(Editor's note: Author Michael Czap submitted this story to MM&F, in the hope that his insights on the popular T-5 transmission would be of some benefit our readers. We also consulted with Tremec to bring you this detailed examination of the T-5 trans.)
The T-5 five-speed transmission has been a staple in the late-model Mustang world, and it has been a favorite for those who are swapping a late-model drivetrain in their vintage Ford as well. Countless Fox-body Mustang pilots have powershifted their way to low-elapsed times, street racing glory, and unfortunately, broken or bent shift forks.
I've been rebuilding a lot of T-5 transmissions lately, for both friends and other people, to make some money on the side. If you have been in this hobby for any length of time, you know a good V-8 spec T-5 is a valuable commodity. Since handling several dozen of these transmissions over the years, I've noticed several trends that have caused me to raise an eyebrow.
The biggest transmission killer I have seen is the use of the incorrect fluid. All of the traditional manual transmissions (Top Loaders, Muncie's, Borg Warner T-10s) used 75w-90 gear oil. The early non-world-class ('83-'84) T-5, evolved to use standard 10W30 motor oil. These transmissions use brass blocker rings that are most often incorrectly referred to as synchronizers. To set the record straight; if the rings are lined (no clutch teeth) they are called friction rings. Synchronizers are the hub and sleeve assemblies that operate friction rings.
Since the mid '80s, more modern transmissions make use of thinner automatic transmission fluids, such as the Dexron III/Mercon recommended for the world-class (WC) T-5 (1985-present). The reason for this fundamental switch in fluid types is the result of new technology being utilized in the synchronizers themselves. They consist of a fibrous clutch material lining bonded to the friction rings of the synchronizer assemblies. The clutch material composition in the earliest WC T-5s was similar to the clutch packs in every single automatic transmission. As time went on, Tremec refined the friction lining materials to the specific design parameters of cone clutch synchronizer systems. Many people refer to these linings as the carbon-fiber linings. Since the implementation of this friction material, ATF has been the required lubricant in a manual transmission.
Back when Ford introduced this, it produced several positive benefits. It enabled much smoother and quicker shifts, as now their new highly effective, three-piece synchronizer assemblies basically doubled the amount of friction material on First and Second gears. Third and Fourth gears now employed only a single lining of this clutch material, as shifting into these gears does not work the transmission as hard.
The measure of transmission efficiency became more important. That is, the notion of spin-loss and its control was a new focus. The thinner ATF allowed the gears in the transmission to speed up and slow down much more quickly, increasing the crispness and speed of the shifts. But by far the biggest impact of the thinner ATF was to extending fuel mileage. That was very high priority in the mid '80s with the fuel crisis scare.
I'm not that old (38), but when I was eighteen, I worked in a small local garage. I remember that using ATF in these manual transmission cars seemed quite an unorthodox practice. It contradicted everything people previously understood about manual transmissions up to that point. If people did not abuse the little T-5 and serviced it with the correct ATF, its service life was quite remarkable with the abuse and performance most owners could dish out. The big problems arose when people started deviating from Fords service recommendations. They stuck to their hard-headed ways of, "Every other manual transmission I've ever worked on used regular gear oil. Why should this one be any different?" Then very soon after servicing up their WC T-5 with some traditional gear oil or other additive, more serious shifting problems usually followed.
 This is a three-piece synchronizer assembly. Note the friction material is only bonded
 Here are a few three-piece synchronizer assemblies. The good unit is on the left, the
 On the right, this First gear synchronizer assembly is worn to the point that the ears
 The unit on the right is the Second gear synchronizer assembly. It is so worn that it
 These pictures show how the orientation inside the transmission can change with good a
 These are one-piece blocker rings used on Third and Fourth gears. The ring on the left
 The good one-piece blocker ring is shown installed onto the back of the input shaft clutch cone. Note the top portion is level with the input shaft and the pointer is showing the gap created between the clutching teeth.
