Michael Czap
December 31, 2013

While the Top Loader transmission will always hold a place in our hearts for the untold gear exchanges it has made in our classic Fords, many people are turning towards overdrive transmissions for better fuel economy and added strength. Tremec remains the go-to standard for brand new gearboxes, but if you like to have a more hands-on and/or budget-conscious approach, rebuilding a used unit might be right for you.

As a longtime Fox Mustang enthusiast, this author remembers the early ’90s 5.0 craze well. The Fox Mustang was a clean slate. The engine wasn’t that great, nor was the suspension, brakes, rear axle, chassis, or transmission for that matter. But Ford built plenty of durability into those stock components—the engine, as well as the rearend, could handle just about everything you could throw at it in that time period. It became apparent, once you added some horsepower and started running slicks or power shifting, that the T-5 transmission would start showing its weakness. It was not so much a weakness, but more to the fact that it was being asked to perform at a level beyond its intended design.

Later T-5s were the strongest, and soon the ’93-’95 Cobra T-5 “Z” internals were the hot ticket to build the highest torque rated T-5s of the time at 330 lb-ft. But the bolt-ons still kept coming and the horsepower levels kept rising. Nobody had much to offer in the way of a heavy-duty transmission that would handle the newfound levels of horsepower and torque. Some resorted to installing Top Loader four-speeds for track duty, and the more affluent racer sprung for boxes from Jerico and Liberty. There was nothing really affordable or practical for the street until the Tremec 3550 and TKO transmissions debuted.

At the time of the 3550’s release, the GM folk were all about the T-56 six-speeds in their new F-bodies. But to the Ford gang, the five-speed 3550 was a godsend. Torque ratings were very conservatively rated at 350 lb-ft for the 3550 and 375 lb-ft for the later TKO. Ford even saw fit to install the 3550 into its ’95 Cobra R behind a 351 Windsor. Times were pretty good for a while.


Tremec transmissions may seem daunting to work on, but they actually require fewer special tools to rebuild than a T-5 does. Simple handtools and a pair of expanding ring pliers are about all that is usually needed.


Back then, these transmissions retailed for around $1,200, and you still needed to buy a new bellhousing as well as perform a few modifications to the car to get it to fit. Adjusted for inflation, that price has easily doubled for the new Tremec TKO 500 and 600 series transmissions.

These transmissions have been coming up for sale more and more frequently in the recent years, but unfortunately, many have been abused and lived hard lives. Let’s face it; someone didn’t pony up the cash for an expensive transmission because he wanted to install it behind a stock 5.0 daily driver. No, more likely the owner blew up his T-5 because of the power upgrades and/or racetrack use, so an upgrade was required.

1. After purchasing the transmission, we disassembled it, inspected the components, took stock in what it needed, and placed everything neatly in a storage container until the replacement parts arrived.
2. For the novice transmission rebuilders out there, rebuild DVD’s can be purchased from vendors such as Hanlon Motorsports, and will guide you step-by-step through the entire process.
3. This simple synchronizer, or more accurately, “blocker ring” kit also includes a small assortment of other parts and can be purchased for around $100. This kit came included with the transmission, as the previous owner thought that was all that was needed. Not so much. There are aftermarket carbon-fiber-lined synchronizer kits that can be substituted. They tout faster shifts at higher rpm than the original components.
4. Severe-engagement tooth wear was evident on the 1-2 and 3-4 sliders. The 1-2 slider assembly is a big-ticket item at almost $300 by itself. Here you can see the difference between the old warn down sliders and the new $130 3-4 sliders. The old teeth aren’t merely rounded off, but heavily chipped as evident by the jagged edges.
5. Clean the mating surfaces gently with razor blades, scrapers and Scotchbrite pads. Power tools can quickly gouge the aluminum and are not recommended. These transmissions have precision-machined surfaces and have much tighter build tolerances than you might think. It doesn’t take much to cause case or cover misalignment.
6. The countershaft drops right in without the need for any special tools.
7. Don’t lose or damage this bearing. It’s made in Germany and costs $150 to replace.
8. The Reverse gears and Fifth-Reverse shift arm are installed next.
9. Don’t fear these needle bearings. Once you use some grease to install them, things quickly start shaping up. Tremec transmissions have very robust bearings and rarely ever need replacing.
10. Sticky with grease, the bearings are now installed onto the Fifth gear clutch cone.
11. Here we have the Tremec 3550 (upper) and TKO (lower) mainshafts. The 3550 shafts can break where they neck down toward the yoke splines, or even twist the splines themselves. Bringing a driveshaft yoke to test for twisted splines is a good idea when evaluating a used 3550/TKO 500 for purchase. The early TKO mainshaft can be retrofitted into a 3550 by simply machining the tailshaft housing to utilize the larger C-6–style tailshaft bushing. These mainshafts also store perfectly in old camshaft boxes.
12. Here’s a closer look at the differences of the 3550 versus the early TKO mainshaft, with a penny used for reference.
13. The mainshaft build starts here with it clamped in a padded vise.
14. Before you start reassembling the gears, make sure you clean up the burnished and polished surfaces left by the old brass blocker rings, as shown on this Second gear. This will ensure a fresh surface so the new rings can bed in quickly and more evenly.
15. Here is the mainshaft ready to install.
16. Here, we are installing the assembled mainshaft into the case, with the addition of the Fourth gear clutch cone. Yes, it does fit, eventually.

