There was a time when the dual-purpose car was the norm rather than the exception. Driving your hot Chevy to work all week and racing on weekends was a way of life for many hot rodders. As the competition got tougher at the racetrack many dual-purpose cars were forced to choose, street or strip. This brought about an onslaught of dedicated race cars. One of the defining reasons for the death of the dual-purpose car was the final drive gear. To be competitive at the track you need a low final gear, while driving to work you were wishing for a tall gear.
Enter the overdrive, a system that was popular in the ’30s through the ’50s but most overdrive transmissions, including the ancient Columbia two-speed rearend, were not up to the rigors of racing.
When Brent Vandervort of Fatman Fabrications decided to build a tribute car of the old Smokey Yunick ’57 Chevrolet, he knew it would be a four-speed car like the original. It’s not a faithful reproduction of the famed No. 13, rather a tribute car that embodies the spirit of the original with plenty of horsepower in a relatively basic ’57 sedan. Underhood, a potent 427 small-block stroker motor fills that requirement. At first the car was enjoyed on the street, hitting a lot of street rod–style events and local cruise nights and then the autocross challenge came into the picture. Competing at a number of autocross events has become very popular and after a few passes it became obvious the car needed a much lower rear gear.
The problem with changing to a 4.11:1 gear was Vandervort is still an avid enthusiast and that means he drives his hot rods. It was obvious the dual-purpose car was back, but luckily today there are options to allow you to drive a long distance to an event, and then, with the flick of a switch, convert to a race gear. Enter Gear Vendors’ Overdrive.
The well-engineered Gear Vendors...
The well-engineered Gear Vendors kit is very complete, and installation is well within the range of the average shade tree mechanic.
This unit couples to your existing four-speed or automatic transmission and can handle the horsepower and torque of big-horsepower powerplants; interestingly, Vandervort was originally introduced to these units several years ago when he installed one on the company truck for towing. The unit proved to be trouble free, tough, and effective. More recently, a call was made to Gear Vendors and the following week we were at Fatman Fabrications helping to unpack one overdrive unit.
The Gear Vendors unit is compact, well engineered, and relatively simple to install. Other than shortening the driveshaft, fabrication is minor and the installation is well within the abilities of your average enthusiast.
Vandervort builds his hot rods the old-fashioned way; if he has something on hand he uses it, and that explains the Ford Top Loader four-speed mated to the 427ci small-block. Once again, no problem for the team at Gear Vendors, they sent all the right pieces to mate an overdrive to the rear of the Top Loader.
This is the actual overdrive...
This is the actual overdrive unit that bolts to the new tailshaft housing supplied for your transmission. The unit is strong enough to handle modern-day horsepower and torque.
We were impressed with some...
We were impressed with some of the details on the Gear Vendors unit, things like a seal retainer on the overdrive output shaft.
After removing the driveshaft,...
After removing the driveshaft, the gear lube was drained from our Ford Top Loader transmission that is coupled to the 427ci small-block.
The transmission crossmember,...
The transmission crossmember, the Hurst shifter, and mounting plate were the next items to be removed. The shifter was left hanging by the shifter rods, and the transmission was supported with a transmission jack.
After removing five bolts,...
After removing five bolts, the tailshaft housing is pulled away from the transmission case and slipped over the output shaft.
With the tailshaft housing...
With the tailshaft housing removed, the output shaft and speedometer gear can be seen. We spent some time with a gasket scraper and Scotch-Brite to clean the old gasket off.
After getting the tailhousing...
After getting the tailhousing mounting surface perfectly clean, we test-fit the Gear Vendors housing. In our case, the speedometer gear prevented the new housing from seating properly, however this will vary with different transmissions.
The speedometer gear is held...
The speedometer gear is held in place with a simple lock ring. We expanded the ring to remove it and slipped the speedometer gear off the output shaft.
With the speedometer gear...
With the speedometer gear removed from the transmission output shaft the new Gear Vendors aluminum tailshaft housing slipped into place perfectly.
The supplied gasket between...
The supplied gasket between the transmission case and the new tailshaft housing is slipped over the output shaft and installed dry.
The splines on the transmission...
The splines on the transmission output shaft were coated with white lithium grease prior to installing the new housing.
The new tailshaft housing...
The new tailshaft housing is bolted in place, and we slipped the coupler over the splines. This coupler connects the transmission to the overdrive unit. The deep end of the coupler goes toward the transmission case.
The coupler must be adjusted...
The coupler must be adjusted to minimize front-to-rear movement on the two splines. We used a straightedge across the round gasket on the rear of the tailshaft housing to determine how much the coupler must be shimmed.
These round 0.020 shims will...
These round 0.020 shims will be inserted in the deep end (main transmission side) of the coupler, but first we must determine how many are needed.
With the straightedge held...
With the straightedge held across the face of the tailshaft housing, insert shims between the straightedge and coupler until the gap is closed. The shims should not touch the straightedge; we are looking for an endgap less than the thickness of one shim (0.020).
After determining the proper...
After determining the proper number of shims, remove the coupler, insert the round shims in the deep end of the coupler, and then reinstall the coupler.
Give the coupler a gentle...
Give the coupler a gentle tap with a soft hammer and check the end of the coupler for proper clearance one more time. Now we are ready to bolt up the overdrive unit.
We installed the original...
We installed the original transmission mount to the new tailshaft housing. The holes in the new housing are relatively shallow so make sure your bolts are not too long or you may pull the aluminum threads out.
The flat-machined surface...
The flat-machined surface on the side of the housing is designed to accept the same Hurst shifter that mounted to the original tailshaft housing.
We used the same bolts, spacers,...
We used the same bolts, spacers, and brackets to mount the shifter; it was truly a bolt-on application.
After placing the round gasket...
After placing the round gasket over the eight 1/4-28 studs the overdrive unit is mated to the tailshaft housing. Be certain the overdrive unit is seated by hand before using any wrenches. Note: Do not pull the unit in place with the nuts.