GM Turbo 350 / 350C Identification & Tech Info
The Turbo-Hydramatic 350 (TH350, THM350) is GM’s medium-duty three-speed automatic. It came out for the 1969 model year and was used in cars and light trucks through 1986. The 350C is the lock-up converter version, built from about 1980 to 1986 (some parts listings start at 1979 or run to 1988). Use this page to confirm whether you have a 350 or a 350C and to find the parts we stock for each.
TH350 at a glance
| Item | TH350 / 350C |
|---|---|
| Type | Three-speed automatic, longitudinal (rear-wheel and four-wheel drive) |
| Model years | TH350: 1969 to 1986. TH350C (lock-up): about 1980 to 1986; some parts listings start at 1979 or run to 1988. |
| Replaced by | 700R4 (four-speed overdrive) |
| Gear ratios | 1st 2.52, 2nd 1.52, 3rd 1.00, reverse 2.07 |
| Case length | 21-3/4 in., measured without the tail housing |
| Tail housing | Commonly 6 in. on cars and 9 in. on trucks; four-wheel-drive units use a short output shaft and an adapter instead |
Name check. GM’s later “3L30” name belongs to the Turbo 180, not the Turbo 350. The TH250 is a Turbo 350 derivative that uses a band with an external adjuster in place of the intermediate clutch pack, so many, but not all, parts are shared.
How to identify a TH350
- Oil pan. 13 bolts, nearly square with one corner cut off (five sides). A Turbo 400 pan also has 13 bolts but has an irregular, many-sided shape.
- Kickdown. A kickdown (detent) cable runs from the throttle linkage to the right side of the case. A Turbo 400 has no cable; it kicks down electrically.
- Modulator. The vacuum modulator is on the right (passenger) side of the case.
- Length. The case is 21-3/4 in. without the tail housing. A Turbo 400 case is 24-3/8 in.
- 350C. A 350C has an electrical connector on the driver’s side of the case for the lock-up circuit. A plain 350 has no connector there.
350 vs 350C: the lock-up differences
The 350C locks the torque converter in third gear at cruising speed. Lock-up is switched electrically through the case connector and releases when you brake. The gear train is the same as a 350, but the converter, input shaft and drum, pump and valve-body parts for the lock-up circuit are different. A non-lock-up converter will not simply drop into a 350C. Order by the version you have:
| Part | TH350 (non-lock-up) | TH350C (lock-up) |
|---|---|---|
| Torque converter | Non-lock-up converter (GM350) | Lock-up converter, 1600–1800 stall (GM16) |
| Input drum with shaft | Non-lock-up, 1969–1979 (1604B1) | Lock-up, 1979–1984 (1604B2) |
| Valve body separator gaskets | Without lock-up, 1969–1980 (1276-Kit) | With lock-up, 1980–1986 (1277-Kit) |
| Sealing rings | Sealing ring kit (44175BT) | 350C sealing ring kit, 1980–1986 (K35901E) |
| Oil filter | Felt filter (1279A) | 350C nylon filter, 1980–1988 (1279B) |
| TransGo reprogramming kit | 350-1&2 (does not fit lock-up units) | 350C-1&2 (350C only) |
Common wear points
- Intermediate sprag outer race. The intermediate overrun (sprag) clutch and its outer race are the weak link, especially with firm 1-2 shifts or extra power. Sonnax HD outer race (35723HD-02) · upgraded HD outer race (A44663HD) · intermediate sprag (A44652).
- Bushings. The TH350 is hard on bushings, the direct drum bushing in particular. Replace them all at rebuild: master bushing kit, 1969–1986 (1388).
- Case lugs at the center support. If the case lugs that hold the center support and low/reverse clutch are damaged, a case saver repairs them: case saver, 1-piece (A44761B) · Adapt-A-Case center support case saver (A74761DAC).
- 1-2 accumulator and servo bores. Sonnax notes that worn 1-2 accumulator and intermediate servo bores cause slipping 1-2 shifts. Sonnax 1-2 accumulator sleeve kit (35719-01K) · intermediate servo sleeve kit (35709-01K).
TH350 parts we stock
Rebuild kits
- TH250 / TH350 gasket and seal overhaul kit (1280A)
- Turbo 350 custom performance master rebuild kit (SMTH350)
- Turbo 350 basic rebuild kit (BKTH350)
- TH350 / TH250 thrust washer kit (1107)
- TH350 / TH250 Torrington bearing kit (44201A)
Clutches, bands and sprags
- Alto High-Energy friction module (AL032752B)
- Red Eagle high-performance friction module (AL032752HP)
- Forward / direct friction plate (AL032714)
- Intermediate band (1333)
- Wide low/reverse roller clutch, BorgWarner (1030A) · standard low/reverse roller (1030B)
Pump, filter and pan
- Pump repair seal kit, 1969–1986 (PK350-KIT)
- Pump body bushing (1388B) · Sonnax HD pump bushing (34034T-01)
- Pump gear set (A44530AC) · pump washer shim kit (44200F)
- Oil pan gasket, Farpak (1335A) · cork (1335) · filter and cork gasket kit (1279)
Shift kits, governor and modulator
- TransGo SHIFT KIT (SK350) · Superior shift correction kit (STP-K350)
- Superior governor shift point package (STP-K018)
- Governor gear kit (A44684) · governor screen (44014)
- Modulator, red stripe (1450) · TH350C modulator (84160R)
Browse by part type: rebuild kits · frictions · steels · bands · bushings · pumps · hard parts · shift kits · torque converters.
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