Q: Hi, James. Just wanted to say that I enjoyed your article on the Muncie transmissions in the December 2016 edition. It was very helpful in understand the stamping on the Muncie transmissions.
I was hoping as I read it that you would mention the metal part number tags apart of the documentation process. I have been doing some research on these transmissions, also. I’ve been concentrating on the metal tag part numbers that are usually attached to a top rear casing bolt.
My reason for researching these tags was to find out why GM stopped using them on the 1973 Corvette. I own a 1973 L48 with a M20 Muncie. All the data I’ve found show these tags were present on Muncies through 1972. They were also used on other GM vehicles during this time. My question to you would be if you know why the tags stopped being used beginning in 1973 on the Corvette? My theory is that there were only two choices for the four-speed during the 1973 run. The M22 Rock Crusher was no longer available as an option. And, eventually, the Muncies were replaced by BorgWarners in 1974.
There were usually two “broadcast code” letters added to the end of the part numbers on the tag. These letters did coincide with the broadcast code letters called out on the build sheet in 1973. For example, my metal tag would read as follows: 6271516WD. Wide ratio Muncie. In the block labeled Trans on my original buildsheet the letters WD are present. This is the same tag that would have been attached to the 1972 wide-ratio Muncie. I have also found that some of the broadcast letters were stamped on top of the cases in black letters for the most part. But some owners also report their letters were yellow.
Would you be able to shed any more light on this? Inquiring minds want to know? These tags are another interesting part of the Muncie identification process.
A: Jim, you do know your Muncie transmission tags. When I wrote the Muncie transmission article I was hoping no one would open that can of worms and ask about the Muncie transmission tags. I am going to provide you with what knowledge I have on this subject.
Numerical Production Part Number
First, the metal tag starts with a numerical production part number. These production part numbers can be found in the AIM (Assembly Instruction Manual) for your year vehicle. The AIM is one of the most important technical references you can have in your Corvette technical library. The AIM was supplied to the assembly plant by Chevrolet Engineering as an assembly aid and it contains part numbers, illustrations of how all the parts are to be assembled, the fasteners and torque values, assembly sequences, adhesives, sealers, lubricants, functional test and processes.
Now this is where the can of worms is first opened. The AIM was updated throughout the production run and was frequently revised with the latest updated information sheets. What you will get in the reproduction AIM book is usually the last revision. So it is possible that the number you are looking for may be in one of the AIM sheets that was updated through the production year run.
It is sometimes possible to tell if revisions have been made to the AIM sheet if you look in the lower right portion of the sheet. There, a section called the Title Block can be found and this block shows the history of the sheet, the references from which it was created,and the sheet’s revision history. However, if the sheet is redrawn, the history can be lost.
From my research over the years, the AIM can sometimes be incomplete and in some circumstances inaccurate. There were more numerical production part numbers used than were ever shown in the AIMs that I have researched.
Also, sometimes the numerical production part number along with the two-digit broadcast code for a certain vehicle model was used in other models. For example, 3978766WO was used in a 1970-’71 Chevelle LS6, the same number was used in the W-30 Oldsmobile Cutlass.
The first contributing factor was which Muncie four-speed transmission was to be installed in a particular application: an M20, M21 or M22.
The second contributing factor has to do with getting the speedometer to read correctly. Other than the model of the transmission (M20, M21 or M22), the installed speedo gear combination was the only difference in that transmission model.
There were two different drive gear choices that could be pressed onto the output shaft at the Muncie assembly plant. Both gears had eight-teeth but were different diameters so they could mesh with the different speedometer drive gears, which would be installed at the assembly plant and depended on what the vehicle’s rear axle ratio was to be.
The broadcast codes are the alpha characters on the transmission tag at the end of the numerical production part number. These letters match the broadcast sheet (buildsheet) for the vehicle in the block labeled TRANS.
I have seen transmissions that also had these broadcast codes stamped on the top of the transmission main case in black or yellow letters. This is not found on every unit.
Not all transmission tags have the broadcast code stamped on them. Some transmission tags will only have the production part number stamped on the tag. Notice this tag’s production part number is listed on the AIM photo above.
This is a subject where there’s not a lot of information, but, Jim, during my research I found transmission tags consistently on Muncies through the 1972 run. I have seen original Muncie Transmission tags on the 1973 Muncie transmissions but for the most part I have seen more of these missing on original, unrestored cars during the 1973 model run.
I did put a lot of research into this topic several years ago and have compiled a list of transmission tag part numbers, including some of the numbers in the mid-year running AIM changes. If any of our readers are interested in getting a copy of this list or can contribute more information to this subject please let me know. I am always searching for more knowledge.
Jim, I hope this helps answer your question. Thank you for putting me to the challenge on this one.