This is from the Burly4X4 website that has been shut down. Stefan helped me find the text, so I didn't have to retype it all. I had the pictures printed for instructions when I did this mod, and scanned them for this thread. Here goes... Ford C-4 Automatic Transmission Valve Body Modifications. (Stage 4) When working with the valve body, keep in mind that there are many sharp surfaces to cut your fingers on. Also, work in a very clean environment and be careful not to lose any of the small parts such as balls and springs. The following tips apply to all 1970 and newer valve bodies except the "Pinto" style units. Be sure to correctly identify your valve body before attempting any modifications. Remove the filter and pickup from the valve body. I will leave these pictures out, since they are basic, and only post the pictures that show relevant information. Remove the pressure relief spring and valve. Seperate the upper and lower halves of the valve body, and the seperator plate. The first modification is to enlarge two holes in the seperator plate. This will firm up the shift. The two holes are indicated in yellow. Use a 5/32" drill bit to open them up, then de-burr them by hand. Next, modify the intermediate servo accumulator valve. This valve is located in the upper corner of the valve body as shown in the first photo below. Remove the parts (cap, valve, and spring) from the valve body as shown in the middle photo. Install the valve and spring in reverse order as shown in the 3rd photo below. Remove the ball from the location in the lower valve body indicated in the image in the first photo below. Also remove the rubber plug from the location in the upper valve body indicated in the 2nd photo below. These mods also help to firm the shift. After cleaning the parts with solvent, dry them with compressed air. Re-assemble the valve body and seperator plate using a new gasket and be sure you don't leave out the balls indicated below in the lower (first image below) and upper (second image below) halves of the valve body. I forgot to download these two pics, and just modified other pics with the HERE. Ignore the finger that was in the pics before I modified it. Before installing the new filter, remember to drop the pressure relief valve and and spring back into the lower half of the valve body. MY ADDITION From my experience with a 35 year-old transmission, there was a really bad 2-3 flareup after I did this mod. When I got under the car to adjust the intermediate band, it had worn so much that I had to turn it in 3-4 turns to get it to seat. So, before you even try to drive the car, be sure to adjust your intermediate band. An excessively worn transmission, such as mine, might need it to be adjusted tighter than normal. Correct adjustment is to torque it down to 10 ft/lbs, then back it off 1-3/4 turn and lock it. I ended up just backing it out 3/4 or 1 turn and almost all the flare is gone. I still know it is there, but others in the car do not notice it.
The reason that you get a flare with those mods is that they are over done and incomplete. 5/32" is too much for a street mod. There are two more restrictors that need to be drilled and other valves that need to be modified. I will show the complete valve body mod in the "how to hot-rod your C4" complete with spring specs to safely increase the line pressure.
My flare was due (in most part) to 55,000 miles on the tranny and 35 years, and nobody ever adjusted it. I just never noticed it until I did this mod, then got under there, and was totally amazed that it was that the intermediate band was so loose... Also, I drained the 35 year-old trans fluid out an it had a good thick layer of the gray sludge (fine metal/clutch dust, I would guess). Then I refilled it with fresh fluid. I have heard that taking all that sludge out makes everything spin more freely, causing all kinds of slips. This may have added to the flare??? This was the first service this transmission has ever had, to my knowledge... Also, this is only the "Stage 4" method of 5 stages that Burly listed. I only downloaded the 4th, since that is what I used on my car. Mine is 98% street car, maybe 6-10 1/8th passes a year, so I did not do the more aggressive Stage 5 (which may have had the other modifications PaulS is talking about).
I have done seveal for my own use and for friends who drag race. Very nice results, shifts are extremely hard and instant. Course there are also other mod's to the clutch packs and clearance's are very close including the lateral movement of the gear train. Have had no flare using a billet servo or even the H one. Not sure the longevety of the stage four mod's in a street driven daily driver though. I also recommend the use of Castrol Type F fluid in them as well as powerglide's for racing. Seems to give less trouble and on teardowns, the internals all look very close to new after many passes on the trusty C4. All this depends on the condition of the hard parts and who built the entire tranny. JMO
The stage five mod is the best for all out racing applications, in my opinion. Although a full manual valve body would be my choice, for the money and learning experience, the stage five would do for most weekend warriors.
If your tranny has wear on it you should expect a flare in the 2-3 shift. The more wear in the direct clutches the worse the flare is. A "flare' is like going into neutral between shifts - it stops pulling in second and the engine races up and then it slides into high and away you go. It causes glazing of the band drum and wear in the direct clutches. If your tranny is still tight it is a shortcut to the mods that should be made to make it work properly. (see "How to Hotrod Your C4") The modifications are more complex but they work right in every tranny I have ever made them on.
I installed a B&M "shift improver kit" in a C4 many years ago and they provide you with a new seperator plate. The instructions told you to drill out those same two holes in the new plate to give your trans the "strip" or "stage 2" shift. Well I ended up with the same bad flare up that you guys are describing. After talking with a transmission shop guy I took out the new seperator plate and put the stock one back in but I think I only drilled one of the holes but with a smaller bit than what came with the B&M kit(this is the only part I cannot remember fully). The flare up problem was completely cured and the shifts chirped the tires nicely.
