Transmission/Drivetrain/Rear-Axle FAQ

1. What is a Powerglide transmission?
2. When was the THM350 transmission first offered?
3. When was the THM400 transmission first offered?
4. [25-Aug-1999] Update: What were the stock transmission gear ratios for the Muncies (and how do I ID a Muncie)?
5. [25-Aug-1999] Update: What is that whining noise coming from my Muncie M22?
6. [06-Oct-1998] New: What are the differences in 4-speed shifters?
7. [10-Nov-1998] New: How do Camaro transmission crossmembers change with application?
8. [14-Jan-2000] New: How were replacement engines and transmissions coded?
9. What are some transmission trivia?

A: It was the "base" 2-speed automatic transmission on the small-block Camaros, and very popular. It is a reliable, smooth transmission and capable of very respectable performance. The Powerglide in "built" form was the preferred drag-race transmission for quite a few years, and is still popular for drag racing.

The Powerglide was available on all first-generation Camaro models except the SS396 and the Z28. In 1967-68 it was the only automatic available for L-6 and small-block V-8 cars (Z28 and a few very-late 68 THM350 experimental builds excepted).

Q: When was the THM350 transmission first offered?

A: The THM350 wasn't officially offered as an option (RPO) until 1969, although some 1968 327/275 Camaros did manage to get this 3-speed automatic transmission as part of an internal GM test fleet. The only way to identify if your 1968 car has an original THM350 is by transmission date code. When the THM350 was released in 1969 it outsold the Powerglide 78,849 units to 66,423. However it was only applied to 6-cylinder and small block V-8 (Z28 excepted) cars, as the high-torque Camaro big-block engines were too powerful for the THM350. The transmission model number comes from the nominal rated torque, 350 lb-ft of torque.

Q: When was the THM400 transmission first offered?

A: The THM400 first appeared on GM passenger cars in 1965. For first-generation Camaro it was only available on, and was the only automatic transmission for, big-block V-8 engines. The model number comes from the nominal transmission torque capacity (400 lb-ft of torque).

Q: [25-Aug-1999] Update: What were the stock transmission gear ratios for the Muncies (and how do I ID a Muncie)?

A: The primary Muncie 4-speed transmission was marketed under Regular Production Option (RPO) M20. M20 was the generic RPO for a 4-speed, so note that the Saginaw 4-speed was also marketed under RPO M20, but with slightly different gear ratios in a significantly different case. In addition to the M20, a close-ratio 4-speed was sold as M21, and a Heavy-Duty close-ratio 4-speed was sold (at a much higher price and much more limited application) as M22.

The buyer had no choice as to the transmission manufacturer; use of the aluminum-case Muncie or the cast-iron-case Saginaw was determined for you by the factory as a function of your model. M21 and M22 transmissions were even more limited to specific models, with details of the limitation depending upon the model year.

The Camaro 4-speed manual transmissions ratios for both Muncie and Saginaw, with other Muncie data, are as follows:

            Maincase     Gear Ratios     Input  Grooves  Cluster   Output
  RPO Years Casting  1st  2nd  3rd  4th  Spline (Input)    Pin     Spline
  -----------------------------------------------------------------------
Muncie
  M20 63-65 3851325  2.56 1.91 1.48 1.00   10   None    7/8-inch      27
  M20 66-67 3885010  2.52 1.88 1.46 1.00   10    2        1-inch      27
  M20 68-69 3925660  2.52 1.88 1.46 1.00   10    2        1-inch      27
  M20 70    3925661  2.52 1.88 1.46 1.00   10    2        1-inch      27
  M20 71-74 3925661  2.52 1.88 1.46 1.00   26    2        1-inch      32

  M21 63-65 3851325  2.20 1.64 1.28 1.00   10    1      7/8-inch      27
  M21 66-67 3885010  2.20 1.64 1.28 1.00   10    1        1-inch      27
  M21 68-69 3925660  2.20 1.64 1.28 1.00   10    1        1-inch      27
  M21 70    3925661  2.20 1.64 1.28 1.00   10    1        1-inch      27
  M21 71-74 3925661  2.20 1.64 1.28 1.00   26    1        1-inch      32

