Turbo 350 (TH350) Information Thread

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NickTransmissions

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Hi Everyone,

Figured I'd start an all-encompassing TH350 Transmission thread, similar to my 700R4 thread, to serve as another repository for information about these transmissions. Covers servicing, rebuilding, parts selection and performance/heavy duty upgrades for your TH350. This should be helpful for those of us who own Square Body Pick-ups, Burbans and K5s with a TH350 installed. Please feel free to add your own tips, tricks and anything else that you found helpful or ask questions as you need to and I’ll respond as soon as possible.

If you would like parts added to the below 'Machining and Assembly Tips' and/or the ‘Parts List’ sections, feel free to post them in the thread and I’ll incorporate into the list(s). Meanwhile, I’ll update this first post with additional information and links as I go.

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A Few Rebuild Considerations
  • You DO NOT need to purchase and install any shift kits to obtain desired performance from these transmissions if the transmission is on the bench for overhaul
    • Kits are perfectly fine for when trans is in the vehicle, healthy and an overhaul is not necessary
    • Shift calibrations (timing/feel/firmness) can be adjusted via feed hole sizing in the spacer plate, clutch clearance, check ball omissions, 2-3 accumulator circuit deletion (HP only), 1-2 acc spring deletion (HP only) and governor mods
    • Dual-feed the direct clutch via the case for all high performance applications - see my TH350 build-up on a budget video or TH350 Rebuild PT1 below for instructions covering this procedure
  • Fluid - Use Dex III except perhaps in street/strip or full race where Type F may be more advantageous given it provides for a bit firmer shifts, all other things equal
  • Carefully inspect the crescent in the pump body - bodies w/worn crescents should be replaced
TH350C Teardown and Inspection (Everything covered here applies to the non-lock up TH350 as well, apart from the aspects that are unique to the 350C)
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TH350 Performance Rebuild - Step by Step
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TH350C - Converting to Accept a Non-Lock Up Torque Converter
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TH350 Rebuild - Parts Selection

PartLocationRationalePart No (if app)
Detent CableKick-down hook-up (case/valve body)Always install a new kick down cable - pre-existing cable is likely worn and may not hold adjustment upon overhaul, resulting in shifting issues
Screen-style filterValve BodyAlways install a screen style filter, never install the cloth variety as they tend to clog easily
Vacuum Modulator and modulator valve
- Inspection/Service
- Adjustment
- Precautions
Case - ExternalReplace modulator w/new, especially if unit has original modulator and it's the original black can style (keep it as a paper weight if in good shape)

Check valve for wear - if valve drags in bore coated w/trans fluid while removing with a pencil magnet, use 600-1000 grit to polish valve in a drill - dragging will cause part-throttle drivability issues

Adjustment and Impact to shift behavior:
- Clockwise: Slightly later shifts, slight increase in firmness
- Counterclockwise: Slightly earlier shifts, slight decrease in firmness

Three precautions

No upshifts / harsh or erratic shifting - Common cause is insufficient engine vacuum to modulator (among other things)

Transmission Fluid Smell in Exhaust + White Smoke - Modulator diaphragm has failed, allowing trans fluid to be sucked into the engine at the manifold and burned during combustion

Winter Weather / Freezing Conditions - Always check for water freezing in modulator; if not caught and water freezes in modulator or vacuum line, transmission line pressure will spike to north of 600 PSI and case will crack in the rear
Install new governor gear and calibration kit for high performanceGovernorReplacing the driven gear and installing a governor kit will ensure the governor is calibrated and functioning properly.

If governor valve doesn’t move on it’s own, remove gear and valve then polish bore - re-seat valve to confirm freedom of movement

*DO NOT interchange a 700R4 governor in place of the original as trans will not shift correctly
Sonnax Boost Valve kitValve BodyReplacing the worn factory boost valve with a Sonnax boost valve kit ensures line rise remains stable/sufficient, especially in high stall/high performance applications350-LB1
35754-01K
Aluminum 3rd gear accumulator pistonValve BodyAlways install an aluminum 2-3 accumulator piston in all TH350s and TH400s - plastic ones like to crack
1-2 Accumulator springCase (1-2 Acc bore)Always replace this spring on overhaul as it’s very common for them to break, resulting in a harsh, late 1-2 shift

Race/Street Strip - leave 1-2 acc spring out for very high stall (4500+ RPM or so) applications if you cannot get 1-2 shift sufficiently crisp w/all other mods/adjustments
TCC Lock-up Solenoid (TH350C only)Aux Valve BodyInstall a new lock up solenoid on overhaul
Pump - GearsPump BodyInstall new gear set if existing set is worn - check clearance
Thicker gear sets were introduced in 1978 in TH350 and TH350C; they are appx .110" wider than previous years’ gears
BushingsInternals / Case / Ext HousingAlways replace all bushings in the transmission as the TH350 is hard on bushings

