The TH400 (Turbo-Hydramatic 400) is one of the most legendary American automatic transmissions. Built from 1965-1990, it's bulletproof, simple, and the go-to choice for serious horsepower applications. Here's everything you need to know.
TH400 basics
- 3-speed automatic (no overdrive)
- Hydraulic control (no electronics)
- Years: 1965-1990 production
- Torque capacity: ~450 ft-lb stock, 1,000+ ft-lb when built
- Weight: ~135 lbs
- Length: 24.5"
- Applications: Chevy/GMC trucks, Buick, Cadillac, Olds, Pontiac, AMC, Jeep, plus many race applications
Why TH400 is legendary
Bulletproof construction
- Cast iron case (or aluminum in later years)
- Heavy-duty internals
- Simple design = fewer failure points
- Proven over 50+ years
Strong stock components
- 30-spline output shaft (HD)
- 5-pinion planetary
- HD direct clutch
- Strong sun gear
Easy to modify
- Wide aftermarket support
- Race-proven combinations
- Simple to upgrade
Wide power range
- Can handle 350 HP stock
- Can be built for 2,000+ HP
TH400 variations
Standard TH400 (long tail)
Standard car/truck length output. Most common.
TH400 truck (long tail with shifter)
Used in pickups and SUVs.
TH400 short tail
For applications requiring shorter overall length.
TH400 SM465 / SM420 (with 4WD adapter)
For 4WD applications. Different output spec.
Common failures (when they happen)
The TH400 doesn't fail often. When it does:
1. Direct clutch wear
The most common wear point. Burns or wears under load.
Fix: Quality friction pack during rebuild.
2. Forward clutch wear
Secondary high-wear point.
Fix: Quality friction pack.
3. Sprag failure
The TH400 has multiple sprags. Eventually fail.
Fix: HD sprag replacement.
4. Pump body wear
Standard high-mileage failure.
Fix: Pump rebuild or replacement.
5. Valve body wear
Less common than other GM automatics, but happens at very high miles.
Fix: Sonnax bore correction or replacement.
6. Modulator failure
Vacuum modulator can leak or fail.
Fix: Cheap replacement ($20-50).
7. Governor weights worn
Affects shift timing. Standard wear item.
Fix: Governor weight replacement or rebuild.
Building a TH400
Stock-replacement rebuild
- Quality master kit
- Alto Red Eagle frictions
- Stock-replacement HD components
- New torque converter
- Total parts: $400-700
Performance street build
- Alto Power Pack or Raybestos Stage-1 frictions
- Extra friction plate in direct clutch
- HD direct clutch sprag
- Performance valve body (Trans-Active, B&M, ATI, or similar)
- Performance torque converter (2,400-2,800 stall)
- Total parts: $1,200-2,000
Race / pro mod build
- Race-spec frictions throughout
- Extra plates in direct clutch (where possible)
- Billet input drum
- Billet hub
- Race valve body (trans brake compatible)
- Race-grade torque converter
- HD intermediate band
- Total parts: $3,000-6,000
Pro Mod / Top Fuel-style build
- Race-spec everything
- Specialty machine work
- Custom valve body
- Specialty torque converter
- Total parts: $6,000-15,000+
Brand recommendations for TH400
Alto
- Red Eagle and Power Pack frictions
- Quality master kits
- Recommendation: Yes
Raybestos
- Stage-1 and GPZ
- Recommendation: Yes for performance
Sonnax
- Bore correction kits
- HD upgrades
- Recommendation: Yes for quality rebuilds
TransGo
- Shift kits for TH400
- Recommendation: Yes
B&M
- Trick Stick valve bodies
- HD torque converters
- Recommendation: Yes for performance street
ATI Racing
- Race-grade everything for TH400
- Industry standard for race builds
- Recommendation: Yes for race
Coan Engineering
- Race-grade TH400 builds
- Recommendation: Yes for race
TCI
- Performance valve bodies and converters
- Recommendation: Yes for performance
Hughes Performance
- Performance and race builds
