would recommend you get at least 2 additional sun gear bushings individually also.
In the photo is shown the 2 common pilot bushings for the output shaft, a metal bushing and the plastic insert sleeve. The plastic sleeve design is very durable, you will find professional builders have their own preference.
Once the case has been cleaned, all threads checked and/or repaired, and the rear case bushing installed you can start assembly.
Photo shows accumulator cover, spring and piston (snap ring not shown). This is located on right side of trans case. Check the bore in the case for scratches or other damage if the spring was found broken.
Right side of T-350 case. Pressure ports circled.
- Rear port (yellow) is main line pressure. You will see pressure here in all gears for most valve body configurations. Some race-only valve bodies may not have pressure at this port or the others.
- Middle port (red) is 2nd gear pressure.
- Front port (blue) is third gear pressure.
Low-reverse piston install
The first item to go in the case will be the low/reverse apply piston. Its function is for oil to force it towards the front or bell housing side of the trans, applying the low/reverse clutches. This allows reverse when applied simultaneously with the direct (high gear) clutches, and allows manual low gear when applied with the forward clutches. Manual low gear creates engine braking on deceleration. The low/reverse clutches do not apply in automatic range 1st gear. The low sprag (low roller clutch actually) causes first gear when only the forward clutches are applied.
In the rare application that does not need reverse or low gear engine braking the low piston and low clutches can be deleted, and the feed holes in the case blocked.
Install new seals on the low piston, these are square cut seals, not lip seals.
Then push it into the rear of the case evenly by hand, aligning it properly so it will seat (there is a "tang"). Look at the park pawl notch to help index it.
Then install the return spring and retainer using the tool pictured.
Output shaft install
Next you install the output shaft.
Be sure you have the output ring gear to case bearing in the case. Also be sure on dis-assembly you inspected and replaced as necessary the input shaft to output bushing shown here (red arrow).
Rear planetary unit install
Next goes in the output planetary to output ring gear bearing or thrust washer (depending on year).
Then the planetary unit. Replace the bushing in this planet.
Note here: If you have a light duty core with low count low pack, unless it is a transbrake application or will be manually shifted into low for engine braking often, fewer clutches is OK. In some cases where you are after every last bit of ET, using fewer clutches can cause less frictional loss in 2nd and high gear. "Turbulator" steel plates from a 4L60E will also help and are a direct drop-in. I am using Alto red frictions in this unit, however a stock tan clutch (I prefer Borg Warner) is more than adequate for even very high HP cars. A Hi-Energy Borg from a 4L60E is also a good choice.
Once the low clutches are installed, you can install the anti-clunk spring (or case-saver). Both are pictured, the case-saver is solid and spreads the load over a larger area. Hold in place with assembly gel or vaseline and then install the low support. Be sure the low sprag inner race is not brinelled or has any unusual wear.
Once it is pushed into place, install the snap ring, with the ends on each side of the spring or case-saver. Now is the time to air check the low clutches. You do so by applying pressurized air to the passage at the rear pan area of the worm casting in the case.
Next you install the 4-tang thrust washer into the large splines of the low sprag inner race (picture above), and then the sun shell and gear assembly. Before this step, for HD use, you would have wanted to install additional sun gear bushings, 2 on each end, and re-drilled the lube holes. Shown here.
Install drive shell and front planetary unit
Drop the sun shell/gear in, spinning in the output carrier until it falls in place.
Install the input planet next, pictured below with thrust washer installed.
Now, To prevent excessive end play, a pump thrust washer can be installed on top of the input planet, then the regular thrust washer. Then install the small outside snap ring to hold it all in place. Check the output shaft for proper rotation, there will be some drag, especially in one direction. Check for excessive output end play. If end play is excessive, it can be tightened up by dis-assembling and shimming up the output ring gear bearing from the case.
Now you drop the input ring gear on, spinning into place.
Then install the 3-tang thrust washer onto the top ring gear.
Direct drum assembly
For racing and heavy towing use, you will want to machine the direct clutch piston down approximately 0.160" to allow for one extra friction and steel plate for a total of 5 frictions, and 5 steels in the directs. This can be accomplished on a brake lathe.
Stock measurement is 0.835", I cut them to 0.700". Ideally you would pre-assemble, and check clearance using a 4 clutch pack. Measure the additional thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb on most auto trans friction clearances is ~0.010" clearance per friction. So a 5 friction direct setup would work great with 0.050" to 0.070" clearance. It will work with less, I wouldn't go less than 0.040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, busted lip seals, and unsatisfactory shift quality.
If the piston is too low, the bottom flat steel plate can drop below the grooves in the drum and bind up the piston. You can leave out center cushion seal if dual feed is used.
