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Reverse Input Drum Fix 1987 & Up 
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Joined: Fri Jun 29, 2012 4:23 pm
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Starting in 1987 they went to a "feed/bleed" setup, similar to the 2004R forward apply clutch setup that started in 1987 also. This was supposed to make for a smoother engagement. They changed the type of apply piston from steel to aluminum. This helped to eliminate the piston from running on after the 1-2 shift. The steel piston was known to keep spinning for a brief time after a quick 1-2 WOT shift. This would tear up the drum, put metal into the oil, and could wear away the snap ring land and ruin the spring retainer in extreme cases. The lighter aluminum piston stopped the "run on" almost entirely. The feed/bleed system has a hole in the aluminum piston (more oil coming in larger feed hole) behind the apply piston than is escaping through the hole (smaller bleed hole in the piston) that slowly gets covered up when the piston starts moving, now the piston can apply the clutches completely. GM went to a "Belleville" (diaphram) type of steel on top of the aluminum piston instead of a "waved" steel as in the earlier setups or the 2004R forward "feed/bleed" setup. The problem GM created by using the Belville apply steel was it tore up the drum, added a lot of metal to the oil, and in extreme cases caused a delay or no reverse at all due to the Belville apply steel hanging up on the torn up splines. The solution was to change the Belville apply steel to a waved steel as used in all 1982 - 1986 reverse/input drums. This will solve all the problems that the Belville setup created, but when you do this you will find that you now have way to much clutch clearance. You can reduce it one of two ways. The easist way is to change to the early 1982 - 1986 type Borg Warner clutches to get the desired .040" - .070" clutch clearance. They measure on average .010" thicker per clutch (four clutches total .040"). Or you can use a thicker top pressure plate. When doing this, always use the "Turbulator" type of steels that came in all 1996 and up 4L60E transmissions. This will free up a slight amount of horsepower, a slight gain in gas mileage, and reduce the oil temperature a little.

Mon Aug 13, 2012 7:11 am
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