C4 Corvette ZF 6 Speed Manual Transmission
1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996
This part is a good, used ZF 6 Speed Transmission which can be installed on any of the following Corvette models:
Body & Trim
Base, Collector’s Edition, Grand Sport
Base, Indianapolis 500 Pace Car
40th Anniversary Edition, Base
Black Tag vs. Blue Tag ZF 6 Speed Transmission
What is the difference between a black tag versus blue tag ZF 6 Speed Transmission? This has always been a question surround the transmission with the later C4 Corvette models. The name of the transmission is derived from the color of the manufacturer’s tag (which is located on the side of the transmission). It is either a black tag or a blue tag. Black tag transmission units were manufactured in West Germany, by ZF, from 1989 to Mid-1993. At this point in time, General Motors obtained a license which allowed them produce this transmission within the United States. Thus, blue tag transmission units were born. They were manufactured in the United States from Mid-1993 to 1996.
The biggest difference between the two units is an internal modification. Due to complaints surrounding gear whine, gears in the blue tag transmissions were cut at a steeper angle than those in the black tag. Essentially, this led to a quieter transmission but also resulted in a marginally weaker transmission housing for blue tag units. Overall, the torque rating for a black tag unit is around 450 lb-ft, while the rating for blue tag unit is around 400 lb-ft.
From ZF regarding ZF transmission noise and differences between the ZF used in early and late ZR-1 Corvette models:
“All S6-40 transmissions installed in the C4 Corvette contain 100% helical gears. In fact, all of the gearboxes ZF produces for passenger car applications contain helical gears, the primary purpose of which is to increase gear contact ratio and thereby minimize gear noise.”
“There is one major difference between the Black Tag (1989 to Mid 1993) and Blue Tag (Mid 1993 to 1996) C4 Corvettes – the contact ratio for the later years was further increased in order to reduce the gear noise. There, however, was a very slight reduction in torque capacity of the gearbox (from 610 Nm to 540 Nm) when ZF increased the gear contact ratio in the later year transmissions.”
“The noise you hear in 1st gear is commonly termed “1st gear growl”. This noise is inherent to the design of the 610 Nm gearbox and in no way is detrimental to the reliable function of the transmission and your ZR-1 Corvette. With the increased gear contact ratio in the 540 Nm gearbox, ZF also was able to reduce the “1st gear growl”.”
In 1993, the part number for the ZF changed from 10186865 to 10255122. In the second generation ZF, the gear ratios stayed the same, but there was an increase in helical pitch of all the drive gear teeth which helped to reduce the whine. The 2nd, 3rd, 5th and 6th speed gears also went to a crimped spline-fit method of fastening the synchronizer clutch body to these speed gears.
GM Part Numbers:
Blue Tag (Mid 1993 to 1996)
12455189, 10242260, 10255122
Black Tag (1989 to Mid 1993)
12351965, 10235121, 10186864, 10186865