The HD applications should also get a heat treated intermediate sprag race. It is darker colored than a stock piece. The intermediate sprag race and intermediate sprag (actually roller clutch) take a brutal amount of force on a 1-2 shift at high rpm. Even more so with harsher shift calibrations. It is the single weakest part of a T-350. This is not to say a T-350 is not tough; they are under-rated and can reliably handle in the 600 HP or lb/ft range based on my experience.
An even heavier duty direct drum/sprag assembly is available. It uses a 36 element sprag and a machined drum, it will withstand more torque than a stock unit or one w/just a hardened race. These units can cost upwards of $300.00.
I also like to always use a NEW intermediate roller clutch for HD builds. My theory on a long lived setup is that the springs that force the rollers against the race must be in good condition to help the rollers grab quicker and more evenly on apply.
The parts go on the direct drum.
Assembled direct drum sprag and race with retainer.
The direct drum should use a wider bushing even on stock rebuilds. Stock is 0.500" a wider 0.750" is available in some bushing kits. Most bushing kits include the wide one. There is a lot of load on this bushing, and it should ALWAYS be replaced on overhaul.
Also check the inside bore of the drum for wear from the sealing rings. A small amount of wear can be honed out. Visual shiny spots with no detectable wear are OK.
Next you will install the direct piston lip seals, and install the piston in the drum.
Take note of the orientation of the seals, the lip will face towards the "oil" side of the piston, not the clutch side.
This is a high performance T-350 and I did NOT install a lip seal in the drum that would separate the two halves of the direct piston.
This is part of the process of "dual-feeding" the direct clutches. On a stock rebuild you would normally install this lip seal. You can do this on any rebuild but you must also block a passage in the case near the pump to prevent loss of pressure to the reverse circuit.
Dual Feed direct drum
Dual feeding doubles the area of the piston that has pressurized fluid on it in 3rd gear, more than doubling the capacity of the clutch. It is accomplished by most valve body kits without doing so internally but requires a "transfer" plate and gasket to be added under the support plate in front of the valve body.
I prefer internally dual feeding because it leave out the additional gasket and eliminates that one extra possibility for a pressure loss resulting in burnt 3rd gear clutches (yes it has happened to me). All transbrakes will require dual feed of the direct clutch, as well as many manual valve bodies.
Here is the passage that must be blocked. I usually tap this hole with a 3/8" tap, and cut off the end of a 3/8" bolt. I then slot the head of the bolt so I can use a flat-tip screwdriver.
I don't thread the passage completely to bottom. I want the "plug" to seat on the unthreaded portion.
May also use roller from a direct drum or sprag to drive into the passage, will not be able to remove!
Also of note, if you are using almost any aftermarket kit, these kits use a feed plate between the support plate in front of the valve body and the separator plate. This feed plate dual feeds the directs and plugging this hole is unnecessary. Also most manual valve bodies, trans-brakes, etc. will have you omit the direct drum center seal and the sealing ring on the stator but plugging of this hole is also unnecessary due to the design of the separator plate.
Don't assume you need to plug this hole. Only plug this hole if you plan on building a unit yourself.
You can omit the center seal, plug this passage, enlarge the 2nd and 3rd feed holes in the stock separator plate and you will have greatly increased the torque capacity of a stock TH350 as well as gained a firmer shift.
Even firmer shifts can be accomplished with check ball removal and modifications to the accumulators. I recommend NOT modifying the 1-2 accumulator. Retain the cushion spring and do not block the circuit. The 2-3 accumulator can be blocked by several methods.
Removing the 2-3 accumulator spring and using a spacer (nut, washers, cut down section of tubing) between the piston and the E-clip is the "shade tree' method. You can also block the feed in the valve body to the 2-3 accumulator.
Installing the direct piston will require some patience, a lip seal installed or 0.010" feeler gauge, and a trick I use is a section of plastic cut from a 3 liter soda bottle to use as an outside installer tool. Set it in the drum, and it will force the lip seal into the drum without using a tool around the entire circumference. You may however need to work the exposed part of the seal into the drum with the feeler gauge or seal tool.
Use care using the lip seal tool, I use a push-in technique more than a work-around-the-drum technique. If you slide around the drum, and it catches the seal it will cut it and then you must get a new lip seal and start again. Good lighting, patience, and technique are key here, and a good air check.
Once you have the piston seated in the drum you can install the return springs and retainer.
You can buy or make a tool similar to the one pictured or use 2-3, 5" or larger C-clamps (much more of a PITA).
Forward drum inspection
Forward drum procedures are almost identical. No center seal to worry about on a TH350, it typically already has a 5 clutch pack, except light duty applications. Same 0.700" thickness piston for 5 frictions and steels. Removing the wave plate will cause harsh forward engagement. Reduce clearance of the stack to 0.040".
Forward clutch pack clearances can be tighter than usual; however it must have some clearance. If too tight it can creep in neutral.
