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MODIFYING

TURBO COUPE DISC BRAKE REAR AXLE SWAP

For All 1983-88 Cougars / Thunderbirds
Revised September 9, 2016

More than likely you've got the 7.5", open-gear rear axle with the 2.73 gear in your car. While decent for fuel economy it's rather lukewarm for performance. If you don't have the Traction-Lok option, you're missing out on greatly increased traction on slick surfaces. And we all know how marginally adequate that drum brakes are. If you're pining to upgrade and want to keep 4-lug wheels, we've got the swap for you.

Turbo Coupe Rear Axle

A rear axle swap is an efficient way to get what you want. Now if you can find a 1988 Cougar/Thunderbird 8.8" rear axle with drum brakes and Traction-Lok, then that's one alternative. Problem is, they're extremely difficult to find, and only come with the 3.08 gear from the factory, at best. And the Mustang 8.8" is not as wide from the factory as one made for our cars, so that would necessitate new rims with a greater offset or tricky axle shaft/brake swaps. But, if you want a factory-width 8.8" axle with a great 3.55 or 3.73 gear, Traction-Lok, axle dampers and disc brakes to boot, then you need look no further than to our sister car, the 1987-88 Thunderbird Turbo Coupe.

Because the Turbo Coupe was powered by a 2.3L 4-cylinder engine it required a really tall gear to get the car moving. Therefore, you're going to get an excellent gear from the factory. The automatic Turbo Coupes have the 3.73 gear; manual cars came with the 3.55's. There is hardly a difference between the two gears, from a seat-of-the-pants perspective, but the difference is there. The downside to the whole conversion, besides a little plumbing, is that you have the potential to lose approximately 1-2 mpg with the taller gears (although several people have reported an actual increase in fuel economy). And, the 8.8" rear axle is a bit heavier than the old 7.5". This should not deter you, however, from aspiring for this conversion. The benefits far outweigh everything else. This is fast becoming one of the most popular swaps for our Cougars due to its relative ease of installation and plentiful sources.

There is a Troubleshooting section at the end, so if you run into any problems during or after installation, or you just want to check things before you install, the information is here for you.



Parts Needed


You will need the following items from an '87-'88 Thunderbird Turbo Coupe:

  • Rear Axle Assembly
  • Emergency Brake Cable Brackets
  • Quad Shocks and Brackets (optional)

You will need the following new items:

  • Brake Master Cylinder
  • Proportioning Valve * (see below)
  • Proportioning Valve Plug * (see below)
  • 4 feet of 3/16" Brake Line
  • 14" of 1/4" Brake Line
  • New Rear Brake Caliper Hoses (check old ones first)
  • New Emergency Brake Cables (check old ones first)



Rear Axle Assembly

This is the axle and all related components (rotors, calipers, etc.). Most of the time the rear rotors can simply be sanded down and reused, so you may not have to buy new ones. The calipers can freeze up, so you may want to have them checked just in case. (See the Troubleshooting section below for more info on these). The axle tag, located on the differential (aka "pumpkin") cover, will help determine the axle ratio—3.55 or 3.73—with a number and letter code. On the bottom row of the tag, you will see a code like '3Lxx', where the 'xx' represents the last two digits of the axle ratio and the 'L' indicates a locking center.


Emergency Brake Cable Brackets
Turbo Coupe Emergency Brake Cable Bracket

Since the Turbo Coupe has disc brakes, all the emergency brake cables will be different from your stock ones and will also need to be moved slightly. You will need all the related brackets for the cables as well as the intermediate spring assembly. Once you're under the car it's self-explanatory. These are proving to be some of the toughest parts to find for this conversion, so be alert.


New Emergency Brake Cables

Emergency brake cables will be needed to adjust the calipers periodically. The TC rear axle uses three cables: one that attaches to the pedal, and both driver and passenger side cables. These cables have been discontinued by Ford but aftermarket cables are available at good auto parts retailers. Before installation, it's recommended that you further lubricate the cables which will help prevent any freezing.

Raybestos lists these part numbers:

  • Driver's Side - BC93634
  • Passenger's Side - BC93638
  • Pedal to Junction - BC93658


Axle Damper (Quad) Shocks and Brackets (optional)
Quad Shock & Bracket

If you want to further increase handling and help control the twisting action of the rear axle, you can also get the Turbo Coupe axle damper (a.k.a. "quad") shock setup for your Cougar. These shocks are horizontal, not vertical like your main shocks, so they can greatly help to tame the axle's behavior. The Turbo Coupes all used them and the bracket is there on the rear axle for them. All Cougars and Thunderbirds from mid-1983 through 1988 have threaded holes in the rear frame rails for accepting the axle damper shock mounting brackets, so all you need is to bolt that bracket in and you're all set up. The cool thing about quads is that they don't behave like normal shocks; once you push or pull it in a direction, it stays there and does not retract under gas-charged pressure. Therefore they're easy to put in. They also last a good long time, at least twice as long as standard rear shocks. The stock axle dampers from the Turbo Coupe just may be fine to reuse. If not, they're usually inexpensive at parts stores; you may need to order them for a Mustang GT as an "axle dampener". It's also important that you try to get the stock bolts for the quad shocks. They're all discontinued and they're also metric.