Use a small blind hole puller to remove the pilot bushing if your output shaft has one installed.
- Some have a plastic seal at the front while other models have nothing at this location as there’s a teflon seal on the pilot end of the input shaft for those units

High Performance - pump stator to input shaft clearance check - max clearance is .0008 per Ron Sessions
Various
Extra clutch/steelForward DrumFor high performance, machine the piston to allow for the installation of an additional flat steel and friction disc
Extra clutch/steelDirect DrumFor high performance, machine the piston to allow for the installation of an additional flat steel and friction disc
High Rate Return SpringsIntermediate & Direct DrumHigh performance/high RPM applications require stiffer springs in the intermediate and direct drums to mitigate the risk of centrifugal apply while in 1st and/or 2nd gear
Direct Clutch Drum bushing (Wide)Direct DrumOriginal factory bushing was too narrow, causing the drum to **** sideways on the stator. Steel sleeve was introduced on stator starting in 1975 and ‘ready rest’ wide sealing ring in 1977 Buick TH350s to help mitigate this problem but wider bushing is essential to prevent cocking entirelyVarious/readily available on-line and via local trans parts suppliers
Sonnax HD Intermediate Outer Sprag RaceDirect DrumNever reuse the factory intermediate outer sprag race. Factory races were overhardened and can fracture in mild applications let alone high performance/heavy duty
Clutch moduleCase/DrumsI suggest high energy frictions for all rebuilds, including mild street/strip and more specialized clutch discs for full race w/high horsepower
Steel moduleCase/DrumsFactory steel module is sufficient for most applications; Kolene-coated or lindered steels are a nice option for high performance or heavy duty useVarious
Intermediate BandCase - Direct DrumUse a kevlar-lined band w/weld-reinforced band anchor points for anything high performance
Sun Gear Reaction Shell aka Drive ShellCaseConsider using a 'Vega' sun shell (aka 250C) for weight reduction and better fluid circulation in the case - works nice for high performance
Center Support / Case SaverCaseInstall a 88+ 700R4 or 4L60E center support assembly for added strength - it’s ‘drop-in’ meaning no other mods or prep has to be done to accommodate this change
  • Purchase a new Borg Warner low roller clutch and install it into the center support prior to installing the center support into the case
  • Install a case saver instead of the ‘anti-clunk’ spring for anything high performance
Low Reverse SteelsCaseUse 97+ 4L60E tubular, vented steels in the low reverse for high performance or heavy duty - double check to ensure steel plate thickness matches your original TH350 steelsVarious


Machining, Reconditioning and Assembly Tips

ProcedureRationale
Pump Working Surfaces (Body and Cover)Machine the pump cover flat to restore it’s working surface to factory new; remove .010” of material

Machine pump body deck by .010 to restore surface to perfectly flat; machine rotor pocket on vertical mill to achieve gears-to-deck surface clearance

Install new gears if existing set is worn and measured clearance is outside of spec (many of them are worn and not in spec)
Pump - Selective washer to Torrington Thrust Bearing ConversionAll pumps from 1969-1974 came with a babbitt style selective thrust washer for the pump-direct drum face thrust surface however it’s more desirable to have a bearing

You must have the pump cover machined to accept the bearing and/or shim to ensure your end play is within spec upon reassembly
Take the pump, pre-existing thrust washer and bearing w/shim to your machinist and have him machine the pump cover, removing the appropriate amount of material to maintain proper end play.

Additional shims can be added if end play is too loose.
Torrington Bearing inspectionReplace bearings if they make noise, feel ‘sandy’ or otherwise appear damaged - otherwise they should be fine to reuse
Intermediate Clutch - Piston MachiningSuper high performance/heavy duty - Machine piston and backing plate to allow for the installation of one additional clutch disc and flat steel
Intermediate Clutch - Clearance adjustmentSand off/remove all friction material off of an old low-reverse clutch disc and either break in half or file off inner teeth - install modified clutch disc under piston to tighten up intermediate clutch clearance if you have too much

Machine piston and/or backing plate if clearance is too tight

Watch my TH350 Setting Up the Intermediate Clutch for more information
Forward Drum - Piston MachiningFor high performance, machine the piston to allow for the installation of an additional flat steel and friction disc - omit any cushion plate if installed

DO NOT over-machine such that the piston surface falls below the bottom groove in the drum - if the bottom steel gets caught in that groove, you will have no forward movement
Forward Drum - Lube ports in hubEnlarge the lube ports to promote more efficient fluid circulation between forward and direct drums
Direct Drum - Piston MachiningMachine direct drum piston to allow for the addition of one additional steel plate and one additional clutch plate.

