TH350 rebuild tech
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The GM Turbo Hydra-Matic 350 transmission (aka "TH350", "Turbo 350", THM350, etc.) was used between 1968-'86 in RWD and 4WD GM domestic vehicles. The TH350 was still produced many years beyond 1986 for export sales. 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, Turbo Hydra-Matic 350 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 It has good pictures, a lot of good info, but has some outdated info.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 tutorial. That said, the following should help give a better understanding of the TH350 and possibly help when a question arises.
- The following page was inspired by an original Hotrodders forum thread by jakeshoe, titled TH350 Rebuild Tech.
 Variations of the TH350 family
There is also a TH350C and TH250C version that has a lockup torque converter (TCC). Look for the end of the input shaft to have a smooth polished end with O-ring on it, similar in appearance to the 200C/2004R and 700R4/pre-LSx 4L60E. 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. Same also holds true with the oil pump - "C" is cast into the pump cover and pump half.
Don't confuse the TH350C 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.
Hughes, TCI and Boss Hog make TH350C torque converters.
 Rebuilding the TH350
 Bushings and thrust washers
A TH350 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 TH350. Might as well order a bushing kit, as well as a thrust washer kit. It is also recommended to get two additional sun gear bushings individually.
In the photo is shown the two 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.
 Case prep
Once the case has been cleaned, all threads checked and/or repaired, and the rear case bushing installed you can start assembly.
 1-2 accumulator
While it may be true that removing the accumulator spring from the 1-2 servo will firm up the 1-2 shift, leaving the 1-2 accumulator spring out is not recommended. One reason is the accumulator protects the failure-prone intermediate sprag and another is the spring keeps the cover loaded against the snap ring. Photo shows accumulator cover, spring and piston (snap ring not shown). This is located on right side of transmission case. Check the bore in the case for scratches or other damage if the spring was found broken.
Right side of TH350 case. Pressure ports are 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
 Transmission reassembly
 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 bellhousing 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.Check planet pinion gears for side to side movement. Clearance of 0.030" to 0.050" is common. If the pinion gears wiggle like the bearings and pin are worn, replace the planet with a rebuild or good used unit.
Now you can install the low/reverse clutches, beginning with a steel, then a friction, alternating, ending with a friction.
Note: 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.
There are Alto red frictions in this unit, however a stock tan clutch ( like Borg Warner brand) 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). Hold in place with assembly gel or vaseline and then install the low-reverse support.
Be sure the low sprag inner race is not brinelled or has any unusual wear. A center support from a 1987-present 700R4 or 4L60E (including the aftermarket case saver with a bolt-on outer plate held with torx head bolts) can be used in place of the stock center support since the roller clutch is wider.
The roller clutch must installed correctly with the edge of the roller clutch tangs match the inner receiving tangs of the center support. Install snap ring. There is a snap ring on both sides of the roller clutch.
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 below), and then the sun shell and gear assembly.
Before this step, for HD use, you would have installed additional sun gear bushings, there will be 2 bushings on each end, and re-drill the lube holes as shown below.
 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 torrington bearing installed. The design of the OE planet determines bearing or thrust washer use. A bearing will not fit a thrust washer design planetary unit with out machine work.
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.
Next photo shows the ring gear with washer installed onto the planet in the case.
 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", machining the piston to 0.685" to 0.720" will allow the use of 5 frictions using 0.070" steels. 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 for most auto transmission clutch pack clearance is 0.010" clearance per friction. So a 5 friction direct setup would work well with 0.050" to 0.070" clearance. It will work with less, but no less than 0.040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, failed 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.
Heavy duty applications should also get a heat treated intermediate sprag race. The heat treatment makes the race less brittle. A heat treated intermediate sprag race is darker colored than a stock part. The intermediate sprag race and sprag (actually a 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 TH350. This is not to say a TH350 is not tough; they can reliably handle power in the 600 HP or lb/ft range if properly assembled using the right parts.
Several companies offer an even heavier duty direct drum/sprag assembly. It uses a 36 element sprag (similar to what is used in the TH400) and a machined drum; it will withstand more torque than a stock unit or one with just a hardened race. These units can cost $300.00 or more.
It is recommended to always use a new intermediate roller clutch for HD builds. The 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 acceptable.
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.
Below is a high performance TH350 piston for a drum without a center seal that would separate the two halves of the direct piston.
 Dual feeding
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.
Internally dual feeding is preferred because it leaves out the additional gasket and eliminates that one extra possibility for a pressure loss resulting in burnt 3rd gear clutches.
All transbrake applications will require dual feed of the direct clutch, possibly as well as manual valve bodies.
Shown below is the passage that must be blocked. One method is to tap this hole with a 3/8" tap, and cut off the end of a 3/8" bolt. The head of the bolt can be slotted so a screwdrive can be used.