Pay attention to this polished stub of the input shaft, it goes into the bushing that is in the end of the output shaft. It is a high wear area. Be sure it is smooth. Minor scoring can be smoothed down. May consider use of teflon bushing, only a lubrication passage, no pressure, no lateral loading.
Notice the scraped and split low reverse friction in the intermediate piston cavity. This slightly tightens the intermediate clutch clearance to achieve cleaner 1-2 shift.
Intermediate piston installed.
Sealing rings on the pump stator. Notice the missing sealing ring 2nd from bottom. You can remove this ring if dual feeding.
Pump gear install. Note the orientation of the tangs on the inner gear. They are offset. They should face away from the seal. Mis-installation will result in pump failure and converter damage.
Notice the dowel pins threaded into the case, you use these and the case to align the pump halves.
This is an example of how I air check the forward and direct drum. After everything is assembled, I place the drums on the pump (above a hole in the bench) with all sealing rings in place, thrust washers or bearings.
Use a rubber tipped air nozzle to apply air to the passages around the pump circumference. As you apply air to the correct passages, you will see and hear the drums apply the clutches. Be sure there are no leaks from the lip seals. There will usually be some minor leakage at the sealing rings. Iron rings air check better than teflon on the bench.
Drums installed in the transmission case. Ready for intermediate pressure plate install. Ensure the lugs of the direct drum are engaged with the sun shell. They should be slightly below the top edge of the sun shell. If you do not get complete engagement, the input shaft will not turn when you install the pump and begin to tighten it.
Brass screened filter allows for more flow. Avoid the cloth looking material filters for the T-350
Here's a pic of the feed holes. These regulate the amount of fluid, or how fast, it can apply the clutches. Drilling them bigger allows a faster apply. However, bigger is not always better.
Remember that as the load increases on the trans, so does the pressure. The increase in pressure will cause the fluid to move faster and shifts to be firmer. If you drill the holes too big the shifts become too harsh at light throttle openings. This causes undue wear on the internal transmission parts, rear differential and axles, drive line and u-joints.
The ideal situation is a shift that gets progressively harder with greater throttle openings. I recommend drilling the 2nd feed to 0.125" and the 3rd feed from 0.125"-0.140"
Also note the 2-3 accumulator hole marked in the upper portion. This hole can be blocked but it is easier to block the passage in the VB or the 2-3 accumulator piston as previously mentioned.
Photo of the T-350 governor and cover. Note the direction of the plastic drive gear teeth. If the plastic gear breaks or wears out, there will be no upshifts since the governor is not rotating.
Valve body gasket check
Always check the valve body gaskets against the separator plate to see if any holes in the plate are covered by the gasket. Place the gasket behind the plate and hold both up at light to check for covered holes.
This gasket on a T-350 sits against the valve body. Notice the open line in the gasket that will parallel the support plate that bolts on at front of the valve body.
Racing valve body
Coan reverse manual valve body. Notice the machining of passages. Please note the DIY cannot just mill the valve body as valves have also been changed in the valve body.
Fastener size and torque values
- Pump Cover to Pump Body- (5/16-18) 15 ft/lb
- Pump assembly to case- (5/16-18) 20 ft/lb
- Valve body and support plate- (5/16-18) 13 ft/lb
- Oil channel support plate to case- (5/16-18) 13 ft/lb
- Parking lock bracket- (5/16-18) 29 ft/lb
- Oil suction screen- 40 in/lb
- Oil pan to case- (5/16-18) 13 ft/lb
- Extension housing to case- (3/8-16) 35 ft/lb
- Modulator retainer to case- (5/16-18) 12 ft/lb
- Inner Selector Lever to Shaft- (M10-1.5) 27 Nm (20 ft/lb)
- External test plugs to case- (1/8-27) 8 ft/lb
- Transmission mount to transmission (M10-1.5) 48 Nm (35 ft/lb)
- Speedo sleeve retainer on extension housing- (M6.3-1.0) 17 Nm (150 in/lb)
- Detent cable to case- (M6.3-1.0) 8.5 Nm (75 in/lb)
- Nut on outer end of shift selector shaft- (M10-1.5) 27 Nm (20 ft/lb)
- Converter to Flexplate Bolts- 35 ft/lb
- Torque converter dust cover pan to transmission case- 110 in/lb
- Transmission Case to Engine- 35 ft/lb
Oil cooler line connectors to transmission case
- Straight Pipe Fitting-25 ft/lb
- Tapered Pipe Fitting- 15 ft/lb
- Oil Cooler Pipe to Connectors- 10 ft/lb
Other torque values
- Gearshift bracket to frame- 15 ft/lb
- Gearshift Shaft to Swivel- 20 ft/lb
- Manual Shaft to Bracket- 20 ft/lb
- Detent Cable to Transmission- 75 in/lb
- Intermediate Band Adjust Nut- 15 ft/lb
If the dipstick is missing or not correct, the fluid level should be from even with the pan to ¼” above the pan when the fluid is hot.
