The TH350 is the small-block partner to the TH400. Lighter, smaller, less torque capacity, but plenty for the millions of GM cars and trucks that used it from 1969 to 1986. If you own a classic GM with a small block, you probably have a TH350. If you're building one for a hot rod, you're probably building a TH350.
Here's the full guide.
TH350 basics
- 3-speed automatic, no overdrive
- Hydraulic control (no electronics)
- Years: 1969-1986 production
- Torque capacity: stock about 350 ft-lb, built versions 600+
- Weight: about 120 lbs
- Length: about 27 inches
- Common applications: small-block Chevy, small-block Buick, small-block Olds, small-block Pontiac, GMC trucks
Where the TH350 went
The TH350 went into almost every GM rear-wheel-drive car and light truck with a small block from 1969 through 1986. Including:
- Camaro (1969-1981)
- Chevelle / Malibu / El Camino (1969-1977)
- Nova (1969-1979)
- Impala / Caprice (1969-1986)
- Monte Carlo (1970-1984)
- Firebird / Trans Am (small block)
- Buick Regal, GS, Skylark (small block)
- Olds Cutlass, 88, 98 (small block)
- Pontiac Bonneville, LeMans (small block)
- C/K pickups (small block, 1/2 ton)
- Blazer, Jimmy, Suburban (1/2 ton)
- GMC vans
TH350 vs TH400 - the key differences
People mix these up constantly. Quick reference:
| Spec | TH350 | TH400 |
|---|---|---|
| Length | 27 inches | 27.5 inches |
| Weight | 120 lbs | 135 lbs |
| Torque capacity (stock) | 350 ft-lb | 450 ft-lb |
| Bell housing | small block only | small or big block |
| Tail housing | shorter | longer |
| Pan shape | square | rectangular, offset |
| Pan bolts | 13 | 13 |
| Trans cooler ports | left side | left side |
The pan is similar but not identical. The bell housing pattern is the giveaway — TH350 only fits small block.
Common TH350 failures
1. Direct clutch wear (most common)
The direct clutch carries the work in 3rd gear. Wears out around 100-150K miles in stock applications, much faster in performance use.Symptoms: slipping in 3rd, no 3rd gear
Fix: direct clutch pack replacement during rebuild
2. Forward clutch wear
Similar to direct clutch, wears at similar mileage.Symptoms: slipping in 1st and 2nd, eventual no-forward
Fix: forward clutch pack replacement
3. Intermediate band wear/break
The TH350 uses a band for 2nd gear. Band wears, eventually breaks.Symptoms: no 2nd gear, slamming 1-2 shift, or harsh engagement
Fix: intermediate band replacement, often paired with band servo update
4. Front pump bushing wear
Front pump bushing wears, causes pump seal leak and low line pressure.Symptoms: front seal leak, slow engagement, possible low pressure across all gears
Fix: pump bushing replacement during rebuild
5. Vacuum modulator failure
TH350 uses a vacuum modulator to control line pressure based on engine load.Symptoms: harsh or soft shifts, shifts change with throttle position incorrectly
Fix: $20-60 modulator replacement, often fixes shift complaints
6. Kickdown cable issues
Stretched, broken, or misadjusted kickdown cable causes no passing-gear kickdown.Fix: $30-80 cable replacement and adjustment
Rebuild kit recommendations
Daily driver TH350 (stock or mild)
- Master rebuild kit with Alto frictions - All steels - Pump bushing - All seals and lip seals - Vacuum modulator - Filter and pan gasket - Cost: $250-450 in partsThe TH350 rebuild is one of the cheapest auto trans rebuilds you can do — simple design, few parts, lots of aftermarket support.
Street/strip TH350 (350-500 HP)
- Master rebuild kit - Raybestos Stage-1 frictions in direct clutch - TransGo HD2 shift kit - B&M or TCI performance servo - Performance torque converter (2,400-3,000 stall) - Cost: $500-900 in partsStrong street TH350 (500-650 HP)
- Full clutch upgrade (Alto Power Pack) - Sonnax billet input drum - Hardened input shaft - Manual valve body OR reverse-pattern street/strip valve body - 3,000-3,500 stall converter - HD pump - Cost: $1,000-1,800 in partsDrag race TH350 (650+ HP)
- All HD parts above - Billet drums for forward and direct - Reverse pattern manual valve body - Transbrake - Custom 4,000+ stall converter - Cost: $2,000-3,500 in partsWhat's NOT in the TH350 that's in modern transmissions
No overdrive
TH350 is a 3-speed. Final drive is 1:1 direct. For highway cruising, gearing must be done in the rear axle (3.08 or numerically lower for daily use).No lockup torque converter
TH350 always runs through the converter. No mechanical lockup at cruise. Means more fluid heat and slightly worse fuel economy than overdrive autos. Acceptable trade-off in classic applications.No electronics
Everything is hydraulic. The TH350 doesn't care what year your car is, what ECM you have, what TCM software is loaded. Drop it in, plumb it up, drive.TH350 to 700R4 swap (very common in classics)
Adding overdrive to a classic by swapping TH350 → 700R4 is one of the most popular drivetrain upgrades for hot rodders. Key considerations:
- Same bell housing pattern — drops right in for most small-block applications
- Slightly different length — driveshaft may need minor adjustment
- Crossmember — different mount location, requires modification or replacement
- TV cable — 700R4 has a Throttle Valve cable that MUST be set correctly or the transmission burns up
- Lockup wiring — 700R4 has lockup torque converter that needs 12V signal at OD lock
- Cost: $1,500-3,000 for a quality rebuilt 700R4 + swap parts
For long highway driving, the overdrive swap improves fuel economy by 15-25% and drops engine RPM at cruise by 25-35%.
Fluid
Dexron III (original spec). Dexron VI works fine as substitute.
Capacity: about 10-11 quarts total, 4-5 quarts in pan.
Service interval: 50,000 miles for normal use, 30,000 for performance/towing.
What makes the TH350 great for builds
1. Cheap cores — $50-200 for a junkyard pull
2. Cheap parts — full rebuild for $300-500 in stock parts
3. Strong enough for most street builds — handles 500 HP small block daily
4. Lots of aftermarket support — Sonnax, B&M, TCI, TransGo all make parts
5. Simple to work on — first-time rebuild is doable in a weekend
6. Light — saves 80+ lbs vs 4L80E in performance applications
When NOT to use a TH350
- Big block applications — torque capacity isn't there. Use TH400.
- Heavy towing — same reason. TH400 or 4L80E for tow rigs.
- Modern fuel injection requiring electronic trans — TH350 has no electronics. Need 4L60E or 4L80E.
- Diesel applications — too much torque, wrong duty cycle.
- Highway daily driver in stock car — no overdrive. Either accept high cruise RPM or swap to 700R4/4L60E.
TH350-C (lockup variant, 1979-1984)
A subset of TH350 production got a lockup torque converter for fuel economy. Identified by:
- Electrical connector on the case (for lockup solenoid)
- 1979-1984 production
- Most applications: full-size GM cars and light trucks
If you're rebuilding a TH350-C, the rebuild kit and procedure is similar but you need the lockup-specific solenoid and the lockup torque converter. Slightly different valve body too.
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Need TH350 rebuild kits or performance parts? Shop our TH350 catalog. Master kits, Alto frictions, performance servos, manual valve bodies, shift kits, converters. Free shipping over $70. Same-day ship in-stock.
Related guides:
- TH400 complete guide
- 4L60E vs 4L80E comparison
- How to identify your GM transmission