Borg-Warner Overdrive - Top GearWritten by on July 1, 2010 Borg-Warner's Old-School Overdrive
Before overdrive automatics and five- and six-speed sticks became common, there was another way to get a tall top cog for highway cruising: a Borg-Warner overdrive behind a three-speed manual transmission. First introduced in '34 Chryslers and DeSotos, they were subsequently used by most Chrysler products and practically every other manufacturer, including Ford, GM, Hudson, Kaiser/Frazier, Packard, Studebaker, Willys, the list goes on. The Blue Oval bunch had an affinity for the B-W overdrive and first offered it in the Lincoln/Zephyr in 1941. It became optional on Ford and Mercury cars from 1949, and it faded from the scene in cars as the '60s came to a close. Ford pickups continued to offer overdrive into the '70s.
Relatively compact, the B-W unit used a planetary gear set that fit in between the transmission and the tailshaft housing and offered a .70:1 ratio (.72:1 for the R-11) overdrive that was available in Second and Third gear, effectively offering five forward speeds: First, Second direct, Second overdrive, Third direct, and Third overdrive.
Operating the overdrive is simple enough-a dashboard-mounted cable control is used by the driver to manually put the transmission in or out of overdrive mode. Pulling the handle out takes the transmission out of overdrive and operates like a conventional three-speed. Pushing the handle in engages the overdrive and an electrical control system consisting of a solenoid, governor, and kick-down switch, take care of actual shifting.
Driving a car with overdrive does require you to understand the system's operation and may require some adjustment in driving style. With the overdrive engaged (handle in), the transmission will freewheel below 20-32 mph (depending on the car's rearend gearing), which means the engine will drive the wheels, but when you lift off the throttle the wheels won't spin the engine-the car will coast as there is no engine braking. However, once the car is up to the required miles per hour, the governor closes the electrical circuit, then when the driver momentarily releases the throttle the solenoid locks the sun gear of the planetary set and the transmission shifts into overdrive (which also means there will be compression braking when you let off the gas). Shifting out of overdrive happens one of two ways: slowing down under the governor cut-in speed causes the solenoid to release. Or, if the throttle is floored the kick-down switch is activated. This does two things; it interrupts power to the solenoid and the ignition circuit momentarily. With power to the solenoid cut you would think it would release the sun gear and the transmission would revert to direct drive. But under load the solenoid pin is "trapped" and can't release, so shutting down the ignition system briefly (it's said to be one or two crankshaft revolutions) the load on the solenoid pin is released, it immediately withdraws releasing the sun gear (which means the transmission is in direct drive) and the ignition system is back on all in the blink of an eye.
Thanks to the governor, the transmission will go into overdrive once cut-in speed is achieved, so around town in traffic using overdrive in Second gear is a natural-it's like having an automatic to a certain extent as the transmission will shift back and forth between direct and overdrive as the speed dictates. On the highway, cruising in Third overdrive and stomping on the gas pedal will cause a downshift to direct for passing.
One other peculiarity of these overdrives should be noted. When left in overdrive mode, more than one parked car has been known to roll away. Most transmissions had an internal mechanism to lock out overdrive in reverse, which would keep this from happening, but the only other way to keep the car from rolling in any other gear was to disengage overdrive and make sure the parking brake was operational.
During its production life, two versions of the B-W overdrive were built: the R-10 and the R-11. The R-10 had a three-pinion planetary gear set and was adequate for most street applications while the R-11 used four pinions and was tougher in general. In fact, coupled with a T-85 three-speed, R-11 overdrives were found behind some pretty stout big-blocks, including FE Fords and even a few Studebaker Golden Hawks.
Over the past three years we've had a '49 to early '51 Mercury style overdrive transmission in daily use behind a Flathead in a '31 Ford pickup that has served us well. Recently, we picked up another and decided to freshen it up as the trans in the A was starting to sound as though some bearings had gone as far as they could. We found all the parts we needed from Mac Van Pelt at Van Pelt Sales who specializes in '28-59 Ford transmission parts and repairs. (Van Pelt also authored a book The Ins and Outs of Early Ford Transmissions. That's a must for anyone tearing into one of these gearboxes.)