T-350 dimensions and tailshaft (extension housing) lengths
You will find the T-350 with 6", 9" and 12" tail housings. There is also a 4WD output (seen below).
Ratios of the T-350 were consistent throughout its production:
- First gear- 2.52:1
- Second gear- 1.52:1
- Third gear- 1.00:1
- Reverse- 1.93:1
An aftermarket low-gear planetary is available that will lower the first gear ratio to 2.75:1.
Valve body modifications
Removing check balls and opening up orifices in the separator plate is also a common modification to firm up shifts. Just remember the one check ball that needs to stay is the one just below the modulator.
Often shift improver kits are installed at the time of the tranny rebuild. Various manufacturers make shift improver kits, Trans Go has a good reputation.
Shift point changes
The following is from Dean Mason of TransLab Engineering:
Modulator adjustment is not about setting the shift timing where you want it, especially at max throttle. Modulator is for LINE PRESSURE BOOST, and here's how it works:
The modulator is a spring loaded vacuum canister. At high vacuum, the force of vacuum opposes/cancels out the spring force. As vacuum drops, the spring exerts an increasingly greater force against the modulator valve, until at near zero vacuum maximum line pressure boost is achieved.
Now, line pressure boost is needed when torque is increased, so that clutches and bands won't slip. But we don't need boost sitting in park or neutral idling in the parking lot. This just causes premature pump gear and sealing ring wear. Therefore, line pressure should not be boosted at idle. If you use your car as a daily driver, the best setting is that which provides maximum boost during acceleration without unnecessarily high line pressure when it is not needed. That is, NO BOOST AT IDLE, BUT AS SOON AS YOU BEGIN TO ACCELERATE (that is, you drop 1" of vacuum off idle) you want RAPID/RESPONSIVE BOOST.
Here's what you need to know: WHAT IS THE VACUUM READING AT IDLE IN DRIVE? The following work is most easily done on a lift with all 4 wheels off the ground and the emergency brake firmly locked!
We want the engine fully warmed up for this test, and take the reading at the tranny end of the vacuum line. Unplug the modulator hose from the vacuum modulator assembly. Attach a vacuum gauge to the hose. Start the engine, TURN ON THE AIR CONDITIONER, put the shifter in DRIVE, and write down the vacuum reading on the gauge.
Next, get yourself a pocket screwdriver and a new modulator "elbow" (keep the elbow in your pocket). INSTALL PRESSURE GAUGE ON MAIN LINE PORT. Plug the engine side of the vacuum line (so you won't have a vacuum leak). Get a hand operated vacuum pump and attach it to the modulator. Get your little screwdriver and punch it thru the rubber hose (yes that's right!) about 1/2" behind the modulator stem. This way you can adjust your modulator inside the hose with it attached and the engine running!
Now, what was the vacuum reading you wrote down? Let’s say your engine pulls 16 in/Hg in D, HOT, with AC on. We want to drop ONE in/Hg of vacuum, then start to increase line pressure. So, start the engine (neutral is OK for this part), raise idle to 800-1000 rpm, and pump up 15" on your vacuum pump (you may need someone to help maintain a steady 15" on the pump with the screwdriver punched thru the hose due to very minor leakage).
Typically a SBC 350 will run about 60-65 psi at idle, but there are variations in PR springs. If it is much higher, back the modulator adjustment out until there is no more "drop" on the gauge. Now, start to screw IN on the modulator adjuster until you see the pressure gauge start to lift and STOP right there. To check this, pump up 20", watch gauge and start to bleed vacuum off very slowly and the gauge should start to increase exactly at 15".
If you took your initial readings correctly, you now have a perfectly adjusted modulator. For a slightly steeper boost rate with higher max line pressure, use a 2" modulator instead of the 1 9/16" can size and adjust it exactly as described here. Once you have set it this way you are finished with the adjustment procedure.
Detent cable adjustment
Unclip the cable locking mechanism and then press the accelerator pedal to the floor. Be sure there's nothing under the pedal and the linkage is allowing full throttle at the carb. Floorboard the gas pedal and the detent cable should just pull tight as the throttle reaches wide open. THAT'S IT! If it doesn't pull tight, readjust, bend bracket, or whatever is necessary to achieve this. If it doesn't pull tight you will not have full detent at wide open throttle. If it pulls tight BEFORE full throttle, you'll break the cable end.
Now that the detent and modulator are set correctly, we are ready to think about shift scheduling. ROAD TEST THE CAR. You will need to know minimum and maximum throttle shift speeds for the 1-2 and 2-3 shifts, the RPM for the max throttle shifts, and the highest speed you can get a 3-1 and a 3-2 kick down. Also, what is the axle ratio.
From this point we work with governor weights, springs, and shift valve springs.