DO NOT over-machine such that the piston surface falls below the bottom groove in the drum - if the bottom steel gets caught in that groove, you will have no third gear
Check BallsLeave out all check balls except for the modulator check ball (that must stay in for proper line pressure / line rise) for firmer shifts when running a high stall converter - pair this with the one below for optimal shift feel/calibration (TH350C - keep check ball #5 in place for the converter clutch)
Separator PlateDrill separator plate hole sizes to for 1-2 feed and 2-3 feed for crisper upshifts from 1st gear to 2nd gear and from 2nd gear to 3rd gear per application and preference

Increase above hole sizes incrementally based on final drive ratio, vehicle weight, overall tire diameter and increase in converter stall over stock rpm stall speed (1800-2000 RPMs for most V8 applications)
1-2 Accumulator Spring DeleteLeave this spring out on very high stall applications for a sufficiently firm 1-2 shift if spacer plate hole sizing, check ball omission and use of Type F fluid fails to produce sufficiently crisp 1-2 shifts

DO NOT delete this accumulator circuit all together as it will cause a mis-time apply condition w/the intermediate sprag and potentially destroy the sprag / sprag race and direct drum
2-3 Accumulator DeleteBlock off the 2-3 accumulator feed orifice at the bottom of accumulator piston pocket via a 3/8 cup plug for high stall applications

*Install a light blue 4L60E stock forth accumulator return spring in place of the original TH350's spring to keep some tension on the accumulator piston, preventing the small c-clip from breaking or dislodging

* tip courtesy of Richard Crich @ Precision Transmission, Amarillo, TX
Front Planet - Torrington BearingHigh performance - replace any front planets that come w/ babbitt style thrust washers with planets that are set up for a Torrington bearing - both planet and bearing must be swapped (you cannot install a bearing onto a planet designed to accept a thrust washer)
Rear Ring Gear / 4L60E 4 or 5 pinion rear planetYou can modify a 4L60E rear ring gear to install into a TH350. This will allow you to install a 700R4/4L60E four pinion planet or 4L65/4L70 5-pinion rear planet for added strength in HP or HD applications
Low Reverse Clutch Pack - Race Only ModsInstall flat, tubular (97+ 4L60E) steels in place of three friction discs for race-only applications where the vehicle is never driven on the street - this reduces drag when going down the track (do this in 700R4s and 4L60Es as well for track-only use)

Reverse will still work but you do not want to use it nearly as frequently as a street-driven vehicle
Gear CommandBlock off manual-low control valve in the valve body in-board to prevent its movement with a large checkball/ball bearing or slug/spacer to bind the spring (or use a larger spacer in place of the spring)

-This enables back-shift to first gear at any speed (recommended for dedicated off road racing/trail riding, bouldering or other non-traditional applications)

*DO NOT perform this mod unless you know exactly what you’re doing as back-shifting to first at high speeds can destroy the transmission and potentially cause an accident on the road.
TH350C Conversion to non-Lock upTwo approaches: Book Method and Hasty Method - Watch my TH350C Conversion to Non-Lock up video for details/how-to


TH350 and TH350C Check Ball Location and Descriptions
* High Performance TH350: Leave out all check balls except the modulator check ball (#3)
** High Performance TH350C: Leave out all check balls except the modulator check ball (#3) and converter clutch lock up check ball (#5)
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Source: ATSG check ball Book, Vol 1

References
- TH350 Automatic Transmission Service Group Manual
- Turbo Hydra-matic 350 Handbook, Ron Sessions
- GM Turbo 350s: How to Rebuild and Modify, Cliff Ruggles
 

Fat 454

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Hi Nick, just had an initial read through of your post. Thank you for taking ( considerable ) time in putting this together for the forum. Hopefully it will become a "sticky" and will help out a lot of other members. I came across this as we are looking at potentially getting ( another !! ) crewcab for our "fleet" for my son to use for work. It apparently has a 454/ th350 combo with no reverse, although we have not looked at it yet. I have good experience with manual boxes ( SM465, M20 / 21 etc. ) however not familiar with slush-boxes, so was looking up info on "no reverse - th530" and came across your post. We have a couple of 350 boxes in need of rebuild. I have the Ruggles book on re-building autos, so we can probably give you some meaningful feedback on your thread once we dive in. Thanks again.
 

