External Hydramatic Band Adjustment

For this job you will need some wrenches and a good, accurate tachometer. Making these adjustments without a good tachometer is almost impossible. Here is the step-by-step procedure:

1. Block the wheels and set the brake.

2. On cars so equipped, remove the plate that covers the access hole in the front floor pan. This plate is located to the left of the transmission hump. if you do not have this plate, the bands of your transmission are only adjusted internally. This requires special tools and should not be attempted.

3. Without the special band-adjusting tool you may have to work partially from underneath the car. I would put a good jack stand under the rear wheels and not rely on the brake to hold the car in place:

4. Warm up the engine by running it neutral range.

5. Connect the tachometer and set the selector for operation in either the six or eight cylinder setting.

6. Set the transmission selector in Drive. For 1950-51 Oldsmobiles put the selector in Low range.

7. Adjust the carburetor idle screw until an engine speed of 700 RPM is reached

low we have to deal with the band adjustment screw and its locknut. The easy way to do this is to use the band-adjusting tool J-26881, as described in the shop manuals. In talking to old-line dealership mechanics, they are unanimous in saying that their bosses bought very few of the special tools recommended by Oldsmobile. J-2681 may have been an exception, but let's take a look at what the tool is supposed to do and then figure a way around it.

The tool consisted of three parts. (1) A tube about ten inches long with a 3/4' hex socket at the bottom engaged the band adjustment lock nut, and a large handle at the top gave the effect of a .ratchet handle: This loosened and tightened the lock nut. (2) a shaft ran through the tube and this shaft had a socket at the bottom which engaged the square-head band adjustment screw. A 3/8" twelve point socket will turn this adjustment screw. These two parts allowed the mechanic to loosen the lock nut, make an adjustment, and then hold the adjusting screw while tightening the lock nut so as not to upset the adjustment just made. This seems a very simple function, and it would be if you were doing it with no front floor in the car. It is the limited access through the access hole that causes a problem.

The third part of J-268 1 was a Veeder-Root type counter, with a reset knob to allow the mechanic to reset it to zero. This device is like the odometer on the dash, and with each turn of the inner shaft in adjusting the band adjustment screw, this thing counts the turn. The assumption was that a mechanic could not concentrate on his task sufficiently to count the turns himself. The third part we can easily ignore.

Lacking the J-2681, there are a couple of approaches around the problem. Loosening the lock nut with a 3/4" socket is the first step, and making the adjustments with a 3/8" socket is the second step. Tightening the lock nut while holding the adjustment screw from turning is probably a two-person job. Cane person could hold the adjustment screw while the second tightens the lock nut with a %" crow-foot on the end of a ratchet extension. An alternative is for the lock nut to be tightened from below the car while the adjusting screw is held from above.

For those who really like to deal with tools, take an old 3/4" socket and cut the top off. Weld a 10" long section, of steel pipe on the top, and weld a steel rod or angle a foot or so long across the side of the tube, near the top; to serve as a handle. Make it so that a 3/8" socket on an extension will fit through the tube and the cut-off 3/4" socket and you are in business.

Now we can return to the adjustment process.

8. Loosen the lock nut on the front band adjustment (the one closest to the engine) while holding the adjusting screw.

J. Turn the adjusting screw counterclockwise until the engine speed goes from 700 RPM to between 900 and 1,000 RPM. The drum of the front planetary is now spinning freely. If the engine speed does not increase, it is an indication that the band is slipping under normal driving conditions and internal inspection and adjustment is required.

10. Now slowly turn the adjusting screw clockwise until the engine speed drops to 700 RPM .

11. Repeat the above procedure three times. The object in loosening and retightening the screw is to locate the exact point at which the bad stops the drum from spinning.

12. When you get back to the 700 RPM the fourth time wait at least 30 seconds.

13. If the engine speed increases at all, turn the screw clockwise 1/10 turn until he engine speed stays at 700 RPM for at least 30 seconds. This indicates that the front drum is being firmly held.

15. For very early Hydramatics, the lock nut is tightened with the adjustment screw at this setting, as described in step 17 and step 16 is skipped.

16. For 1949-51 units, while holding the lock nut stationary, tighten the adjusting screw exactly 5 turns

For 1952 units tighten the adjusting screw exactly 6 turns. Here it is best to check the shop manual for your car to get the proper adjustment.

17. Hold the adjustment screw stationary and tighten the lock nut. This nut should be torqued to 40-50 foot pounds.

16. Recheck for a 700-RPM tachometer reading.

Repeat this procedure for the rear band adjustment with two exceptions. Make the adjustments with the transmission in neutral. After getting the 700-RPM to hold steady for 30 seconds, tighten the adjusting screw exactly two complete turns. Then put the transmission into Drive range and tighten the lock nut while holding the adjustment screw in place. This lock nut should be torqued to 40-50 foot pounds.

With the transmission still in drive, reset the carburetor idle adjustment to the specified idle speed, which is 375 RPM for the early Rocket engines.


Never flush the Hydramatic transmission- If the fluid seems bad, fill and refill several times, letting the fluid come up to operating temperature each time before refilling.

1. Clean around the fill hole, which is under the floor pan on the right side of the transmission prior to 1951, and through the transmission dipstick from 1951 on.

2. Pour eight quarts of the proper fluid into the transmission, using a filler funnel.

3. With the engine in neutral, run it at fast idle for 1 to 2 minutes.

4. Then, while running at slow idle, add enough fluid to bring the level to just below the "L" mark on the indicator.

5. Run the engine at fast idle for three minutes to bring the fluid to operating temperature.

6. The at slow idle add fluid to bring the level to the "F" mark.

7. With the engine idling and the fluid warm, check to make sure that the transmission is not overfilled. Be careful not to overfill the transmission, as this will cause the fluid to foam.

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