928 Driveline Information
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928 5-speed Drive Shafts
All 928 5-Speeds have 25mm drive shafts in their torque tube regardless of model year.
928 Automatic Drive Shaft Differences
The 928 automatics have two different sized drive shafts according to model year. From 1977 through 1986 the 928 had a 25mm drive shaft installed in the torque tube. Sometime in 1987 Porsche changed it to a 28mm drive shaft. However they kept the splined areas of the drive shaft at 25mm which necessitated the drive shaft to be necked down. This neck down has been found to be a shear point for the 28mm drive shafts which will be discussed below.
We have had problems with customers who have ordered our 928 torque tube rebuild kit for a certain model year only to discover the wrong size drive shaft is in their 928. This is usually due to the prior owner replacing the torque tube with one from a different model year. Both the 25mm and 28mm automatic drive shafts will fit all model year automatic 928s. Below are a few pictures to help distinguish which drive shaft you have in your 928.
The picture below shows the engine end of the 25mm automatic drive shaft on top and the 28mm automatic drive shaft on the bottom. Notice the pronounced necked down area of the 28mm drive shaft after the splines.
The picture below shows the transmission end of the 25mm automatic drive shaft on top and 28mm drive shaft below.