  M22 65    removed  2.20 1.64 1.28 1.00   10   None      1-inch      27
  M22 66-67 3885010  2.20 1.64 1.28 1.00   10   None      1-inch      27
  M22 68-69 3925660  2.20 1.64 1.28 1.00   10   None      1-inch      27
  M22 70*   3925661  2.20 1.64 1.28 1.00   10   None      1-inch      27
  M22 71-74 3925661  2.20 1.64 1.28 1.00   26   None      1-inch      32
*Colvin reports that 454 Chevelle M22 in 1970 used the 1971-74 splines

Saginaw
  M20 6-cyl          3.11 2.20 1.47 1.00
  M20 V8             2.54 1.80 1.44 1.00

To distinguish between Muncie types, if the Muncie was built after 21-Oct-1968, the plant assembly-date stamp contains one of three letters at the end of the build code:

  A = 2.52:1 Wide Ratio     (M20)
  B = 2.20:1 Close Ratio    (M21)
      or
      2.42:1 HD Close Ratio (MC1 3-speed)
  C = 2.20:1 HD Close Ratio (M22 4-speed)
      aka, "Rockcrusher"

This suffix was used on all Muncie assembly plant stamps from 21-Oct-1968 through 1974. See the transmission decoding FAQ for other details. The assembly plant stamp is normally located on the passenger side of the transmission, arranged vertically just in front of the tailhousing joint.

If the input shaft is visible, the number of circumferential grooves on the input shaft will also generally indicate the Muncie type. Be careful, as 1963-65 M20 Muncies are like later M22s in that they have no input shaft grooves; however, the early M20 has only a 7/8-inch diameter cluster pin, while the M22 has a 1-inch pin.

To distinguish an early (pre-suffix) M22 from the other Muncies when the transmission is on a car, note that all M22s had the lower, forward, passenger-side maincase boss drilled and tapped for a magnetic drain plug. This wasn't done (by the factory) for M20/M21 until 1970. While this boss could be drilled for a plug on a M20 after it left the factory - if the boss is undrilled then the case definitely was not part of a M22.

The M22 gears have a shallower mesh angle to reduce thrust load and heat. If the gearbox cover is removed, the difference in the angle between the M22 gears and the gear angle used in the other Muncies is discernable. Note in the photo below that on the M20/M21 the bottom of the next tooth starts below the top of the preceding tooth (i.e., they overlap), whereas on the M22 the angle is such that the teeth actually have a small gap between the top of one tooth and the bottom of the next.

Muncie 4-Speed Transmission Gear Angles
(Photo courtesy of Year One, Inc.)

For additional details on other transmission component casting numbers, component casting dates, and internal transmission details, see Colvin's Chevrolet By The Numbers series, listed in the CRG References.

Q: [25-Aug-1999] Update: What is that whining noise coming from my Muncie M22?

A: This noise is normal and has to do with the angle at which the gears mesh. You'll usually hear it in 1st, 2nd, and 3rd gear (but not 4th) after you've accelerated and you then release the gas pedal, letting the engine brake the car. It has almost the same sound in reverse. The M22 gears have a shallower mesh angle, as shown in this photo that compares Muncie gear angles. The shallow angle causes the "whine" noise, and the sound is the reason for the popular nickname for this transmission, the "Rockcrusher." The lower angle increases the load carrying capacity of the gears at the expense of the increased noise. The reason for absence of the noise in 4th gear is that 4th is a direct output from the input (1:1 ratio), and there are no significant gears involved.

See the Muncie FAQ for other information on M22 and other Muncie transmissions.

Q: [06-Oct-1998] New: What are the differences in 4-speed shifters?

A: A more detailed CRG research paper on both 3-speed and 4-speed shifters by Martin Foltz is nearing release, and will be linked here when it comes out.

• 1967-68 Camaro 4-speed transmissions (Saginaw transmissions as well as Muncie) used Muncie shifters. (The Saginaw required different shift rods than the Muncie.) This stock Muncie shifter was commonly replaced with the aftermarket Hurst Competition-Plus shifter, and is distinguished from the Hurst by being mounted by a bracket to the transmission crossmember and further supported by a longitudinal stabilizer. In contrast, the Hurst shifter is mounted directly to and "floats" with the transmission housing.
• The Muncie shifter in 1967 was generally stamped "MUNCIE" on the handle, (we are still researching this - while there was only one part number in 1967 there are unconfirmed reports of unmarked handles). In 1968 there were two part numbers. The Muncie shifter on the Saginaw was not stamped, while the Muncie shifter on the Muncie transmission was stamped "MUNCIE".
• 1969 Camaro 4-speed transmissions (Saginaw transmissions as well as Muncie) used a Hurst-supplied shifter. (The Saginaw required different shift rods than the Muncie.) The Hurst shifter was similar to the over-the-counter Hurst Competition-Plus model, but differed in the following ways:
  • Slip-in "bayonet" style round handle instead of bolt-on square handle.
  • No adjustment bolts.
  • Big-block cars used a unique shifter mounting plate.

