General Motors · 1966–1985
Turbo-Hydramatic 425
Turbo-Hydramatic 425 (TH425 or THM 425, later 325) was an automatic transmission developed and produced by General Motors. The THM425 was based on the design of the THM400, with most parts being identical and some others interchangeable with minor modifications. In the THM 425, the internal parts spin the opposite direction; for example, the helical angle of the planetary gears is "reversed" and the one-way clutches freewheel in the opposite direction. The THM425 was developed for the 1966 Oldsmobile Toronado and the 1967 Cadillac Eldorado. After the 1978 model year, both lines replaced the THM425 with a lighter-duty transmission known as the THM325 (using components sourced from the THM200). Starting in 1979, all longitudinal engine front-wheel drive vehicles used the THM325. In 1982, an overdrive was added to the THM325, turning it into the THM325-4L (4L means 4 forward speeds, Longitudinal). Production of this transaxle continued until 1985, being phased out when GM moved to transverse-engine FWD layouts in 1986, when all vehicles using the THM325-4L switched to more-conventional transverse engine mounting. THM325's bellhousing pattern (arrangement of bolt holes and shape of the transmission’s engine-side mounting flange) used the 1967-90 Buick-Oldsmobile-Pontiac-Cadillac V8 pattern throughout its entire lifecycle. Vehicles that used the THM 425/325: THM425 1971–1979 Cortez Motor Home 1966–1978 Oldsmobile Toronado 1967–1978 Cadillac Eldorado 1973–1978 GMC Motorhome 1973–1978 GMC TransMode multi-purpose vehicle 1972–1978 Revcon Motorhome 1989–1993 Vector W8 THM325 1979–1981 Cadillac Eldorado 1979–1981 Oldsmobile Toronado 1980–1981 Cadillac Seville THM325-4L 1982–1985 Buick Riviera 1982–1985 Cadillac Eldorado 1982–1985 Cadillac Seville 1982–1985 Oldsmobile Toronado
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Specifications
- name
- Turbo-Hydramatic 425/325/325-4L
- Production
- 1966–1985
- Manufacturer
- General Motors
- Class
- 3-speed longitudinal automatic transmission for THM425/325, 4-speed longitudinal automatic transmission for THM325-4L
- Successor
- 4T60
- Related
- Turbo-Hydramatic 125, Turbo-Hydramatic, Turbo-Hydramatic 180
Specification summary from Wikipedia (CC BY-SA 4.0). Check the linked sources for citations.
More photos
Sources
- Wikidata — https://www.wikidata.org/wiki/Q7853782CC0 1.0 · retrieved 2026-10-01
- Wikipedia — https://en.wikipedia.org/wiki/Turbo-Hydramatic_425CC BY-SA 4.0 · Text © Wikipedia contributors; revision 1372412269 · retrieved 2026-10-01
- Wikimedia Commons — https://commons.wikimedia.org/wiki/File:Ypsilanti_Automotive_Heritage_Museum_May_2015_057_(1965-78_THM_425_transmission).jpgCC BY-SA 4.0 · Photo: Michael Barera · retrieved 2026-10-01
- Car Model Index — https://models.autoculturesociety.com/Compiled record. Sources that do not permit reuse are not copied; this index is credited instead.