For the sake of simplicity and space I primarily cover a C4 teardown in this book. C4 teardown technique generally applies to the C6 as well, though there are some distinct differences in the ford c6 transmission and ford c4 transmission. You’re going to be looking for the same elements of wear regardless of the type of vintage Ford automatic you’re working with. Much of this may also be applied to the FX, MX, and FMX transmissions; the basic design and function isn’t much different than the C4 or C6, aside from the planet package mostly, which is different on the FMX.
When automatic transmissions fail, there are physical reasons that can be identified when you pay close attention to details. Most of the time, transmissions fail from wear, tear, and neglect. As a matter of practice, motorists never service automatic transmissions until they do fail. However, transmissions need the same kind of attention engines do. They need regular fluid and filter changes if you want longevity.
Dirty fluid is a transmission’s greatest enemy because it damages seals and causes excessive wear. And as fluid does its work, it deteriorates over time from heat and additive breakdown. As seals deteriorate, so does line pressure, which causes slippage and failure.
There are also irregularities in parts and assembly technique that can cause failure. Improperly machined or incompatible parts, dirt, or grit missed during assembly, and improper assembly of parts can cause failure. And, if missed a second or third time, failure is inevitable again unless you catch the problem this time. The buck stops with you, the transmission builder.
Transmission failure is normally a chain reaction—a series of small events and disruptions that ultimately lead to failure and a tow truck. The normal pattern of failure is at first seal wear/damage, which leads to weaknesses in hydraulic pressure, which leads to clutch and band slippage, which puts contaminants (friction material and metal) in the fluid. This damages seals further, hinders control pressure, and causes slippage and failure.
There are also types of mechanical failure, such as roller clutches and bushings, which fail with great regularity. When a roller clutch falls apart, you’re not going anywhere. Clutches and bands burn up from slippage and extremes of stress such as towing or racing. Transmissions have an incredible job to do and must do it for thousands of miles, often without the benefit of service.
Transmissions also fail due to improper assembly technique in transmission shops. Whether it is the mass rebuilder or an independent shop, plenty gets overlooked in the course of a transmission teardown and rebuild, which leads to the same kind of failure again and again.
Study wear patterns and anything else out of the ordinary during teardown. Look for parts that haven’t functioned well together, evidenced by scoring and other damage. Just because the transmission functioned normally doesn’t mean it was properly assembled. Mass rebuilders, for example, toss parts in bins during the rebuild process with reckless abandon, clean them up, machine, or replace as necessary, and put transmissions together haphazardly. Transmission failure under warranty is considered part of the cost of doing business. As a matter of economics, rebuilders reuse hard parts that should be machined, rebuilt, or tossed in the recycle bin so they can never threaten another transmission’s lifespan. Carelessness also means generations of transmission parts thrown together that aren’t always designed to work together.
Ford is infamous for annoying engineering changes, dozens of them over the production life of a unit design, which means you must pay very close attention to parts intended to work together and parts that were never intended to be together. The C4, for example, had at least three major production changes between 1964 and 1971 that are very significant, which means you can wind up with parts that don’t even fit, let alone work well together. Also, changes came well after 1971 as durability requirements increased in the late 1970s.
Step-1: Remove Servo and Piston
Disassembly begins with external parts like the neutral safety switch, kickdown linkage, and any brackets. First remove the low-reverse servo cover and piston (C4 and C5 only). Inspect the piston for damage and a worn seal. Replace the low-reverse piston in any case.
Step-2: Remove Throttle Valve
Remove the throttle valve (vacuum modulator), including the valve body control rod (pin) inside the case. C4 transmissions from 1964 to 1971 have screw-in throttle valves. From 1972-on, expect to see a pressed-in throttle valve with retaining bracket. The same can be said for a C6 with a screw-in throttle valve from 1966 to 1971, and then press-in from 1972-on.
Step-3: Remove Pan & Valve Body
Pan and valve body are next, using a 1/2- inch socket for pan bolts and both 3/8- and 7/16-inch for valve body and filter.
Step-4: Remove Tailshaft Housing
The tailshaft housing comes off next, using a 9/16-inch deep-well socket.
Step-5: Remove Governor (Important!)
Remove the the governor and check for freedom of valve movement. Both primary and secondary valves should move freely, detected by a rattle. There should also be a tiny filtration screen in the tailshaft governor flange. But be prepared; most are gone and you may need to replace yours. See Transtar or your local transmission parts supplier for this screen.
