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Tech Alley               "Where you find all the answers."

Automatic transmission to TREMEC TKO big block conversions

When it comes to big blocks a T5 will not hold up very well unless you use a T5z, and are not making more than 450hp.  In most cases however, it is cheaper to run a TREMEC TKO transmission. 


When converting from an automatic to a five speed there are a couple of ways to execute the swap. 

  • Use the stock big block 3 or 4 speed bellhousing with a shortened input shaft TKO, that will bolt direct to the bellhousing.  Or use a standard length input shaft TKO and use a spacer/adapter plate. There is a added benefit to shortening the input shaft, the driveshaft will remain the same length saving you time and money by not having to shorten the driveshaft.  However, in early Mustangs the shifter will be moved further forward, requiring additional material to be removed from the tunnel opening.
  • When using a stock bellhousing, all the stock clutch linkage can be used.  This is perhaps the least expensive option as fewer components are changed.
  • When the stock linkage is not available, another option is a hydraulic kit.  We offer hydraulic kits that use either a internal slave throw out bearing setup or an external style.  The kit comes with a master and all the hardware to connect the master to the slave. If header clearance is an issue, and you wish for a smoother movement, a hydraulic kit is the way to go.
  • Use a aftermarket scattershield for added safety or when a stock bellhousing is not available. All the same linkage options apply as the stock bellhousing. The aftermarket scattershields do offer different motor and transmission bolt pattern combinations that were never available before. They are heavier however, as they are made of solid steel and require more clearance for the larger flange.
  • Another linkage option is the new Quicktime clutch cable operated big block scatter shields.  We recommend these scatter shields for those of you wishing to use our clutch cable kit in your early Mustangs with big blocks, as well as, any late model Mustang using a cable kit. This bellhousing works very well in Factory Five Cobra kit cars when installing a big block.  It will work with the stock pedal and cable. Call us for details or to place an order.

Since the TREMEC transmission mounting point is further back than stock 3-4 speeds, a TREMEC cross member is needed for early Mustangs.  The cross member is designed for use with the stock early transmission mount, so don't make the mistake and purchase a late model T-5 transmission mount as it will not work.

For 2x4 trucks, moving the stock cross member back will solve the mounting difference.  

In late model Mustangs, the stock cross member needs to be moved back on the outer mounts. This can be done by cutting the weld, sliding the cross member and re-weld or swap it out with an auto cross member which has this done already. 

The shifter on a TKO lines up with the shifter hole in the floor in early Mustang and Camaro. Just a little of the floor has to be removed on the front and right side of the opening. The stock TKO shifter has two threaded holes to mount almost any kind of shift lever desired. We offer both stock like appearance shifter lever, boot and ball. For those who like the Hurst look of the 60's, there are several chrome levers to choose from. Hurst also offers five speed pattern shift balls in white and black or T-handles found in 69-73 Mustangs. 

Be sure to save the automatic transmission speedometer cable, as it will be reused. You will, however, need to install a T5/TKO speedometer driven gear, which are available in a variety of tooth counts to match your differential gear and tire combinations. Installing the new speedometer driven gear is simple, it requires removing the c-clip and pulling the old gear off and sliding the new one on. 

Automatics have a neutral safety switch, so that the car can only be started in park or neutral. Manual transmissions don't need, the switch, which must be by-passed or the motor will not start. On early Mustangs the four-plug harness sticks out of the firewall above the master cylinder. Two of the wires control the reverse lights; the other two control the neutral safety switch. Splice two of these wires together and you've bypassed the neutral switch. 

The starters from early Fords with automatic transmissions work just fine in most cases. The issue is usually the starter snout length. If you find yours is too long, replace it with a manual application starter.