Brake Master Cylinder

Rear discs can require additional brake fluid capacity, therefore may you need to replace the master cylinder. For 1983-86 cars, you will need to replace your stock one with a 1984-86 Crown Victoria master cylinder (1-1/8" bore). For '87-'88 cars with the aluminum body/plastic reservoir, you can more than likely reuse yours with no modifications. Several people have reported that it works just fine, with no low pedal feel or difference in braking whatsoever. Check the Troubleshooting section below for more info on this.


Proportioning Valve

Your stock proportioning valve is pre-metered for drum brakes (around 70% of the brake fluid to the front, 30% to the rear). Rear disc brakes, with their increased fluid volume, can handle more of the braking duties. This means you need to decide what to do with the brake proportioning. There are two schools of thought: gut yours and install an adjustable prop valve, or replace yours with a prop valve from a factory rear-disc Ford automobile.

Stock Turbo Coupe Proportioning Valve

Now several people have reported that they used a stock 1987-88 Turbo Coupe factory disc brake proportioning valve with no problems. Same with a (Fox) Lincoln RWD Continental. We've even heard from a person that used an old 1978 Lincoln Versailles prop valve. So long as the Ford car had factory rear discs you should be able to use it. Remember that you will be stuck with the factory brake bias; there is no adjusting that valve. But for most people that probably won't be much of an issue. Best of all, it should be a simple remove-and-replace deal.

Adjustable Proportioning Valve

The adjustable proportioning valve will give you a little more control over the front/rear bias. Essentially you will gut your stock prop valve of its spring and plunger, and install a fixed plug on the end (see below). Your stock prop valve then becomes a simple brake line junction where all the lines meet. The adjustable prop valve goes in-line, between the rear port on the master cylinder and the stock prop valve, and the rear brake line goes into the new prop valve. A good inexpensive unit can be purchased from Summit Racing or similar company. You will need some adapters and some short brake lines in order to fit this valve up to the master cylinder. We do not recommend using brass adapters, as they have the potential to leak; use steel ones if possible.

Fixed Proportioning Valve Plug

Disassembled Stock Prop Valve

Inline Adjustable Prop Valve

Left: Currently available as a Ford Racing part (p/n M-2450-A) as well as being available at Mustang performance websites, this fixed plug will reside on the end of your old proportioning valve. After the new master cylinder and prop valve are installed, and before you put in any fluid, you must essentially gut the stock prop valve. The end facing the front of the car must be removed and the spring and plunger inside must be taken out. The new plug will go on, and that's it. Be sure to save the old parts.

 

New 3/16" and 1/4" Brake Lines

1983-86 Cougars have the stock 3/16" brake lines from the master cylinder to the back, along the passenger side framerail. On 1987-88 non-turbo cars the lines can be on the passenger side also; the Turbo Coupe the brake lines ran down the driver's side and were 1/4" to handle the higher volume of fluid. However, it is very expensive and impractical to replace all your car's brake lines. Therefore, new lines on the rear axle itself will need to be installed. One side will use the 4' section of 3/16" line (driver's side on '83-86), and the other side will use the 14"; piece of 1/4" line. You will also need a 3/16"-to-1/4" adapter for the longer side, where the line enters the junction for the caliper hose. This is because you will use your stock brake junction; one side is 3/16", the other side is 1/4". We do not recommend using brass adapters, as they can leak fluid; steel adapters work better. You can purchase a line bending tool for about $7 or so; that'll help you get the funky bends you need to jump the center section of the axle. If this all sounds sort of strange, don't worry—it'll become clear to you as you attempt to hook up the brake lines.


New Rear Brake Caliper Hoses
Rear Brake Caliper Hose

While the rear end will come with brake caliper hoses already attached, chances are they may not be working to full capacity. Brake hoses will naturally decay from the inside out, meaning you won't spot the problem until the erosion is complete. The easiest time to replace them is beforehand; you don't need to do this but it's definitely worth considering. Price is around $30 US each. You will need a T45 Torx tool to remove the stock bolt that holds the brake hose bracket onto the axle housing.



