[63corvette]

I sent this procedure to Bill in a PM and will post it here to let everyone know how I do the shimming.

Bill, sorry I have not responded earlier but I have been out racing the last 3 weeks. I am retired and go out for weeks at a time before returning home. We came home last night.
What I do is with the trans out and on the bench with the pan off is get a good light pointed into the trans in the piston port area on the valve body. The valve body is not square with the piston and opens up at the top of the piston first so you have to look into the trans closely to see when it opens up there.
With the solenoid in the energized position unscrew the solenoid until the piston starts to open a gap with the valve body.
Then screw the solenoid back into the case 1/2 a turn or a little more then measure the gap between the solenoid base and the case. That is the size of washer you need or .010 less including the gasket if you use one. I usually just use a little silicone on each side of the washer. This really helps on the GM cases and not so much on an after market case. I guess the after market case builders add a little more height to the base where the solenoid screws in. I have not found an after market case yet that need shimming.
I build my own washers on my lathe.
Hope this helps.
Good Luck with the trans.
Rick