Step-6: Inspect Governor (Critical Inspection)
Governor assembly has two points of inspection: primary and secondary control-valve pistons for freedom of movement and spring integrity. It’s a good idea to disassemble the governor and do an inspection, taking note of how it comes apart.
Step-7: Remove Servo
Remove the intermediate band servo, checking cover and piston for any damage or abnormal wear. Remove the seals at this time. This is an “A” servo, which is the standard V-8 intermediate servo for compact and intermediate-size Fords. You’re going to want an H or R servo, though it seems everyone wants the “C” 289 High Performance servo. The H servo for big Fords and trucks is plentiful.
Step-8: Remove Band Struts
Remove and inspect intermediate and lowreverse band struts. The C6 (not pictured here) doesn’t have a low-reverse band, but instead a low-reverse clutch package, which is part of the main case.
Step-9: Inspect Geartrain
With band struts removed, you can see the C4’s geartrain package.
Step-1: Remove Pump, Input Shaft & Forward Clutch
The front pump, input shaft, and forward clutch come off next. With pump bolts removed, pump and forward clutch come right out.
Step-2: Remove Intermediate Band
After forward clutch and pump are out, remove the intermediate band.
Step-3: Inspect Intermediate Band (Critical Inspection)
Inspect the intermediate band for worn and burned friction surfaces. A burned intermediate band is a sign of poor adjustment or inadequate servo pressure. It’s a good idea to replace the intermediate band regardless of condition. Be choosy about replacement band quality; this is no place to cut corners.
Step-4: Disassemble Front Pump (Critical Inspection)
Next, disassemble the front pump and inspect for abnormal wear patterns. Mating surfaces should be smooth and void of scoring. Excessively worn gear and pump-cavity surfaces are grounds for replacement or machine work. Scoring adversely affects pump pressure. Inspect pressure relief valves for sticking, proper spring pressure, and any debris.
Step-5: Remove Gear Shell
Remove the Sun gear shell, forward planet, and ring gear and inspect for wear and damage.
Step-6: Remove C-Clip
Remove the tailshaft C-clip next, which releases the tailshaft.
Step-7: Remove Governor Support
Remove the governor support assembly, along with two pressed-in hydraulic lines.
Step-8: Remove Parking Pawl
Remove the parking pawl. Inspect gear, pawl, and thrust washer for damage and wear. Anything marginal (cracks, chaffing, wear) should be replaced.
Step-9: Remove Ring Gear & Thrust Washer
Ring gear and thrust washer are next to be removed. Examine each thrust washer for wear. Plan to replace all thrust washers and bushings.
Step-10: Inspect Forward Planet Carrier (Critical Inspection)
Forward planet carrier does most of your C4’s work during acceleration and upshift. Do a complete inspection, checking planet gears for excess wear and oscillation. Operation should be smooth and without oscillation. Any resistance to rotation (binding) is reason for discard or rebuild. As a rule, this is not a serviceable piece.
Step-11: Inspect Forward Planet Torrington Bearing (Critical Inspection)
Inspect forward planet Torrington bearing for wear. Most of the time, it’s a good idea to replace this thrust while you’re in there.
Step-12: Inspect Rear Planet Carrier (Critical Inspection)
Rear planet carrier gets less wear and tear. However, it calls for the same kind of detailed inspection as the forward planet, checking planet gears for excessive wear and oscillation and the carrier for cracks and other damage.
Step-13: Remove Tailshaft Sealing Rings
At this time remove the tailshaft/governor support sealing rings. These are not gapless rings as found elsewhere in a C4 transmission. Always replace sealing rings in any case.
Step-14: Remove Front Pump Sealing Rings
Front pump/ forward clutch sealing Rings are gapless types. They should also be removed and replaced.
Step-15: Remove Sealing Rings
Remove forward clutch/ front pump sealing ring as it sits inside forward clutch. Contact surfaces should be free of scoring and excessive wear. Any damage calls for machining or replacement.
Step-1: Disassemble Reverse-High Clutch Assembly
Disassemble the reverse-high clutch assembly. This is an early C4 reverse-high clutch with 10 return springs. Later models have one large return spring. Piston seals are removed at this time. Check clutch piston for wear and check ball integrity.
Step-2: Inspect Reverse-High Clutches (Critical Inspection)
Inspect reverse-high clutches for wear and evidence of slippage and burning. Also inspect clutch drive plates for slippage and heat damage. Heat damage shows up as brown, black, or blue score marks. If drive plates look undamaged, all you have to do is dress them with fine sandpaper (280-grit). If they have heat damage, they must be replaced. Not all transmission builders agree with the idea of resurfacing drive plates (steels) because they don’t come out of the box machined to begin with. The same can be said for clutch drums.
