Saginaw Transmission Identificationby Rob Wagner
The Saginaw three- and four-speed manual transmissions transmitted engine power to the wheels on General Motors non-performance and mid-range performance cars. The transmissions were cast-iron and stamped with an application code.
Both versions of the Saginaw transmission featured the case and extension housing sealed with a seven-bolt cover on the side. The gears were fully synchronized and the synchro rings had 30 teeth. Mounted on the cover was the reverse shift lever.
Saginaw transmissions featured application codes on the passenger side of the case near the rear cover. "O" denoted the transmission was equipped with overdrive. "S" identified the three-speed transmission as manufactured in Saginaw, Michigan, from 1962 to 1979. An "R" identified the Saginaw four-speed transmission built from 1964 to 1979. General Motors used the in-house Regular Production Option (RPO) code "M15" to identify the three-speed and "M20" for the four-speed.
Saginaw three-speed transmissions featured gear ratios of 2.85:1 for first, 1.68:1 for second and 1.00:1 for third when matched with L6 in-line six-cylinder engines. For small-block V-8s, like the 350, the three-speed used 2.54:1, 1.50:1 and 1.00:1. The four-speed matched with the straight-sixes were 3.11:1, 2.20:1, 1.47-to-1 and 1.00:1. For V-8s, it was 2.54:1, 1.80:1, 1.44:1 and 1.00:1.
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