MISC. PARTS & INFO


Banjo Bolt

You'll also need new brake line (a.k.a. "banjo") bolts to hold the brake hose onto the rear caliper. Each one comes in a kit with two brass washers (you need one bolt and two washers per side). Always use new brass washers when removing and replacing the brake line from the caliper. These bolts are available at auto parts stores as well as the Ford dealership. One washer goes under the brake caliper end, the other on top, then the bolt goes through all. Be sure to tighten the bolts really hard in order to crush the washers, creating a leak-proof seal.

The Thunderbird Turbo Coupe rear rotors are unique to that axle. You cannot use rear rotors from any other Ford car. The original rotors are made of organic compounds and cannot be easily turned (machined) like a front rotor, at least not without a special organic cutter. All that is usually necessary to clean up an original-style rear rotor is to use 150-grit sandpaper or to use a wire wheel to remove the rust. If the damage to a rear rotor is found to be significant, an all-new rotor must be purchased. New rear rotors are reported to be non-organic, meaning the should be able to be turned in a normal fashion.

Ford Rear Brake Piston Tool

When replacing rear pads with new ones, don't forget to have the piston turned in on the calipers. The notches must be in the 12 o'clock and 6 o'clock positions in order for the pads to fit properly. If you don't want to take the caliper off and have the piston pushed in by someone else, there is a special Ford rear disc brake piston tool that you can use with the caliper still on the axle. It's available at good parts stores. By the way, if you ever do remove the caliper and you're not changing the pads at all, you should simply need to reinstall them. There should be no need to have the pistons turned in.

You'll need new C-clips to hold the emergency brake cable in the caliper housing. They're available in a 20-pack at your Ford dealer, p/n 97413-S.

You will need to use your emergency brakes more often now, if you don't already. The cables help adjust the rear calipers; they need to be adjusted every few weeks or so. Also, this will keep the cables from freezing up due to not being used. If you haven't noticed already, e-brake cable freezing is a common problem with Cougars so if you're spending the money on this rear axle, you need to get in the habit of showing a little love to the e-brakes.

As with all lower (numerically higher) gear changes, you'll need to replace your transmission's driven gear with the correct gear. You have to do this in order for your speedometer to be fairly accurate. Now this has been a source of confusion from time to time, but in reality it's not too difficult to figure out. There are several online calculators where you can input your tire dimensions and drive gear (usually 7 or 8 teeth), and get the approximate number of teeth needed for your driven gear. Most people end up needing a 23-tooth gear. In years past this was a problem because the only 23-tooth gear available was for manual transmissions. While it worked, it also got shredded rather quickly and needed replacing every 5,000-10,000 miles or so. Fortunately companies are now offering specific automatic (AOD) transmission 23-tooth driven gears that should last a lot longer.

The speedometer driven gear is attached to the end of the vehicle speed sensor (VSS) that is bolted to the side of the transmission. One 11mm bolt holds in the assembly, then remove from the transmission housing. You'll see the colored driven gear on the end. One C-clip holds it in place.



FINAL THOUGHTS


This is a rather large job; a good weekend may be enough to get it done. The upper and lower control arms can be reused, as well as the sway bar, if so equipped. Always get NEW brake pads for the back; don't try to reuse old ones. Remember to use only DOT 3 or higher brake fluid; synthetic fluid is very affordable and should be seriously considered. You should need 2 quarts of fluid total. Bleed all 4 brakes—don't forge that air is now introduced to the lines. Make sure you bench bleed the new master cylinder to prime it up for use. You may need to bleed the brakes several times in order to increase pedal feel.

Before you install the new axle, remove the pumpkin cover and check the axle fluid to make sure it's fine. This is the absolute easiest time to change fluid if you need to. Again, we recommend synthetic fluid, with limited slip additive (available at your local Ford dealer). Check the old fluid for any metal shavings—if you find some, the ring and pinion gear may be bad. Get this inspected immediately! After installation, take it easy on the axle for the first few hundred miles just to get the fluid broken in. (Read: please don't take the car to the drag strip right away.) If you hear increased noise from the back while driving it could just be the noise associated with a lower (numerically higher) gear. You may also hear the rear brake rotors working when you hit the brakes; this is normal as well. If you hear a humming or whistling sound back there, though, get it checked ASAP!!!

When setting the front/rear brake bias, remember that you want the back brakes to lock up just after the front brakes, NEVER the other way around. This may take a few tries to get right. Try to get a nice open parking lot to do all your adjusting.



TROUBLESHOOTING


So maybe the rear end is installed and you've set everything up, except you're not stopping that well. Or maybe you have the brakes all the way to the floor and one of your rear wheels is still spinning. You've definitely got a problem somewhere, and this section should help you troubleshoot to find that problem.