For an "old-school" rod with three pedals, an overdrive is a great choice and is probably more appropriate than a contemporary five-speed. And if you're building a car that came with an overdrive, don't get in a hurry to replace it. Most likely you'll like it and agree a Borg-Warner overdrive is a great way to get in top gear.
More Photos From This Article
To remove the overdrive unit from the tail housing, the shift lever retainer pin (arrow) must be driven out with a small drift-the lever is then pulled out, disengaging it from the shift rail.Removing the small cover plate from the top of the extension housing reveals a snap ring (arrow) that holds the output bearing in place. The snap ring has to be spread while the shaft is tapped forward and removed from the housing.On Ford and Mercury transmissions there is a pin that locks the counter gear and reverse idler shafts in the case. With the side cover removed, this pin can be driven out from the passenger side. During disassembly of the transmission it's necessary to remove the shaft and drop the counter gear into the bottom of the case to get the input shaft out.This is the planetary gear set of an R-10 overdrive. The two-bolt flange is where the solenoid mounts; the slotted rod controls the shift fork that engages/disengages the overdrive function on the end of the shaft; the roller clutch that powers the output shaft in one direction and allows it to freewheel in the other.This particular transmission used caged bearings on the cluster gear shaft and on the end of the main shaft (some transmissions use rollers without the cage). New bearings were ordered from Van Pelt Sales.New gaskets and seals will help prevent leaks. The transmission case and the overdrive assembly have separated gear oil supplies.At the top is the Second/Third gear synchronizer assembly, we installed replacement brass rings (below) from Van Pelt Sales.We completely disassembled our transmission, cleaned and checked the case for damage. The first part to go back in is the reverse idler. Like earlier Ford boxes, these can be equipped with Lincoln Zephyr gears, which Van Pelt offers new, although supplies are limited.
A neat little trick is to use a 3/4-inch wood dowel to hold the counter gear bearings, spacer, and thrust washers in place during assembly. The dowel is cut to the same length as the gear assembly and is pushed out of the case when the shaft is installed.In most manual transmissions there are thrust washers, and/or selective snap rings that control the endplay of the gears. The three washers shown here are for the counter gear assembly in our Mercury transmission. Excessive countershaft endplay can cause the transmission to jump out of gear.The bushing in the tailshaft supports the driveshaft yoke. A worn bushing can be the source of driveline vibration.As bearings in the overdrive assembly were smooth and the planetary gear set looked perfect, the overdrive assembly was left intact. The parts to be reassembled on the mainshaft from the left of the overdrive adapter are the First/Reverse slider, Second gear, synchronizer hub, synchronizer sleeve, and main shaft roller bearing. Below are the synchronizer rings, springs, and the three spring retainers.A new bearing was installed on the input shaft before it was put in place. Of all the bearings in the transmission this one is the most likely to require replacement as it spins any time the engine is running and the clutch is engaged-even with the transmission in neutral.When reinstalling the counter gearshaft, mark the end to indicate the location of the hole for the lock pin. With the counter gear properly positioned, the shaft can be tapped in from the rear of the case with a brass hammer at the same time driving the dummy shaft out the hole in the front of the case.Once the counter gearshaft is in place, the retainer pin can be driven in. It's held in position by the side cover.
The hardest part about reassembling the overdrive unit is getting the roller assembly into the ring gear. Heavy grease can be used to hold the rollers in place, then turning the ring gear counterclockwise while slipping it into place works.Another old trick was to use a rubber band to hold the rollers in place. When the sun gear was slipped on, the rubber band was pushed off and was eventually ground up in operation with no harm done.With the tailhousing in place, the transmission is just about ready to see daily service. Note the overdrive fill and drain holes.The governor is driven off the speedometer gear on the output shaft. At a preset rpm, the shaft in the center of the assembly moves toward the cover.The side cover includes a lever that disengages the overdrive in reverse even if the control knob on the dash is pushed in. When parking the transmission should be out of overdrive or left in reverse.This overdrive unit is an R-10 and odds are it's stronger than the transmission it's attached to. The tag at the bottom is the production date, the letter is the month, so this example was made Nov. 2, 1950.