Editing Turbo-Hydramatic (section)

==THM350==
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The Turbo Hydra-matic 350 was first used in 1969 model cars. It was developed jointly by Buick and Chevrolet to replace the two-speed Super Turbine 300 and aluminum-case Powerglide transmissions. So, although it carries the Turbo Hydra-matic name, the Hydra-matic Division of General Motors had little, if anything, to do with its design. The 350 and its 250, 250C, 350C and 375B derivatives have been manufactured by Buick in its Flint, Michigan plant, and by Chevrolet in Toledo and Parma, Ohio and Windsor, Ontario.

The THM350 was also regarded{{by who|date=February 2024}} as a 'three speed Powerglide'{{says who|date=February 2024}} and during its development, was generally called this.{{says who|date=February 2024}} Although it uses the same torque converter as the THM400 (without variable pitch stator) it has a familial resemblance{{vague|date=February 2024}} to the 1962-73 aluminum Powerglide from Chevrolet and was largely derived from the Chevrolet design. An important difference in the THM350 compared to the THM400 is that there is no fixed center support midway through the geartrain; this difference in layout would have permitted the THM350 to be adapted to the Corvair where the drive and driven ends are the same, but this feature was not exploited. Air-cooled versions (with a baffle on the torque converter and air intakes cast into the bellhousing) of the THM350 appeared mid-1972 in the Chevrolet Vega and Nova 6.

One THM350 weak point was excessive end-play between the pump and center support and resulting wobble of the direct clutch drum due to both the end play and use of a relatively narrow bushing in the drum. This weak point can be addressed by using an extra thrust washer between the planetary gear and direct clutch to remove the end play and using a wider aftermarket bushing in the direct clutch drum. Another weak point is the relatively thin center support and the lightweight matching splines in the case. This weakness can be addressed by using an inexpensive aftermarket case saver kit.

Four-wheel drive truck applications for the THM350 used an iron adapter that mated it to the transfer case directly, similar to the THM400. The THM350 adapter was cast iron and used a sliding sleeve to couple the transmission output shaft to the transfer case input shaft with a steel coupler sleeve that was splined to accept both shafts and couple them together. An internal snap ring inside the coupler sleeve controlled the sleeve's position on the shafts, with circular seals in the adapter sealing the transmission from the transfer case.

For the 1981 model year, a lock-up torque converter was introduced which coincided with the new EMC control of most GM cars; this version is the THM350-C, which was phased out in 1984 in GM passenger cars for the 700R4. [[Chevrolet]]/[[GMC (General Motors division)|GMC]] trucks and vans used the THM350-C until 1986. The lock-up torque converter was unpopular with transmission builders.{{says who|date=February 2024}} B&M Racing once marketed a conversion kit for THM350-Cs during the early 1980s until the advent of high stall lock-up torque converters when its overdrive counterpart (THM700R4/4L60) was modified. The standard TH350 is still very popular in drag racing.

===THM250===
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The '''THM250''' is a derivative of the THM350 and was introduced for 1974 in Chevrolets as a [[Powerglide]] replacement. Internally, the THM250 is a THM350 without the intermediate clutch pack and with a band adjuster similar to the Powerglide. The THM250 was usually coupled to smaller displacement engines - the largest a third generation Chevrolet inline six found in the Nova and Camaro (1974 and 75 model year only). During the 1976 model year the THM250 was phased out of production, replaced with the lighter duty THM200. It was later reintroduced in 1979 as the THM250-C in the wake of the failure-prone THM200/200C - the later 250C was further lightened with the use of a sun gear shell used with the THM350 but with 3 holes to reduce rotating mass and the low/reverse piston with 8 cutouts.

{| class=wikitable
|+Gear ratios: THM350, 250, 250C
!Gear
!Ratio
|-
|1||2.52:1
|-
|2||1.52:1
|-
|3||1.00:1
|-
|R||2.07:1
|}