1989 Mustang TKX Transmission Swap: A Stronger 5-Speed for the Fox Body


The Fox Body Mustang is becoming an increasingly popular platform for restoration and resto-modding, and it's easy to see why. It offers timeless style at an affordable price point. The Fox platform also has enormous performance potential for those of us who love to go fast. The 5.0L V8 and Borg-Warner 5-speed manual that came with this 1989 Mustang GT are a great starting point for tuning, but owner Gabe Flores — the Brand Manager here at Classic Industries — wanted performance on par with modern-day muscle cars. So, he teamed up with Mustang Hub Magazine and American Powertrain to install a Tremec TKX transmission that can hold even more power than the stock Borg-Warner T5.

Photos courtesy of Eric Geisert / Mustang Hub Magazine

Step-by-Step TKX Swap with Mustang Hub


In the latest issue of Mustang Hub, author Eric Geisert worked alongside Gabe Flores and Jason Scudellari to document each step of the process switching from the Borg-Warner T5 to the Tremec TKX. The transmission was sourced from American Powertrain and paired with the company's Hydramax hydraulic clutch system, Centerforce clutch package, lightweight Fidanza flywheel, and transmission crossmember, pictured on the table above.


Above: Flores removes the original Borg-Warner T5 transmission. It served the car's lightly-modified 5.0L V8 well, but it won't be capable of holding the massive torque of the 7.3L crate motor this Mustang will soon receive.

Why Swap from a T5 to a TKX?
You may be wondering why swapping from a 5-speed to another 5-speed is necessary at all, and that's a fair question. Flores plans to swap the original 5.0 V8 for Ford's new 7.3 "Godzilla" V8 crate engine, and that will mean a substantial increase in torque output — more than the original T5 can hold. So, the TKX will help this Fox Body Mustang put power to the ground reliably. It also offers better gear ratios, tighter shifts, and the ability to handle higher engine RPMs.


The Fidanza flywheel was installed using an ARP flywheel bolt kit. The green protectant will come off with use, but the team wiped it off with some acetone to ensure a clean surface right away.


Next, the aluminum bellhousing was test-fitted and carefully measured to ensure it was properly centered on the block.


The American Powertrain 26-spline clutch kit was assembled, and each bolt was torqued to spec using a cross pattern. It's important to make sure the alignment tool keeps the clutch disc centered and can slide back and forth easily.


American Powertrain's Hydramax hydraulic release bearing eliminates the factory clutch cable to produce an easier pedal press and smoother engagement. It features an anti-rotation pin that prevents the unit from spinning around the release bearing and damaging the hydraulic lines.


Once the new hydraulic bearing was measured and shimmed, the American Powertrain Tremec TKX was installed on the bellhousing. Its hydraulic lines were routed out the clutch fork opening.


This tubular transmission crossmember will provide solid support for the new TKX. It features adjustable spacers and shims to ensure the correct mounting position and pinion angle.


After installing the hydraulic reservoir under the hood, routing the feed line through a grommet on the firewall, and installing the new clutch pedal, the Mustang was ready for a test drive. Flores said he was very happy with the tighter, more precise feel of the shifts and the responsiveness of the lighter flywheel. And most importantly, the car is one step closer to being ready for that monster 7.3L crate engine!

Be sure to pick up a copy of Mustang Hub magazine to read this article along with more photos and details from the installation process. You can also check out the video below for a closer look at the TKX swap.

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