It was much easier to stockpile Muncie spare parts and cores in the 1970s than it is today. Reliable sources to validate information regarding serial and casting numbers will get scarcer as time passes. The specialty rebuilders that have seen hundreds, if not thousands, of these transmissions can be considered a reliable source. These guys stockpiled a lot of parts! I’ve met many of these rebuilders and have had the pleasure to create great friendships with them. I spent a great deal of time sorting through their parts. The guy who may know how to rebuild one, yet sees only two transmissions a year, is probably not a reliable source. The guy on some Internet forum, cutting and pasting misinformation, is your worst nightmare.
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This is the standard Muncie 4-speed case. It has a seven-digit serial number cast into the case on the passenger’s side. These are usually the first numbers used to start the identification process.
This is Brian Higgins of the SK Tranny Shop. Shops like this are a great place to research castings and part numbers. Few people today will ever be able to amass this number of Muncie parts.
Several times a day I’m asked, by email or phone, how to identify a Muncie. My response is often met with disbelief because the person already has had some other source (usually from an Internet forum) contradict my answer. Some people don’t even have a Muncie. They often get angry because they don’t want to face up to the fact that they got burned and made a bad 4-speed purchase.
Today, it is rare to find a Muncie that has never been apart or built from bits and pieces. Untouched “survivor” Muncies are rare; so are the ones that have correct correlating casting and serial numbers.
Main cases often suffered from broken mounting ears, but the later cases had thicker, stronger mounting ears. Its quite common to see later cases replace early ones for this reason. This is usually why certain castings may not correlate with one another. Tailhousings often get damaged where the reverse shifter shaft lock pin sits, and at both the shifter and main mount bolt-boss locations.
Because date-coded serial numbers and VINs were only stamped (not cast) into the main case, the main case should be your starting point. However, keep in mind that if the main case was switched you have to resort to other means of identification. You can think of them as a system of checks and balances.
Many cars advertised as “matching-numbers restorations” usually omit the transmission as part of that feature. I created a Facebook page at www.MuncieBook.com. This page is in place to support this book and it’s free. Feel free to upload pictures and ask questions. Some of the best Muncie builders participate on that page.
Muncie RPO Codes
Just three types of Muncie 4-speeds are available based on the RPO (regular production option) code used when a car was ordered: the M20 (standard wide-ratio), M21 (close-ratio), and M22 (heavy-duty close-ratio). (The M does not stand for “Muncie.”)
Typically, when you ordered a GM car you had standard options and accessories installed. “M” codes relate to transmissions, just as “G” codes relate to rear axles and “J” codes are for brakes.
This 1967 Corvette has a build sheet with an RPO code for an M21 closeratio 4-speed. Notice the other letters and the type of options they represent. Because M21 is just a code, later Corvettes or other 4-speed cars didn’t have Muncies; those equipped with a Super T10 4-speed could have an M21 or an M20 code. Window and tank sticker resources reveal that there was never an M21 option printed on a sticker before 1966, even though many M21s existed I believe the final-drive ratio determined whether a close- or wide-ratio transmission was installed.
Main Case Casting Numbers
Here is a procedure I use that works by using components as building blocks toward identification.
The patent that was originally filed on November 29, 1957, for the T10/Muncie design was approved 5½ years later on May 7,1963. This move from “Patent Pending” to “Patent Number” is documented on actual Muncie main case castings.
The evolution of case design also helps pinpoint the year a case was released. The main case casting number was used for either a specific year or series of years. Casting numbers are not GM part numbers. Some cases may look identical and have a different casting number but have the same GM part number. Usually a casting number is changed because of a design change. This could be an alloy improvement, a casting tool change, or some physical change.
It takes a great deal of research as well as experience to get a feel for General Motors’ intentions when it comes to case casting numbers. Most of the books and articles I have seen, combined with some bits of Internet folklore, tend to have the same flaws because most information is copied and simply edited and resubmitted. I have done my own research by looking at literally thousands of cases, extensions, and sidecovers to draw my own conclusions. I can guarantee that most people reading this book will never see some of the items I’ve stumbled across.
Muncie 4-speeds pretty much have a solid chronological order regarding the main case, retainer, midplate, and extension housing casting numbers. Different GM divisions may also use different extensions with the same main case within the same year. Casting number revisions are always chronological, which means that a higher number corresponds to a later part.
People often get confused about what a casting number is. Simply put, it is the number that is actually cast into the case when the molten aluminum is poured into the case mold. (A serial number is stamped into the case by hand, usually with numbered and lettered punches.) As a mold is poured, date-coded clocks are cast into most cases that represent the month, week, and sometimes day the case was cast. These casting dates or “clocks” are not generally necessary to determine what type of Muncie you have. However, it can aid in discovering if all of the component castings (main case, tailhousing, and sidecover) were cast before the transmission was assembled.