Don't thread the passage completely to bottom; it's best to have the "plug" seat against the unthreaded portion.
This passage may also be plugged by using a roller from a direct drum or sprag to drive into the passage, but this will be a more or less permanent modification due to the difficulty in removing the roller.
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.
Almost all aftermarket 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, so plugging this hole is unnecessary. Also most manual valve bodies, trans-brakes, etc. will call for omitting the direct drum center seal and the sealing ring on the stator, but plugging this hole is also unnecessary due to the design of the separator plate.
Even firmer shifts can be accomplished with check ball removal and modifications to the accumulators. It is recommend to NOT modify 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 to 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 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. 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 be sure to verify using an 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 below, or use several 5" or larger C-clamps.
 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.
 Pump assembly
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. Remove the second ring from the bottom 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. Incorrect 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 to 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. You can also see the braking band installed.
 Braking band
Install coasting band aka braking band. This band provides engine braking in manual 2nd gear.This slides in the case around the direct drum. Check band for wear at the struts. The servo pin can wear a hole through the strut.
There are HD versions of this braking band. These HD bands should have welded reinforced band struts as show in photo.
 Intermediate clutch
6 cylinder applications use an intermediate piston measuring 1.175"-1.185" thick. The 6 cylinder uses two frictions.
The V8 intermediate uses three frictions. The piston is about 0.990" thick.
The wavy cushion plate should be used.
There are two types of intermediate return spring/retainers. With the early type you had to manually place the loose springs onto the piston's spring locator bosses,then the retainer went over that. The later retainer has the springs attached to their respective retainers (the bosses are not used). So if the intermediate piston is smooth, use the retainer with the springs attached to it. The late retainers will work in a transmission that originally used the early type retainer, but not vice versa.
 Intermediate clutch clearance
After the pump is set into place to test end play clearance, you can also check the clutch clearance on the intermediate clutch pack. Clearance of 0.050" to 0.070" will work well. There are thicker intermediate steel plates available if need to tighten up the clutch pack clearance. Intermediate steels are available (see page 40) in 0.068", 0.078" and 0.089" thicknesses, some manufacturers may use slightly different thicknesses than these.
Also, a scraped low-reverse friction can be used under the intermediate piston if need be. A scraped friction will measure about 0.032".
 Valve body
 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 TH350 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.
 Valve body modifications
Removing check balls and opening up orifices in the separator plate is a common modification to firm up shifts. Just remember the one check ball that needs to stay is the one just below the modulator.
 Separator plate
Below is a picture 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 firmer as the throttle opening increases. Drilling the 2nd feed hole to 0.125" and the 3rd feed hole to 0.125"-0.140" works well for many applications.
Also note above 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 valve body or the 2-3 accumulator piston as previously mentioned.
 Shift kits
Often shift improver kits are installed at the time of the tranny rebuild. Various manufacturers make shift improver kits, TransGo has a good reputation.
 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.
Image below of an OEM TH350 governor and cover.
 Shift point changes
 Governor adjustment
Shift point changes made via the governor weights and springs. Stronger springs and heavier weights will bring earlier shift points. Lighter springs and weights have the opposite affect.
The springs have a greater affect on shift points at light throttle. After about 5/8 throttle the weights have more affect on shift points. Gear ratio and tire size will also play apart of the shift points and changes made.
Brass screened filter allows for more flow. Avoid the cloth looking material filters for the TH350
 Vacuum modulator
The vacuum modulator senses vacuum in the intake manifold as an indicator of engine load. As the throttle is opened, vacuum decreases. The modulator on the transmission will raise the main line pressure in the transmission
 Fastener size and torque values
- Pump cover to pump body- (5/16-18) 15 ft/lb
- Pump assembly to case- (5/16-18) 15 ft/lb
- Valve body and support plate- (5/16-18) 10 ft/lb
- Oil channel support plate to case- (5/16-18) 10 ft/lb
- Parking lock bracket- (5/16-18) 20 ft/lb
- Oil suction screen- 40 in/lb
- Oil pan to case- (5/16-18) 10-12 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)
- Speedometer 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- 20 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
- Intermediate Band Adjust Nut- 15 ft/lb
 TH350 dimensions and tailshaft housing (or extension housing) lengths
You will find the TH350 with 6", 9" and 12" tail housings.
There is also two 4WD output shaft lengths, the shortest and more common is seen below. A longer 4x4 output shaft was used for around 1.5 years. It is a 2WD shaft of 6 inch length that is cut off about 1.250 inches. Used on 208 transfer case with adapter.
 Gear ratios
Ratios of the TH350 were consistent throughout its production:
- First gear- 2.52:1
- Second gear- 1.52:1
- Third gear- 1.00:1
- Reverse- 1.93:1
 Aftermarket parts
- An aftermarket low-gear planetary is available that will lower the first gear ratio to 2.75:1.
 Reference material