Tranny Upgrade Part I (Corvette Servo)


This seems to be one of the most popular tranny upgrades next to a shift kit, and usually done in conjunction with a shift kit.  The Corvette's "YDM" transmission is unique to all other 4L60s (700R-4) in part to its very large 2nd Apply Piston.  Take a look at the picture:

This is the piston housing.  The Corvette's is on the left, and the standard F body is on the right.  There is actually a third size (not shown) that has an even larger piston size used for non-performance vehicles. 

The other half to the servo is the piston.     Once again, the YDM (Corvette) is on the left and the F-body on the right.  Notice the substantial difference in size. The larger piston allows for more holding power.


Installation is a snap, and some good news:  1) you won't have to lower the transmission, and 2) you only loose about a quart of fluid.

First thing you need to do is raise the car up, either with jackstands or a lift, ramps, or whatever you have, just make sure it is safe.  Then locate the servo cover (painted black in this picture).  There is a snap-ring to hold it into place.  I cheated and used the "special GM tool" to depress the cover to remove the ring, but a simple prybar against the transmission tunnel will work, then just pry out the ring, and remove the cover.

The cover has a large O-ring around it and may make it difficult to remove, just make sure you purchase new seals, because this one will probably tear.  When you order the servo, it will come with Teflon rings, but you will need the Cover O-ring, and the Housing O-ring.  Once the cover is off, the trick is to disassemble the servo in the case.  First remove the housing.  Then remove the E-clip, washer and spring.

Cover and 4th (overdrive) Piston removed.

Cover, 4th Piston, E-clip, Washer, Spring, and Housing removed.

Once all of this is removed, you will probably need to employ the services of a friend.  You will need to pry the transmission towards the drivers side (or away from the passenger side).  You may need to take the nut off of the crossmember to allow more movement.  You want to pry it over far enough so that the apply piston will clear the transmission tunnel with the servo attached.  If you have to lower your trans to get it out, just make sure you remove your distributor cap so you don't risk breaking it.  I didn't have to lower mine.


Once out,  you will see that there is a "plate" inside the piston and is spring loaded.  You will need to squeeze this together and remove the snap-ring.  Once apart, the servo will separate from the pin/spring assembly.  Simply install the new servo onto the pin, squeeze the assembly back together and install the snap-ring.  Install the new seals and use ATF to lubricate the assembly.  When you go to put the assembly back in the trans, look out for the spring still in the case.  It may still be on the "nub" or it may have fallen.  If you feel inside the case, you will see the hole for the pin is in a raised "nub" and the spring must rest around this nub.  Then pry on he tranny again and install the assembly.  Once in, push on the assembly, it should feel springy.  If it does, its in right. 

On the left is the Piston.  Make sure you get a new seal (as shown, RED).  The side you see goes in towards the transmission.

On the right is the 4th Piston.  Notice the nub on top.  This goes towards the outside of the transmission.

Install the Piston, and then the 4th Piston.  Once all in place, install the cover (with new seal of course).  Using the same method for removal, depress the cover into the case, and install the snap-ring.  Re-install everything that was undone, lower the car, and check you fluid level and add as necessary. Enjoy!


Part Numbers:

Piston: 8642079

Housing: 8673039