NickTransmissions

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Hi Nick, just had an initial read through of your post. Thank you for taking ( considerable ) time in putting this together for the forum. Hopefully it will become a "sticky" and will help out a lot of other members. I came across this as we are looking at potentially getting ( another !! ) crewcab for our "fleet" for my son to use for work. It apparently has a 454/ th350 combo with no reverse, although we have not looked at it yet. I have good experience with manual boxes ( SM465, M20 / 21 etc. ) however not familiar with slush-boxes, so was looking up info on "no reverse - th530" and came across your post. We have a couple of 350 boxes in need of rebuild. I have the Ruggles book on re-building autos, so we can probably give you some meaningful feedback on your thread once we dive in. Thanks again.
You're welcome, @Fat 454 !
 

77 k 20

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Hello all I have a 77 k20 turbo 350 that's giving me fits. It blows the dipstick up out of the tube about 1-2 inches after driving it even a little distance to town and back about 15 miles. Just running in shop never an issue. I just changed fluid and filter thinking that might be a problem as well as trying to clean the vent on the left side above shifter. Any ideas?
 

AuroraGirl

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Part No (if app)
350-LB1
35754-01K
Various
Direct Clutch Drum bushing (Wide)Direct DrumOriginal factory bushing was too narrow, causing the drum to **** sideways on the stator. Steel sleeve was introduced on stator starting in 1975 and ‘ready rest’ wide sealing ring in 1977 Buick TH350s to help mitigate this problem but wider bushing is essential to prevent cocking entirelyVarious/readily available on-line and via local trans parts suppliers
Sonnax HD Intermediate Outer Sprag RaceDirect Dr
So youre saying im lucky?
 

AuroraGirl

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Damn....that must have hurt, lol.
Probably, it was sittting on the ground near where my dads th350/np203 is on his 1975 square
The trans had a hole in the case and tha was on the ground. I think @HotRodPC told me what part it was before but i forgot
 

HotRodPC

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Probably, it was sittting on the ground near where my dads th350/np203 is on his 1975 square
The trans had a hole in the case and tha was on the ground. I think @HotRodPC told me what part it was before but i forgot
It's a chunk out of the case where the center support is splined into the case with a snap ring holding it in place. You can see the wide splines and snap ring groove. Makes it quite obvious where it came from. I'd imagine some exploded planets or sun shell to cause that damage right there.
 

AuroraGirl

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It's a chunk out of the case where the center support is splined into the case with a snap ring holding it in place. You can see the wide splines and snap ring groove. Makes it quite obvious where it came from. I'd imagine some exploded planets or sun shell to cause that damage right there.
do you think a failed modulator could have caused that? Because I was with my dad in the 75 when he last used it and it started to buck really hard and a friend of his who tried to fix it later(who managed to get the spark plug order WRONG ON SO MANY LEVELS. despite being cast on the intake.....) disconnected the intake source for the modulator (so the time it spent idling where it sits , running pig roughm, im just imagining indsane amounts of lkine pressure and nothing to do with it
 

HotRodPC

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do you think a failed modulator could have caused that? Because I was with my dad in the 75 when he last used it and it started to buck really hard and a friend of his who tried to fix it later(who managed to get the spark plug order WRONG ON SO MANY LEVELS. despite being cast on the intake.....) disconnected the intake source for the modulator (so the time it spent idling where it sits , running pig roughm, im just imagining indsane amounts of lkine pressure and nothing to do with it
No, A modulator didn't cause that. That's a chain reaction catastrophic failure and very uncommon. I'm thinking it may have started if someone forgot the anti clunk spring and over time that center support being rocked back and forth in the case took it's toll, or possibly that snap ring failed.
 

AuroraGirl

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No, A modulator didn't cause that. That's a chain reaction catastrophic failure and very uncommon. I'm thinking it may have started if someone forgot the anti clunk spring and over time that center support being rocked back and forth in the case took it's toll, or possibly that snap ring failed.
gotcha, thank you. d oyou think issues with the np203 and its full time may have done something. Like the differential not modulating front/rear speeds proper or being used in HI-LOC on pavement adn then that sends a lot of force back up the transmission from the road? Just curious
 

HotRodPC

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gotcha, thank you. d oyou think issues with the np203 and its full time may have done something. Like the differential not modulating front/rear speeds proper or being used in HI-LOC on pavement adn then that sends a lot of force back up the transmission from the road? Just curious
Well yes, that is certainly possibly too. It could have been bound up and caused the rear planets to explode. Nonetheless, there was a catastrphic failure somewhere in the back of the case behind the center support.
 

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