Adapting a Saganaw Four-Speed Transmission to a Vintage Chevrolet Truck

Until 1955 three-speed Chevrolet pickups employed what is called a torque-tube, or closed, drive system. The forward end of the torque tube mounts on a ball joint at the back of the transmission; the aft end of the tube is secured to the third member. The driveshaft lives inside of the torque tube, where it is supported by bearings. Power to the drive shaft comes through a splined universal joint at the back of the transmission.

This system isolates the rear springs from drive thrust and from the torque generated as the pinion gear attempts to climb on the ring gear.

Second Series 1955 and later trucks use the Hotchkiss, or open, drive system, familiar to anyone who has looked under a modern rear-wheel drive car. Since the drive shaft is isolated by u-joints, the rear springs must take out the drive thrust and the torque generated as the pinion gear attempts to climb the ring gear.

The type of drive arrangement probably made little difference to the original owners of these vehicles. What the torque tube gave in better rear wheel alignment, it took away in ride quality, since the weight of the driveshaft and tube adds to the unsprung mass. For restorers, the drive system is an essential feature which reflects design practices of Ford and other auto pioneers. For those of us whose hope to get some use from these trucks, the torque tube system is an impediment.

OEM Transmissions

Three Speed

Most pickup trucks were fitted with Saganaw three-speed transmissions, side-loaded for column shift after 1948. The countershaft was supported on uncaged roller bearings (like the optional �heavy-duty� passenger car transmission).

Published sources give inconsistent information about gear ratios. According to the factory, from 1940 Saganaw three-speed boxes all had a 19-tooth input gear; 27T -14T - 22T cluster; 26T sliding second gear and a 29T first and reverse wheel.

No one can afford to make replacement gear sets, but other parts, including bearings, bushings and synchronizer rings are available from A.B.E. Transmission and Gear (outside California 800 821-0308; California 800 631-8756). Gears pirated from another transmission should be replaced as meshing pairs.

Column Shift

The column-shift mechanism was notorious for jamming, even when these vehicles were new. When that happened, one had to get out, open the hood and �pull the ears together.� That is, you tugged on the control box levers until something gave and the levers could be rotated to the horizontal, or neutral position.

Mechanics blamed the control box, but the more likely culprit was the lack of rigidity of the shifter rods which allowed two gears to be half-selected at once. If your truck suffers from this problem, make sure the adjustment is spot on, and try to eliminate as much slop as possible from the mechanism. Wear points on shifter yokes and control box levers can be restored by brazing, and nylon or Teflon linkage bushings seem to help.

Even so, a column shift cannot be forced: the vacuum shift package on 1939-48 passenger cars took second or so to complete a gear change, but rarely jammed.

Four Speed

The floor-shifted four-speed used straight-cut, unsynchronized spur gears and had an almost agonizing low first gear (6.395:1). Early versions were configured for torque-tube drive.

Alternative manual transmissions

The widely spaced ratios of the original three-speed transmissions make shifting gears an experience akin to falling down the side of a mountain. You quickly reach the plateau of peak torque in first, drop to half the engine rpm when entering second, recover, and tumble again during the shift to high.

The four or more closely spaced ratios, full synchromesh and positive gear engagement offered by more modern transmissions effects a kind of driving revolution. It�s a bit of an exaggeration to say that two tons of vintage iron can be made to zip through traffic, but progress is accompanied by a definite rush.

However, exchanging transmissions is an expensive and labor-intensive exercise that involves purchase of a shifter kit (a good one, the only kind to have, costs more than $200), modification or replacement of the transmission cross member and, nearly always, replacement of the rear axle with concomittant modification to whatever driveshaft you might use. Expect to encounter niggling, but real, problems, such as speedo cable incompatibility, poor shifter ergonomics, etc.

Costs are hard to predict. Probably we are looking at a minimum of $1000 for an open-driveshaft conversion and a used, but serviceable transmission. A professionally rebuilt and warrantied overdrive transmission can double the cost of the project.

Overdrive

Vintage trucks were not intended for high speed travel, at least as the concept is understood today. Inadequate brakes, steering and suspension aside, going fast requires about a 30% reduction in engine rpm in high gear. A change of this magnitude calls for an overdrive transmission. (Swapping rear axles or gear sets in the original assembly typically lowers the ratio 10 -15% for the same marginal gain in cruising speed and loss of acceleration.)

A good approximation to the ideal might be a Borg-Warner six-speed, as used on the Z/28, Trans-Am and Chrysler Viper. The GM PN is 10236021. Moving down a notch toward practicality, you might try a Borg-Warner T5 which carries GM PN 10192297 and is salvageable from Cameros and Mustangs. Gear Mart sells rebuilt and warrantied T5s for $650 plus core charge. You might contact them (800-229-2299) for prices on these and other, more exotic, boxes.

The T5 is not a simple bolt-in, but no transmission swaps easily when you come down to it. Ron Cox at Stovebolt Engine can furnish an adaptor plate to mate the T5 to a V-8 bell housing which, in turn, bolts up to the 216/235/261.

Non-Overdrive

Probably the most popular swap is the Saganaw four-speed, originally used in GM medium duty cars and trucks. These transmissions are still plentiful and and go for about $150 in wrecking yards.

The Saganaw makes a �doable� swap that easily accommodates six-cylinder torque and power, features synchromesh low and good ratio spread, shifts so much better than the old three-speed that you could almost believe in the concept of automotive progress. It also fits the 216/235/261 bellhousing without an adaptor plate, mates with existing clutch disc and crankshaft pilot bearing.

On the negative side, the Saganaw lacks overdrive and the cast iron case makes it heavier than it should be. Junkyards charge a premium for aluminum-cased four-speed Muncies, many of which were not driven by old ladies on the way to church.

There are four Saginaw ratio combinations, identified by the grooves or absence thereoff on the outer, splined end of the input shaft.

Saganaw Ratio Combinations

The two-groove Saganaw is preferred, because of its relatively low first gear and evenly spaced ratios. Engine rpm drops 29% between first and second; 33% between second and third; and 32% between third and top.

Patrick�s, whose catalog supplied the ratio data above, has become world headquarters for Saginaw conversions. The company supplies a torque-tube adaptor kit, with shifter, for 1948 - 54 pickups that is claimed to make the swap a bolt-on process. The kit sells for $400, which sounds like a fair amount of money until you calculate the dollar and time costs of modifying the system to open driveshaft. Of course, you are stuck with the torque-tube 4.11 rear end ratio, unless you want to spend another $400 on Patrick�s 3.55:1 ring and pinion set. Early Powerglide passenger cars had the same 3.55:1 ratio.

Saganaw installation

To install a Saganaw in a late Chevrolet factory or home-brewed open-drive system, you will need to:

• Have the driveshaft cut to length and balanced (about $100, plus u-joints).
• Modify the existing cross member in order to install a transmission mount, such as NAPA Belcamp PN 620-1034, or else spring for a purpose-built X-member ($60 from Patrick�s or a �universal� mount at about the same price from other suppliers).
• Purchase a shifter kit. As mentioned above, quality kits, such as Mr. Gasket or Hurst, cost more than $200 and may leave you reaching awkwardly for the shift lever. Patrick�s version costs about the same and is claimed to be ergonomic. Not having seen one, I do not know about its quality.
• Notch the floor pan.
• Figure out what to do about a speedometer. The easiest solution is to purchase an electronic speedometer with an adjustable pulse generator. The orginal instrument can be used with a longer cable, although you might encounter some interference between the cable and the shift mechanism. The speedometer should also be recalibrated.