If you’re lucky, said owner hopefully was an intelligent fellow and installed his new Tremec transmission with care, while heeding to the strict transmission fluid and 500-mile break-in requirements. Again, this was probably not the case. Your author knows of at least four personal acquaintances who hurt their T-5s at the track one week, then melted the plastic on their credit cards the following week by buying and installing a new 3550/TKO, only to drive straight back to the track once it’s all buttoned up.

These folks are the first to complain about the Tremecs having poor shift quality, which was the primary complaint about this series of transmission. The heavy-duty internals also mean that the larger gear mass is just not going to be as quick to shift at high rpm compared to the owner’s previous T-5 transmission. Break-in was not meant to be 500 miles of highway driving, but 500 miles of city-style driving—up and down through the gears giving the brass blocker rings a chance to properly break-in. The shifting would indeed become easier if you followed the directions—go figure.

This author recently picked up an early Tremec 3550 and bellhousing to rebuild. These transmissions have inspection covers that are a lifesaver when buying used transmissions. Although being very clean externally, this unit had signs of heavy wear on the 1-2 and 3-4 sliders, denoting a generally abusive life. Purchasing a used transmission is a huge gamble, especially if you do not know what you are looking for. Never ever take a seller at their word. How often do people sell perfectly good transmissions? Performance parts have been removed from service for a reason. Simply spinning and wiggling the input shaft, while rowing the shifter through the gears is not an indicator of the health of the transmission. Internal inspections are a must. Parts for Tremec transmissions parts are about twice as expensive as for T-5 transmissions, so take that into consideration when striking a deal on a damaged box.

Tremec transmissions may seem daunting to work on, but they actually require fewer special tools to rebuild than a T-5 does. Simple handtools and a pair of expanding ring pliers are about all that is usually needed. The only press-fit part is usually the tailshaft bushing, but a machine shop can make quick work of that task. Follow along as we show you some of the highlights of rebuilding this Tremec 3550.


The early TKO mainshaft can be retrofitted into a 3550 by simply machining the tailshaft housing to utilize the larger C-6–style tailshaft bushing


17. The input shaft gets its new bearings and it’s installed through the front of the box.
18. The new input shaft seal gets installed next. Another nice thing about these transmissions is that they all have steel bearing retainers from the factory, unlike the stock fox T-5s.
19. At the back of the transmission, Fifth gear, along with its shifter fork and the speedometer drive gear, are installed next.
20. New shift fork pads are installed throughout the transmission. A few vendors sell aftermarket brass shift fork pads. If you think you will be putting the transmission through some abuse, they might be a good option for you.
21. Clean the surfaces well and lay down a thin bead of a premium, firm-setting RTV. These are precision machines surfaces, so a little dab will do you.
22. Rub the RTV across the width of the sealing surfaces with your finger. It goes a long way toward preventing leaks. Try not to get any down into the bolt threads. When you install the bolts, the excess can fill all the available space and prevent the fastener from being torqued down properly or even damaging the case threads.
23. Once the shift forks are lined up, install the cover and torque to 20 ft-lb.
24. This tailshaft bushing was quite scored, so we will be replacing it with the one provided in the kit.
25. A fresh tailshaft seal gets installed after pressing in the new bushing.
26. For the final assembly, it’s easier to have the transmission installed into a fixture. This author is lucky enough to have a vise large enough to tackle this job.
27. Once sealer is applied, the tailshaft housing can be installed and torqued to 50 ft-lb. Remember to install thread sealer on the top two bolts, as they protrude into the main case.
28. Mainshaft endplay is checked and brought into spec with the aid of shims installed into the input shaft bearing retainer.
29. The stock shifter parts are cleaned and installed onto the tailshaft. One complaint people have with the early Tremecs is the lack of a breather in the top cover. As the transmission warms up, they complain of leaks out of the input and output shaft seals. The factory shifter has a rubber seal that vents this pressure, but once an aftermarket shifter is fitted, that pressure relief is removed and it makes the seals the next path of least resistance.
30. Here, the transmission covers are installed and this unit is ready to be put back into service. A frequently heard Internet myth is that the early cases have weak mounting ears because they don’t have as much ribbing as later models. This author has seen just as many, if not more, new-style cases cracked compared to the early ones. One could attribute it to poor installation techniques and general abuse that no case would be able to withstand. We’ve seen a high-horsepower car with solid motor mounts, a poly transmission mount, and a weak chassis that has cracked the heck out of a Tremec case—hardly a design flaw.