On shifter knobs:

1967

All shifter knobs were a 2-piece ball: the top 2/3 was black, with the shift pattern printed in white; the bottom 1/3 was chromed and included a threaded locking ring.

1968 and 1969

With a console, the shifter knob was a chrome ball without a pattern. Without the console, the shifter knob was a 2-piece ball: the top 2/3 was black, with the shift pattern printed in white; the bottom 1/3 was chromed and included a threaded locking ring. In 1969 the size of the attach thread was changed from 5/16-inch diameter to 3/8-inch diameter.

Q: [10-Nov-1998] New: How do Camaro transmission crossmembers change with application?

A: All 1967-69 L6 and SBC engines use the same transmission crossmember.

BBC transmission crossmembers are different. The BBC manual transmission crossmember is drilled differently from the SBC version and is slightly different in appearance, having a squared-off opening for the trans mounting bolts with a beveled corner. The SBC crossmember opening is oval.

The BBC THM400 crossmember is distinctly different from all others, being formed from tubing and with a mounting plate welded slightly off-center.

   Distinct Transmission Crossmembers
   ----------------------------------
   67-69 L6 and SBC
   67-69 BBC M/T
   67-69 BBC T400

Q: [14-Jan-2000] New: How were replacement engines and transmissions coded?

A: Quoting from a Chevrolet Dealer Service Information Bulletin dated 14 April, 1969:

The first letter will designate the GM division which produced the engine. C-Chevrolet L-Oldsmobile K-Cadillac B-Buick P-Pontiac

The second letter will designate the type of unit "E" engine or "T" transmission. The number following the letter will designate the model year "9" for 1969. The last five digits specify the service replacement unit sequence number. The group of numbers to be used by Chevrolet manufacturing plants [for engines, CRG] are as follows:

Flint motor plant (L-6 engines) 00001 to 19999
Flint V-8 engine plant 20000 to 49999
Tonowanda motor plant 50000 to 79999

Example: Number CE900175 designates Chevrolet engine - 1969 year, and the 175th unit produced for service at the Flint motor plant.

This numbering system applies to service engine assemblies, partial engines, fitted cylinder cases, cylinder cases, transmission assemblies and transmission cases.

At this time, CRG does not know if extra production engines and transmissions (produced in excess of the assembly plant needs) were also used as service parts.

Q: What are some transmission trivia?

A: In no particular order, some interesting transmission factoids:

• The M20 4-speed, standard transmission, RPO was a generic category. It was translated at the factory into a low-performance Saginaw when teamed with the L6 engine, as the higher-performance Saginaw in the lower-end V8s, and as a Muncie in higher-performance V8s such as in the SS and Z28 models. The line between low-performance and high-performance blurred with time, as the 1967-68 275HP L30 engine received the Saginaw while the 1969 255HP LM1 engine received the Muncie (possibly as a result of in-service failures of the Saginaw).
• The M40 3-speed, automatic transmission, RPO was also a generic category in 1969. It was translated at the factory into the moderate performance THM350 (actually RPO M38) for non-big-block engines, and into the THM400 for big-block engines.
• The base 3-speed standard transmission for the SS-350 in 1967-68 was the 3-speed, column-shift, Saginaw. And it was only available in the column-shift. If a floor-shifted 3-speed was desired, the M13 HD three-speed, available only in floor-shift, was a separate option. The base SS-350 (and LM1) 3-speed was changed to the floor-shifted HD version in 1969 - the column shift standard transmission was no longer available in the SS or LM1 cars.
• The HD three-speed standard transmission was RPO M13 in 1967-68 (manufactured by Warner), but was MC1 (manufactured by Muncie) in 1969.
• A floor-shifter for automatic transmissions was only available with the D55 console. Except the 1968 L35/L34 396 engines with the THM400 transmission, for which a non-console M11 floor-shifter was available. (Note: the CRG would be interested in hearing from anyone with a 396/M40 with a non-console floor shifter.)
• The MB1 "Torque-Drive" two-speed transmission was only available for the L6 engines, and only in 68-69.

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