Step-3: Disassemble Forward Clutch
Next, disassemble the forward clutch by removing perimeter snap-ring, clutches, and drive plates.
Step-4: Remove Belleville Spring
The forward clutch piston is returned to rest by a Belleville spring, also known as a disc spring. A snap-ring retains the C4’s Belleville spring.
Step-5: Inspect Low-Reverse Band Servo (Critical Inspection)
Inspect the lowreverse band servo piston for seal damage. It is a good idea to replace the low-reverse band servo piston as a matter of practice regardless of condition because the seal is probably hard and worn from heat. Seal and piston are bonded into one assembly. Seal leakage (loss in pressure) leads to transmission slippage and failure.
Step-6: Inspect Intermediate Band Servo (Critical Inspection)
Inspect the intermediate band servo piston for wear and check-ball function. Seals must always be replaced. Take note of the type of seals and how they’re installed during disassembly. Some builders suggest keeping the old seals installed during disassembly and cleanup to reduce the risk of installing incorrect seals or installing seals incorrectly.
Step-7: Remove Low-Reverse Band and High-Reverse Clutch Drum
Remove the low-reverse band and reverse-high clutch drum and inspect for wear, heat damage, and scoring. As a rule, low-reverse bands are pretty hardy and hold up quite well. The best low reverse band to buy is new-old-stock Ford or OEM if you can find it, which offers the better friction materials than you see today from the aftermarket. When new-old-stock isn’t realistic, go with the best low-reverse band money can buy. Transtar or Trans-Go are among the best sources.
Step-8: Remove One-Way Clutch (Documentation Required)
Take note of how the rollers and springs are installed when you remove the oneway clutch. Take pictures of this assembly prior to disassembly. It is easy to improperly reinstall rollers and springs. Be extra careful here. The roller clutch’s job is to allow low-reverse drum rotation one way, but not the other.
Step-9: Remove Manual Shift
Next, remove the manual shift and kickdown linkages by removing the retaining nut inside with a 7/8-inch open-end wrench. This frees up the manual and kickdown linkages. Take note of how these “shaft within a shaft” levers are installed. There’s an outer manual-shift-lever lip seal and an inner kickdown lever O-ring seal to remove at this time.
Step-10: Use Flat-Blade Screwdriver (Professional Mechanic Tip)
Manual shift lever lip seal pops out with a large flatblade screwdriver.
Step-11: Inspect Ring Gears (Critical Inspection)
Inspect ring gears for abnormal wear. Leon’s Transmissions strongly suggests disassembling ring gear assemblies because metal and stray friction material gets trapped here, which can later cause failure.
Step-12: Inspect Thrust Washers (Critical Inspection)
Closely inspect the thrust washers, which provide a loadbearing surface between rotating geartrain parts. Measure thicknesses and closely examine these washers for wear. They can be reused if no real wear is found.
The front pump has a nylon thrust washer (number-2), which you should replace if it exhibits heat damage. This one is very discolored from excessive heat damage and should be replaced. Washers number-1 and -2 determine geartrain endplay, which makes them known as “selective” washers because they come in various thicknesses to help set proper geartrain endplay as it relates to the front pump.
Step-13: Remove C-Clip (Professional Mechanic Tip)
Ford’s C4 and C6 transmissions are among the easiest in the industry to rebuild in your home garage. Transmission shops have special spring compressors for clutch disassembly; however, you can do this at home with four C-clamps. This makes it easy to remove the C-clip.
Step-14: Remove Clutch Pistons
Use compressed air to remove clutch pistons. Turn clutch drum face down for this procedure to protect your eyes and face. Piston should pop right out.
Step-15: Note Any Clutch Wear
Clutch wear is a strong indicator of transmission health. When clutches are badly burned from slippage, ask yourself why there was slippage to begin with. The same can be said for band heat damage. Ask yourself why there was slippage and be diligent in finding the cause. Most of the time, slippage is caused by insufficient line pressure at clutches and bands. Surface irregularities can also cause slippage and heat. Hot spots indicate high and low spots— irregularities in surfaces. When in doubt, replace. These steels have excessive scoring and should be replaced.
Step-16: Use Compatible Parts
If you have to replace the input shaft or forward clutch, make sure you’re getting Compatible parts. There are three basic scenarios: 1964–1969, 24-spline both ends; 1970- only, 26/26-spline both ends; and 1971-on, 24/26-splitspline. The best-case scenario is 26/26 if you can find it, which was originally 1970 only. However, the aftermarket can help you with a 26/26 shaft and forward clutch.
