SW Performance Transmissions Install - TH350 Transmission

Smooth transmission performance is a vital part of drivability. All the power and rumbling horsepower in the world is useless if there is a tired and worn transmission delivering gear commands to the differential. High-mileage trucks with several years of hauling and road time under their belts usually warrant transmission attention. One of the classic transmissions found in several older GM offerings is the Turbo 350 three-speed transmission. The Turbo 350 is an economical alternative to more current 700-R4, 4L60E, and 4L80E overdrive units, which can put a hefty strain on the wallet when it comes time to rebuild or replace.

Recently, we noticed that the original Turbo 350 transmission in our '85 Chevy C10 project truck was beginning to slip and leak an excessive amount of fluid out of the front seal. Since we planned on driving this truck considerably after its first show season tour, a fresh replacement gearbox was a must for worry-free driving. The transmission specialists at SW Performance Transmissions in Huntington Beach, California, answered our shifting prayers with a freshly rebuilt Turbo 350 to slide under our C10. Before hoisting the replacement SW Transmission in place, we decided to take an inside tour with owner Steve White of this classic transmission and show what makes it tick.

The TH350 was placed on the jig at SW Performance Transmissions and drained in preparation for rebuild.
The TH350 was placed on the jig at SW Performance Transmissions and drained in preparation
This comparison photo shows the planetary gears from a 700-R4 (left) and the planetary gears from a TH-350 (right). SW Performance Transmissions installed the 700-R4 planetary gears in the Turbo 350. The benefits include a stronger gear train and a lower gear ratio. The TH350 low gear ratio is a 2.52, while the 700-R4 is a 3.06 low gear ratio for increased pulling power and low-end performance. This is especially beneficial in truck applications.
This comparison photo shows the planetary gears from a 700-R4 (left) and the planetary gea
This photo shows the use of the 700-R4 ring gear in place of the Turbo 350 ring gear, allowing it to fit into the 700-R4 planetary and mesh with the 700-R4 gear train.
This photo shows the use of the 700-R4 ring gear in place of the Turbo 350 ring gear, allo

The stock Turbo 350 sprag gear (left) and the SW Trans Turbo 350 sprag gear (right). The SW Transmission sprag gear uses bigger rollers and a bigger cage, making it a more durable unit.
The stock Turbo 350 sprag gear (left) and the SW Trans Turbo 350 sprag gear (right). The S
A heavy-duty, intermediate, hardened sprag race is used in SW Performance Transmission applications to keep everything free of cracks and breakage when shifting into second gear.
A heavy-duty, intermediate, hardened sprag race is used in SW Performance Transmission app
The pump assembly of the TH350 accumulates excess wear on the ring lands and rings. This usually indicates premature wear of the clutches.
The pump assembly of the TH350 accumulates excess wear on the ring lands and rings. This u

This photo shows the wear pattern of the pump gear and the stator support.
This close-up shows the wear produced from the pump gear riding against the crescent.
Examining this close-up shows the drive tangs on the pump gear, which wear out primarily from excess use and mileage. When the drive tangs go south, the pump cannot be driven, and no fluid pressure is produced to allow the transmission to engage and the vehicle to move. Breakage of the drive tangs is a result of poor converter installation and spacing issues between the converter and flexplate.
Examining this close-up shows the drive tangs on the pump gear, which wear out primarily f

A peek at these two pump gears shows a comparison of a fresh gear on the right and a worn unit on the left. Once again, the issue of hydraulics come into play: With no fluid pressure, the transmission will begin to slip.
A peek at these two pump gears shows a comparison of a fresh gear on the right and a worn
This photo shows the high gear (Third) drum ring wear pattern. If the drum ring wears, too much pressure is affected and the clutches get burned up. When the drum ring wears excessively, there is not enough pressure to keep the clutches together.
This photo shows the high gear (Third) drum ring wear pattern. If the drum ring wears, too
SW Performance Transmissions uses a five-pack clutch assembly instead of four to increase durability and longevity and to provide more holding power.
SW Performance Transmissions uses a five-pack clutch assembly instead of four to increase

Closing in on the planetary gears we examined where the wear occurs and learned that wear shows on the gear itself and the surrounding washers. The result, if ignored, is that the gear wears right through the housing, causing excess noise. Pitting is a sign that the planetary gears are on their way out.
Closing in on the planetary gears we examined where the wear occurs and learned that wear
SW Performance Transmissions installed a valve body improver kit for tighter and firmer shifting characteristics. The SW kit features higher tension springs for the governor, pressure regulator, and shift spring to adjust the shift points throughout the powerband. SW modified the separator plate to create more oil flow for the clutch packs and to provide better shifting behavior. The regulator pressure was increased over stock and the holes were opened up for better flow.
SW Performance Transmissions installed a valve body improver kit for tighter and firmer sh
A look inside the transmission housing shows wear from the front planetary gear on the thrust washer. It is important to investigate the washer to ensure it is reusable.
A look inside the transmission housing shows wear from the front planetary gear on the thr

The front ring gear, which surrounds the planetary gear, was inspected for wear and pitting.
The front ring gear, which surrounds the planetary gear, was inspected for wear and pittin
This photo shows wear of the bushing on the front ring gear. When the bushing wears too much, oil can't properly lubricate the planetary gear.
This photo shows wear of the bushing on the front ring gear. When the bushing wears too mu
A peek inside the transmission case shows where the lower sprag assembly sits in the case and how the lugs it sits around wear out.
A peek inside the transmission case shows where the lower sprag assembly sits in the case

Before the transmission went back into the truck, SW Performance Transmissions owner Steve White inspected the dowel pin holes where the transmission lines up with the engine at the bell housing. Quite often, the holes go out of shape from poor alignment or years of use. Here, Steve drilled out the holes for an insert to correct the problem.
Before the transmission went back into the truck, SW Performance Transmissions owner Steve
A sleeve-style insert went into the dowel pin holes to ensure precise fitment when the transmission was mated back up to the engine.
A sleeve-style insert went into the dowel pin holes to ensure precise fitment when the tra
The high gear drum was slid onto the forward drum and input shaft before the assembly was lowered into the case.
The high gear drum was slid onto the forward drum and input shaft before the assembly was

The high gear drum and forward drum mounted on the input shaft were installed into the TH350 case. The forward drum is always active when the vehicle is in First and Second gear.
The high gear drum and forward drum mounted on the input shaft were installed into the TH3
The overrun band was installed. It serves as a brake band to stop the direct drum and provide engine braking in Second gear and slow the truck down.
The overrun band was installed. It serves as a brake band to stop the direct drum and prov
The intermediate Borg Warner clutches were installed.

The case before the pump was assembled and installed.
The pump was assembled on the bench before Steve installed it into the transmission.
An alignment band was used to properly install the pump so it fit perfectly in the case. It's all about proper clearances in the tranny game.
An alignment band was used to properly install the pump so it fit perfectly in the case. I

The pump assembly was tightened to spec with an impact before being installed.
The pump assembly was lowered into the housing and slid onto the input shaft.
After being installed onto the input shaft, the pump assembly was tightened into place with an impact.
After being installed onto the input shaft, the pump assembly was tightened into place wit

Steve muscled the torque converter in place after filling it with fluid.
Each SW Performance Transmission is put on an engine dyno to check for leaks, shift quality, noises, and transmission line pressure. A vacuum pump was used to create a vacuum in the transmission identical to road conditions.
Each SW Performance Transmission is put on an engine dyno to check for leaks, shift qualit