- Recommendation: Yes
Performance modifications
Shift kit (basic)
- TransGo or B&M shift kit
- Firmer shifts, better protection
- $80-150
- Mandatory for any performance application
Performance valve body
- Pre-built performance valve body
- Optimized for street/strip
- $300-700
- Significant performance improvement
Race valve body with trans brake
- For drag race
- Allows full-power launches
- $500-1,500
- Required for serious drag racing
Performance torque converter
- 2,400-2,800 stall for street performance
- 3,200-4,500+ stall for drag race
- $400-2,000 depending on grade
Billet input drum
- Replaces stock input drum
- Mandatory for serious power (1,000+ HP)
- $300-600
HD bands
- HD intermediate band
- Better holding power
- $50-100
Aluminum case (if iron)
- Significant weight reduction
- For race applications
- $400-800
TH400 vs TH350
TH350
- 3-speed
- Lighter (~120 lbs)
- Less torque capacity (~350 ft-lb stock)
- Shorter
- Better for daily driver / small block
- Generally cheaper
TH400
- 3-speed
- Heavier (~135 lbs)
- More torque capacity (~450 ft-lb stock, 1,000+ built)
- Longer
- Better for big block / heavy applications
- Generally more expensive
For high-power applications: TH400.
For daily driver / small block: TH350 is often plenty.
TH400 vs 700R4
TH400
- 3-speed (no OD)
- Bulletproof
- Race-proven
- Higher RPM cruise
- Worse fuel economy
700R4
- 4-speed (OD)
- Lower RPM cruise
- Better fuel economy
- Slightly less HD than TH400
- Needs TV cable setup correctly
For race or HD use: TH400.
For street use with cruise comfort: 700R4 or modern overdrive.
TH400 swap considerations
Why people swap to TH400
- Bulletproof for HD power
- Race-proven
- Wide aftermarket support
- Often cheaper than custom-building a 700R4
Drawbacks
- No overdrive (high cruise RPM)
- Heavier than some alternatives
- Longer (driveshaft modification)
Swap parts needed
- TH400 trans
- TH400-specific torque converter
- Crossmember (if changing)
- Driveshaft (length change)
- Cooler lines and fittings
- Cooler (factory typically inadequate)
- Bellhousing or adapter
- Shifter assembly
Cost
- TH400 swap kit: $1,500-3,000 for trans, converter, and adapters
- Installation: $500-1,500
- Total: $2,000-4,500
Fluid
Dexron III or VI
TH400 works with either. Dexron III is the original spec; Dexron VI is the modern fluid that's backward compatible.
Capacity
About 12 quarts total, 5 quarts in pan.
Service interval
30K severe, 50K normal.
What kills TH400s
1. Skipped service
Old fluid = wear. Service the trans.
2. Heat
Cooler is critical for tow/performance applications.
3. Beating on it without HD parts
TH400 is strong but not unbreakable. Build HD for HD applications.
4. Wrong torque converter
Cheap converter on built TH400 = converter failure.
Cost summary
DIY rebuild
- Stock: $400-700
- Performance: $1,200-2,000
- Race: $3,000-6,000
Shop rebuild installed
- Quality independent: $1,500-2,500
- Performance shop: $3,000-5,000
- Race shop: $6,000-12,000
Built option (most popular)
- Built TH400 from race shop: $2,500-5,000 (without converter)
- Including race converter: $3,500-7,000
The bottom line
For serious horsepower, TH400 is the answer. Built right, it handles 1,000+ HP reliably. Built race-spec, 2,000+ HP. The aftermarket support is unmatched and the design has been proven for 50+ years.
Don't overpay for fancy features — TH400 is a simple, robust design. Quality parts in the right places = bulletproof trans.
Ready to build a TH400? Shop our TH400 catalog. Master rebuild kits, Alto and Raybestos frictions, Sonnax HD parts, performance valve bodies, race torque converters. Free shipping over $70. Same-day ship in-stock.
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