Step-1: Prepare For Disassembly
Although the heavy-duty C6 transmission has similar architecture as the C4, it does have differences that need to be addressed. Bellhousing and main case are a one-piece design. Instead of a low-reverse clutch drum and band, you have a low-reverse clutch tied to the case. This is a mid- 1970s C6 transmission.
Step-2: Note Bolt Locations
C6 valve body removal involves 3/8- and 5/16-inch sockets. Take note of bolt locations.
Step-3: Remove Servo (Documentation Required)
Like the C4, the C6 has an intermediate servo and band. Servo removal is the same as C4 removal using a 1/2-inch socket. Remove servo piston seals and take note of what kind of seals your servo piston has. The most popular C6 intermediate servo is the R servo (the largest) which provides solid band hookup.
Step-4: Remove Pump
C6 front pump removal is carried out the same way as C4, including input shaft removal. When the pump is out remove the forward clutch.
Step-5: Remove Forward Clutch
Forward clutch assembly comes out just like the C4’s and appears identical except for its larger size.
Step-6: Remove Forward Planet Carrier (Critical Inspection)
Forward planet carrier from this mid-1970s C6 is aluminum for weight reduction. Expect to see iron or aluminum. As with the C4’s forward planet, inspect planet gears for wear and damage. There should be no oscillation around each planet gear’s centerline or resistance to being turned. Each gear should roll smoothly without sideplay or much endplay. Inspect the carrier for cracks or damage.
Step-7: Remove Rear Planet Carrier
C6 rear planet carrier works exactly like the C4’s rear planet, which fits right into the sun gear shell. And like the C4’s rear planet, this is also an aluminum carrier.
Step-8: Remove Tailshaft C-Clip
The tailshaft is secured with a C-clip. Remove the C-clip and free the tailshaft.
Step-9: Remove Low-Reverse Hug, Ring Gear & Roller Clutch
After the tailshaft is free remove the low-reverse clutch hub, ring gear, and roller clutch. Ring gear fits inside the low-reverse clutch hub.
Step-10: Remove Low-Reverse Clutch
With snap-ring remove the low-reverse clutches and drive plates. The low-reverse clutch package does the same thing the low-reverse drum and band do in a C4. Clutches are quicker and more durable than bands and drums.
Step-11: Note Parking Pawl
Parking pawl and gear on the C6 is similar to those in the C4—quite visible when the governor support is removed.
Step-12: Remove Roller Clutch (Documentation Required)
Free the one-way roller clutch with a 7/16- inch socket. Again, take note of roller and spring configuration.
Step-13: Inspect Reverse-High Clutch (Professional Mechanic Tip)
Examine the reverse-high clutches for wear and replace if indicated. As with the C4, examine friction and steel surfaces for heat damage. Drive plates should be resurfaced to improve hook up, or replaced if there’s heat discoloration. Heat damage cannot be machined out of steels. When there’s heat damage, temper in the steel is lost and they should not be reused. And because steels are cheap, there’s no excuse for not replacing them.
Step-14: Clean Parts
Use a heated chemical washer to clean all transmission parts.
Step-15: Remove Clutches With C-Clamps (Professional Mechanic Tip)
Like the C4’s clutches, you can use C-clamps for C-clip and spring removal instead of this spring compression tool found in most transmission shops.
C6 Teardown Notebook
Ford took what it learned from C4 development and production and applied it to the new heavyduty C6 transmission first introduced for the 1966 model year. The C6 was a lighter-weight replacement for those older FX and MX iron heavyweights used behind FE and MEL big-blocks and Y-blocks for so many years.
What makes the C6 different than a C4 is its one-piece bellhousing and main case design, plus a low-reverse clutch instead of a lowreverse band and drum. The lowreverse clutch package makes the C6 more rugged and certainly more reliable, making the C6 among the most durable automatic transmissions ever produced.
Ford then took what it learned from the C6 and applied it to the heavy-duty E4OD overdrive transmission that came in 1989. Inside every E4OD are modified and improved C6 internals.
Beyond the E4OD has been the 4R100, which is basically an E4OD. Unfortunately, little if anything from the 4R100 fits your C6.
Brian Fortune, of Tom’s Transmissions, performs the following C6 teardown and buildup. Along with TCI Automotive he shows you how to build a better-performing C6 transmission using E4OD geartrain components, which provide durability and better gearing for improved acceleration. I don’t kid myself about the C6 though; it is not a drag racing transmission because it is too heavy and eats up a lot of power. For tow vehicles and classic Fords, it is an outstanding piece because it was so rugged.