3/4 clutch pack Clearance

Yeah, I'm not a giant fan of the Alto 9 disc stack, either. Sure, you get 9 frictions in there, but you have sacrificed too much thickness on both the frictions and the steels to make them much good at tolerating heat. The Alto kits measure .062" on the frictions and .060" on the steels - pretty thin...

A 700R4/4L60E 3-4 clutch is kind of an anomaly of sorts. It's such a weak clutch design that convention kind of gets thrown out the window to make them last, but shift well. The thing that comes into play is not only lubrication and cooling of the plates (more clearance is better), but the quality of the 2-3 shift (less clearance is better). During the 2-3 shift, the transmission is releasing the band and engaging the 3-4 clutch simultaneously. When the valve body commands the shift, pressure is sent to the back side of the 2nd gear band servo and to the 3-4 clutch piston at the same time. The band begins to release almost instantly, so during the time between the release of the band and the engagement of the 3-4 clutch, you're, quite literally, in first gear for a fraction of a second. This is why it is so critical for the 3-4 clutch clearance to be as tight as possible, while still allowing the plates to lubricate and cool acceptably. The tighter the clearance, the less time it will take for the 3-4 piston and apply ring to travel the distance necessary to begin clamping down on the clutch plates.

In either case, .107" may as well be a mile... That's WAY too much clearance. You're right in that conventional thinking provides appx. .010" per friction in the stack for good lube and cooling. But, even .090" (9 x .010") is way too much clearance. Your 2-3 shift will be a bit of a lazy, slide-shift at low throttle and, under heavy acceleration, you're likely to have some flare, especially as the plates begin to wear and the clearance increases even further. When I use a 9 disc setup (not often), I use the red frictions and kolene steels and shoot for .030-.035", and definitely no more than .040". Transgo recommends .015-.020" in their shift kit instructions, but that's for a stock 6 disc stack. You won't get away with that using 9 thin discs for very long.

At the risk of giving away trade secrets, my preferred stackup, starting at the piston, goes like this:
-Bonded steel piston
-"7" apply ring (a "4" will work if that's all you've got, but pretty much all 4L60E's came with "7"s)
-Stock return springs
-Factory NEW 3-4 apply plate (OEM AC Delco only)
-7 Borg Warner high energy frictions (factory 4L60E plates)
-6 700R4 .076" steels (stock or kolene, your choice)
-Factory NEW 3-4 clutch backing plate (OEM AC Delco only)
-.093" snap ring
-This stack almost always comes out between .022-.025".
-On a high RPM application, I'll drill a .035" relief hole in the input drum at outer edge of the 3-4 piston bore to prevent centrifugal apply, thus dragging the plates unnecessarily.

One more thing... I ALWAYS use the load release springs in the 3-4 clutch. These are important when running a tight stack because they force the apply and backing plates apart when the clutch is released, minimizing drag on the plates between them. If yours are missing, or have lost their tension, get new ones and put them in...

Sorry for the novel...!

They don't necessarily sell a 7 disc "set". You'll just need 7 high energy frictions (you should have gotten 6 of them in the overhaul kit - just order one more), 6 .076" 700R4 steels new apply plate and backing plate. Make sure you order 700R4 steels, as the 4L60E steels are too thick for a 7 disc stack (.106").

I can't stress enough the fact that you need to replace those apply and backing plates, especially the apply plate. They warp and take on a "cone" shape through years of use. This becomes greatly exaggerated if the old 3-4 clutch was burnt up, which is very common. The heat generated when the clutch fails will soften the apply plate and cause it cone badly. If you put a coned apply plate back in, it will only apply pressure to the outer edge of the stack due to its cone shape. I have seen many a transmission guy pull his hair out with repeat 3-4 clutch failures due to not addressing this issue. The plates are cheap (relatively) and readily available from GM. I keep 4-5 in inventory at all times. Plus, they are slightly redesigned to improve their rigidity.

I don't typically iuse the rapid release valves. That clutch vents quickly enough as it is. The whole purpose of the relief hole in the drum is to prevent fluid from being forced against the outside of the 3-4 piston's bore by centrifugal force, causing the 3-4 piston to begin to apply at high RPM (remember, the input drum spins at engine speed). All it takes is a very small hole, right at the very outer edge of the bottom of the bore to relieve any centrifugal pressure and prevent any piston movement. Remember, you should be running a pretty tight 3-4 stackup, so pretty much any piston movement due to centrifugal apply will cause the 3-4 plates to drag. They won't put up with that for very long... I usually drill a .035" hole, which is large enough to relieve pressure, but small enough that the pump will easily overcome the small "leak" when the clutch is applied.

Oh, and, yes, your rapid release valve will probably seal just fine once there's fluid in there. Air will easily find it's way through places that fluid cannot go.

Ever since GM brought out the concept in the late 700's, we put alot of research into what they actually do. GMs theory was to push the top and bottom pressure plates apart so the 3-4's wouldn't centifically apply at high RPMs. If you remember, the tech agency ATSG told us to leave these out of everything.
Ever pull an older 700R4 apart and find the teeth on the 3-4's worn, but the material still look good? Some looked this way after even 200,000 miles...Then, we got load release springs. The reason for GM switching to kolene steels was because of 3-4 clutch failure. They knew the springs caused the pack to apply slower, and the kolene gave it more chance to tolerate the slip the springs brought with them. How many load release spring transmissions have you pulled apart that didn't atleast have hot spotted 3-4 steels? That's because of the slower apply of the 3-4's.
This is made even worse in a high RPM application, when the 3-4's need to come on even quicker. Any delay will allow the engine to climb higher in RPM's and bang the limiter. This can be seen in many data logs that show beginning and ending shift times.
I worked at a shop for many years that was an R&D shop for many aftermarket 700/4L60E parts. If it's out there, we've tried it...LOL We had our own machine shop and made many parts that are now sold through Sonnax. I helped develop the original dual piston Sonnax O/D servo. The prototypes were used in my 700's. We started doing things with 700/4L60E's that other people thought was impossible. Mainly because we found solutions to all of their weaknessses. In the process, we proved the theories wrong on many aftermarket or "performance" parts widely available for these transmissions.
Grab the November issue of GM High Tech magazine. This is the most recent magazine we have a customers car in. The car is a 9 second, 4100# nitroused Impala SS. The 4L60E shifts at close to 7000 RPMs. It's had the same set of 3-4s in it for over 4 years...AND, he's making a 3-4 shift in the quarter mile...LOL. He launches the car in drive and makes a manual 3-4 upshift.
I'm not here to have a pissing contest or try to intentionally try to prove anybody wrong. I'm here to help. If anyone wants to know why I do or don't do something a certain way, I'll try to explain it the best I can. I hope this explanation helps people understand the problems the load release springs create, but the cause of 3-4 clutch failure go far beyond just those springs. GM designed quite a few "flaws" in these transmissions that contribute to their failure as well.

Frank