 The worn synchro shown here is installed onto the same input shaft. Note how far down onto the clutch cone it is oriented. The clutching teeth are resting on the input shaft tooth surface. What you can’t see is that there is no friction surface contact, as the blocker ring cannot come down any further onto the cone. Much like a ball cap hitting your ears before it gets tight onto your head
The way synchronizers work is by slowing-down/speeding-up its particular gear as pressure is applied via the shifter handle. Once the gear speeds are matched, the clutching teeth become "synchronized" and the transmission will drop into its next higher or lower gear. So synchronizers are effectively little brakes. That is why slow steady shifts usually get you into gear very crisply as the synchronizer has the time to slow the gears enough for smooth alignment. When you shift too quickly, the transmission will sometimes grind or get a few nibbles of the clutching teeth before it slips into gear, as the synchronizer has much less time to slow down the gears inside the transmission. The ATF works very well with this matched friction lining on the synchronizers to allow a smooth and effective shifting action for tens of thousands of upshifts and downshifts.
Now, back to the use of thick 90W gear oils... For the most part, the lubricating and friction properties of gear oil is quite different than that of ATF. Using gear oils will quickly saturate into the porous fibers of the clutch material of the WC T-5 synchronizer assemblies. Once that happens, the synchronizer is permanently contaminated and will never regain its previous performance.
To be more precise, it's actually the chemical sulfur that is what attacks the organic friction material itself. The sulfur actually breaks down the chemical bond of the minute particles composing the organic friction lining (The "glue holding the fibers of the lining together, not the glue holding the fiber material to the core ring). I have used every petroleum solvent I know of, as well as heated ultrasonic cleaners and have not been successful in restoring a contaminated synchronizer. Most people know all too well the lingering stench that gear oil has around the garage because of the sulfur content. Even after the smell is gone, the oil stain in the porous concrete garage floor or asphalt driveway can linger almost forever. Once contaminated, this type of synchronizer can no longer generate the required friction to enabling effective shifting performance. Oftentimes, when a transmission gets noisy from bearing wear or actual gear damage, people install thicker gear oils or additives. While this will make the transmission a little quieter (much like installing 90W gear oil into an engine with a rod knock), doing this unknowingly just contaminated the friction materials of the synchronizer assemblies, and now the damaged transmission rapidly starts destroying itself.
The first indication of this terminal condition will be labored shifting. The driver will now need to shift more forcefully to achieve the previous shift performance. The contaminated friction material in the synchronizers can no longer generate even a fraction of the previous amounts of friction required to synch-up the gears. This excess force can actually start to burn the friction linings—a condition that also drastically alters the friction producing qualities—so they will produce increasingly poorer quality shifts as time goes on. More friction is also required in order to drive the gears now that they are moving through a bath of the heavier gear oil. These excess forcing and shifting efforts usually smash flat or will crack the plastic shift fork pads.
As the friction materials gets worn away, the tolerances of the synchronizers change much the same way you adjust a baseball cap to your head. The eroded clutch material makes the synchro fit further down onto the clutch cone just like a larger sized hat fits further down on your head. Soon it is so far down you will not be able to shift the gears, or the hat will now be so low it will cover your eyes. This is called wear gap. As the material wears away, the gap is reduced until there is none. No wear gap will result in a lack of frictional assist in synchronization, culminating in a full clash shift or grind. These problems continue to compound, and there are not many miles before the shifting quality will degrade to a point that the transmission is unserviceable.
I hope that the data and visual images will convince you of at least one thing—use only approved fluids in the transmission you are working on. The next time you see or hear of someone about to put anything other than approved fluids into their world class T-5 transmission, you can give them a few kind words of advice.
Take a look at the included photos, as they visually point out the effects of using improper lubricants in a WC T-5. There's a reason why manufacturers specify a certain lubricant for certain parts. While no lubricant can prevent damage from improper shifting techniques or abuse, simply sticking to the instructions will provide a long service life in your T-5 transmission.