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