A transmission that is similar, but not the same

General Motors 6T70 and Ford’s 6F50 front wheel drive transmissions are similar but definitely not the same. When these parts start compiling themselves on a shelf, it’s not too difficult for the wrong part to be pulled for a rebuild. This is especially true when you consider that the 3-5-R cushion spring is notorious for breaking in the 6T70, taking out several other parts (Figure 1).

Such was an incident where a 6F50 3-5-Reverse Clutch drum was accidently selected to replace the drum in a 6T70. In stacking up the clutch assembly with a new 6T70 cushion plate in this 6F50 drum, it was noted that the 3-5- Reverse Clutch clearance was too tight (Figure 2).

This was due to dimensional differences between these two drums assemblies (Figures 3 and 4). In fact, the 6F50 uses a thinner cushion spring (Figure 5) than the 6T70 (Figure 6) as a result of these dimensional differences.

This thinner cushion spring requires less distance from the top of the retaining snap ring to the area on the edge of the piston. Therefore, if the 6T70 cushion spring is used in place of the 6F50 cushion, there will be little to no clearance. Likewise, if the thinner 6F50 cushion was used in a 6T70 stack-up, clutch clearance will be excessive (Figure 7).

Although the cushion plates are different, clutch thickness and snap ring thickness are the same for both housings. 

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