FROZEN CALIPER

One of the most common problems concerning rear disc brakes is that the calipers will "freeze". This isn't quite accurate; either or both of the 2 slider pins have rusted shut into the lower caliper housing. This allows the rear wheel to spin freely at idle in drive gear with the brakes on—not very safe on an icy parking lot. The culprit will be a ripped rubber boot on the slider pin that allows water to enter, resulting in rust. You probably will not be able to free the pin, as it will shear off if you try to force it back out with a socket. That is good, though, for at this point, you CAN drill out the remainder of the pin. Just be sure to use a new 11/32" drill bit. It's a long ream (3" or so) but it will save you some money from buying a new lower caliper assembly. On the other hand, if you do wish to buy a new assembly, you will also need a new pin and boot set for these calipers (p/n E7SZ-2B296-A). They come with little packets of grease but you can just use regular bearing or high-temp grease to pack the new pins. Work out all air bubbles from the pin sleeves. After installing the new pins and boots, be sure to inspect the boots every oil change for any holes that could encourage water entry. This is about the only maintenance you need to do on a regular basis with this rear end, save for brake pad inspection.

If your brakes just won't adjust right, or you get a locked-up caliper or two, that usually signifies that the rubber hoses going to the calipers have internally collapsed. This condition will let fluid to the caliper, but not back out, meaning a locked-up caliper. You'll definitely need new hoses for the calipers (replace both at the same time) and then a re-bleeding ot the system. Please, above all, DO NOT attempt to lengthen the adjustable pushrod going into the master cylinder in order to increase pedal feel. Even one thread out from normal will result in too much pressure in the braking system, therefore locking up all 4 calipers in mid-driving. Trust us.


FROZEN PISTON

The piston in each rear caliper is a screw-in type; in fact, you will need to have them screwed in before you put new pads on. There are some rubber seals inside the piston chamber that rarely fail, but can, under extended use or repeated hard braking. If they fail, water again enters and the piston will rust inside the chamber. To fix this, you can purchase a piston rebuild kit from Ford (p/n E7SZ-2L128-A), and perhaps have a brake shop install it for you. In worst-case situations a new caliper may be needed, but a rebuild usually does the trick.


LOW PEDAL FEEL

So after all this effort, the pedal feels a little lower now with rear discs than it was with rear drums?!?!? What's up with this? Well, first you have to understand how rear discs and fluid correlate. A bigger bore master cylinder (like the one from the Crown Vic) provides a bigger reservoir of fluid for your rear discs, but less overall pressure; consequently, pedal feel suffers a bit. If you used your stock master cylinder, that gives you better pedal pressure while still having an adequate reservoir, although it's not as generous as the Crown Vic one. So either way, there are advantages and disadvantages. Having done both, we opted for the Crown Vic master cylinder for the bigger reservoir. As long as you don't have a fluid leak anywhere the stock master cylinder will work fine. But if you're worried about fluid shortage, then the Crown Vic master cylinder is for you; you'll just have to live with the slightly lower pedal feel. However, you can get better pedal feel back by installing stainless steel brake lines in the front. There are several companies that offer street-legal versions for the Mustang, including Russell, for the 11" Mustang front rotors and the stock 10" rotors. There is always the possibility that you may need a bigger brake booster to compensate. The 1994-95 Cobras provide a nice booster for us that fits rather well and greatly helps out.


LOUD SOUNDS

Hear a whole lot of action going on in the rear axle area every time you hit the brakes? Believe it or not, it's normal. Squeaking, on the other hand, means you're either getting low on pads, or there is excessive glazing on the rotors. Since the rotors and pads are both organic, pad dust is attracted to the rotor, and with heat added, you've got a wonderful recipe for glazing. You'll have a tough time removing the glazing with the rotor still on the car; it'll have to be taken off. Easiest way to remove the glazing is with a soft wire brush or 150-grit sandpaper and brake cleaner. Together, the combination will zip right through that foul stuff. Don't worry about roughing up the surface; the pads will actually like that better the next few dozen times you hit the brakes.


LEAKING BRAKE FLUID

If you find some brake fluid puddles, or you're low on rear fluid every time you check it, you obviously have a leak somewhere. The first place to check is the banjo bolts on each rear caliper. If the brass washer(s) were not properly crushed during installation, or if they moved off-center, that will create a leak. Best thing to do is remove the banjo bolt, install new brass washers, and tighten the bolt back up big time. If you're not leaking from there, check the connections at the brake line junction. If nothing still shows up, check under the hood at all the connections by the master cylinder and proportioning valves. Still nothing? Blown brake line somewhere—sorry.

If you still are having some kind of problem that isn't resolved by the notes above, just contact us and we'll try to work it out.

 

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