'82-'93 4L60 Harsh or Delayed Reverse Engagements
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Early 4L60 transmissions ('82-'86) use a reverse input drum that has a rectangular hole where the fluid enters the drum. In 1987, when the auxiliary valve body was first introduced, the reverse input drum feed hole was changed to a smaller round hole (.095") which became the orifice to feed the drum.
If an early square hole drum is used on a 1987-1992 transmission with auxiliary valve body, the reverse engagement will be harsh. The reason for this is that you increased the size of the feed hole to the reverse input clutch, causing the clutch to apply much faster than with the smaller hole drum, which results in a harsh engagement.
In 1993, a reverse feed orifice check ball was added to the valve body, and GM went back to the drum with the rectangular feed hole in the reverse input drum. If a reverse input drum with a round feed hole is used in a 1993 that has the reverse feed orifice check ball, a delay into reverse may result.
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