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The TH350 and the TH400 are the two transmissions almost every GM hot-rodder eventually chooses between. Both are Turbo Hydramatic 3-speed automatics, both bolt to the common GM bellhousing patterns, and to a beginner on a swap meet table they look like siblings. They are not. The TH400 was engineered for big-blocks, trucks, and towing. The TH350 was built lighter and shorter to save weight and parasitic loss in passenger cars. Pick wrong and you either haul around a heavy, power-robbing unit you did not need, or you grenade a light one behind torque it was never rated for.
Two questions settle it fast: how much torque are you actually making at the crank, and do you care about the parasitic drag and weight of the trans. Everything below is the detail behind those two questions.
Quick-Glance Spec Comparison
| Spec | TH350 | TH400 |
|---|---|---|
| Introduced | 1969 | 1964 |
| Gear ratios (1/2/3) | 2.52 / 1.52 / 1.00 | 2.48 / 1.48 / 1.00 |
| Case length | ~21.75 in | ~24.375 in |
| Dry weight | ~120 lbs | ~135 lbs |
| Stock torque comfort zone | up to ~350-400 lb-ft | up to ~450 lb-ft stock, 1,000+ built |
| Tailhousing bolt pattern | 6-bolt round | Square 4-bolt (most) |
| Pan shape | Square-ish, 13 bolts | Irregular "brain" shape, 13 bolts |
| Parasitic loss | Lower | Higher (heavier rotating mass) |
The single fastest way to tell them apart in a junkyard is the oil pan. The TH350 pan is a fairly regular rounded rectangle. The TH400 pan has a distinctive irregular outline with one corner cut at an angle, often described as a shape like Texas or a brain. If you can see the pan, you know the trans.
The Real Differences That Matter
Strength and Torque Capacity
This is the whole reason the TH400 exists. It uses a larger, more robust planetary set, a stronger center support, and beefier clutch packs. GM put the TH400 behind 454s, in 3/4-ton trucks, and in motorhomes for a reason. A healthy stock TH400 shrugs off 450 lb-ft all day, and a properly built one with a hardened output shaft, an upgraded intermediate sprag, and a good valve body will live behind 1,000+ horsepower drag cars. The TH350 is a lighter-duty unit. Stock, it is happy behind a mild small-block making 300-380 lb-ft. Push a stock TH350 much past 400 lb-ft and you start eating the direct clutch drum and the intermediate sprag.
Weight and Parasitic Loss
The TH350 is roughly 15 pounds lighter and, more importantly, has less rotating mass and less internal drag. In a light street/strip car chasing e.t., that lower parasitic loss is worth a couple tenths and a few wheel horsepower over a TH400. This is why serious bracket racers with small-blocks under about 500 hp often prefer a built TH350 or a TH350-based race unit over the heavier TH400.
Length and Driveshaft Fitment
The TH400 is roughly 2.6 inches longer than the TH350. Swapping one for the other in the same chassis means the driveshaft length changes and, on many cars, the crossmember mount location moves. Do not assume a bolt-in. A TH350-to-TH400 swap almost always needs a shorter driveshaft (or the shaft shortened) and often a different or relocated crossmember.
Gear Ratios
The ratios are close enough that they are not a deciding factor. First gear is 2.52 in the TH350 versus 2.48 in the TH400, and second is 1.52 versus 1.48. You will not feel that difference from the driver's seat. Both are 1.00 in third with no overdrive. If you want an overdrive gear you are looking at a 700R4/4L60 or a 200-4R, not either of these.
Kickdown and Control
The TH350 uses a mechanical throttle-valve cable or a detent cable for kickdown, and it is picky about correct adjustment. The TH400 uses an electric kickdown switch instead, which many builders find simpler to wire up cleanly. Neither has a lockup converter in the early versions; the TH350-C and later units added lockup.
Which One Should You Rebuild?
Match the trans to the actual power and the actual job. Here is the honest breakdown:
| Your Build | Better Choice | Why |
|---|---|---|
| Mild street small-block, under 400 lb-ft | TH350 | Lighter, less drag, cheaper to build, plenty strong |
| Light street/strip bracket car, sub-500 hp | TH350 (built) | Lower parasitic loss = better e.t. |
| Big-block street car or heavy cruiser | TH400 | Built for the torque and the weight of the car |
| Tow rig, truck, or motorhome | TH400 | Handles sustained load and heat far better |
| High-horsepower drag / forced induction | TH400 (built) | Only one of the two that lives past 700 hp |
The short version: if you are under about 400-450 lb-ft and want efficiency, rebuild the TH350. If you are making real torque, towing, or planning to add power later, rebuild the TH400 and never think about it again.
Rebuild Cost and Common Failure Points
A DIY master overhaul kit for either unit runs roughly $120-$250 depending on brand and whether you go with a performance clutch/steel pack. Add a torque converter ($130-$400 depending on stall and quality) and, for a hard-launch car, a shift kit ($60-$120). A shop rebuild on a TH350 typically lands around $1,000-$1,600 out the door; a TH400 usually runs $1,400-$2,200 because of the larger parts and the heavier-duty upgrades people order with it.
TH350 weak points to address on the bench
- Intermediate sprag rolls over under hard 1-2 shifts. Upgrade to a heavy-duty sprag.
- Direct clutch drum cracks with abuse. Inspect closely, replace if suspect.
- Low/reverse and forward clutch packs glaze. Refresh frictions and steels every rebuild.
- Governor and valve body get sludged. Clean thoroughly and add a shift-improver kit.
TH400 weak points to address on the bench
- Intermediate sprag (34-element) is the classic failure under big power. Upgrade it.
- Output shaft can twist behind serious torque. A 4340 hardened shaft is cheap insurance.
- Center support bolt backs out; use the correct torque and thread locker.
- Rear ring gear / planetary wears; inspect and replace on high-mile cores.
Whichever you build, replace the pump bushing and seal, the tailhousing bushing, and every rubber seal in the kit. Reusing tired bushings is the number one reason a fresh rebuild leaks or buzzes on the first drive.
Parts and Tools You Will Need
You do not need a transmission shop to rebuild either of these on a bench, but you do need a few specific items. A master overhaul kit, a good torque converter matched to your stall, a shift kit, and a spring compressor for the clutch drums are the core list.
- TH350 master rebuild kit
- TH400 master rebuild kit
- TH400 heavy-duty intermediate sprag
- Transmission shift improver kit
- Clutch spring compressor tool
Building One on the Bench? Parts Ship Same-Day
Once the case is split and the core is inspected, you need frictions, steels, sprags, bushings, and seals that actually fit. Gearbox Insider stocks TH350 and TH400 rebuild components and ships same-day before 2pm Eastern.
Shop Gearbox Insider →FAQ
Will a TH400 bolt up where a TH350 was?
To the engine, usually yes on the common GM patterns. To the chassis, not cleanly. The TH400 is about 2.6 inches longer, so you need a shorter driveshaft and often a relocated or different crossmember. Plan for both before you swap.
Is a TH400 always stronger than a TH350?
Stock for stock, yes, the TH400 handles more torque. But a properly built TH350 with an upgraded sprag and good clutches will out-live a neglected stock TH400. Build quality matters as much as the base unit.
Which is cheaper to rebuild?
The TH350 is generally cheaper because the parts are smaller and people order fewer heavy-duty upgrades with it. Expect a TH400 rebuild to run a few hundred dollars more.
Do either of these have overdrive?
No. Both are 3-speeds with a 1.00 top gear. If you want an overdrive for highway rpm, look at a 700R4/4L60 or a 200-4R instead.