Serial Numbers
Serial numbers are stamped into the case. It is a date code for when the transmission was assembled based on the model year, not calendar year.
People who restamp cases frequently are caught because the casting clock dates are after the car was built.
Serial numbers for Muncie 4-speeds always begin with the letter “P,” which stands for Muncie Plant (not “passenger car.”) “M” and “N” were used to identify 3-speed Muncies; “O” identifies the Saginaw 3-speed with overdrive.
Serial numbers from 1963 to 1966 included only the month and day. For example, P0101 indicates January 1. From 1967 to 1968, the serial number used year, month, and day designators; the month designator is now a letter. For example, the serial number P8A01 is January 1, 1968.
If you have a Muncie dated with a December build date it was actually built the previous year. An example is the date code P8T13. This is for a 1968 production car. The “T” stands for December and 13 is the day.
To confirm this simply look at the VIN. It usually begins with 18S101350 or 28N12950. This means the Muncie was assembled December 13, 1967, for the 1968 model year. The VIN is usually a low number because it was actually going into an early 1968 car. The 1969–1974 Muncies received a ratio designator at the end of the serial number: “A” for M20, “B” for M21, “C” for M22. An example is P2D23B. This is April 23, 1972, M21 ratio. It’s also not uncommon to see double stamps or sometimes even double letters at the end.
Date code format for 1963–1966 model years was just a month and day. Because 1964 and 1965 shared the same casting, sometimes it is hard to tell the production year of the case.
The 1967 and 1968 vehicles received a year, month, and day designator. Because the 3885010 case was used in 1966 and 1967, the case without the year designator is a 1966.
The 1969–1974s received a year, month, day, and ratio designator. Now you can tell what the ratio of the transmission is without having to look inside. It’s still a good habit to double-check the gears to confirm the ratio because often the ratios were switched around.
Main Cases
Let’s take a look at how the Muncie main case evolved from 1963 to actually going into an early 1968 car. 1974.
1963
The first Muncie case was number 3831704. It had a 7/8-inch countershaft bore and used a 6207-type front bearing. All ball bearings have standardized numbers used by the bearing industry. The 6207 bearing’s dimensions are 35-mm bore, 72-mm outside diameter, and 17-mm width.
The Muncie case (3831704) replaced the T10 4-speed in the 1963 Chevrolet Bel Air and Corvette by mid-May of that year. The T10 front bearing size was reduced in diameter in 1963 only. This meant that the transmission’s front bearing retainer and register bore of the bellhousing were also smaller in diameter. The smaller 6207-type bearing replaced the larger 6307 bearing in 1963.
The first Muncie copied the smaller bearing and retainer combination of the T10. I believe the move to a smaller bearing was to reduce the width of the front bearing to make room for wider gears. By 1963, wider gears improved the T10; this meant that the front bearing had to get narrow to compensate for the size difference of the front portion of the countergear.
The 6307N bearing has a 35-mm bore, an 80-mm outside diameter, and is 21 mm thick.
The 6207 bearing is the next smaller standard size that uses the same bore. However, it is thinner by 4 mm and smaller in diameter by 8 mm. Because the Muncie was really GM’s design improvement of the T10, the 7/8-inch-diameter counter-shaft was also used in 1963.
By late 1963 the rare 3839606 case started to show up in cars with a larger bearing bore. The fact is General Motors couldn’t use a standard 6307-style bearing with a width of 21 mm because the gears were already tooled and made. Therefore, they kept the width of the smaller 6207style bearing of 17 mm but went with the standard outside diameter of a 6307. This is called a narrow 6307 bearing (PN N307LOE).
This Tech Tip is From the Full Book, MUNCIE 4-SPEED TRANSMISSIONS: HOW TO REBUILD AND MODIFY. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
SHARE THIS ARTICLE:Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://www.chevydiy.com/muncie-4-speed-identification-guide/
This is the 1963-only 3831704 main case with “PAT. PEND.” cast above “General Motors Corp.” Notice that the drain plug boss was not drilled out. The casting clock codes are at right above the fill plug. The clock looks like a mathematical fraction with a 1/2-inch circle around it; they were a way to track any quality-control issues. The number represents the month and the dots represent the week. Most of the time the other clock is blank, but occasionally it represents numbers and deviations to the original blueprint. Later cases seem to only have one clock. A “4” next to the casting is the mold pattern number.
New Departure Hyatt (NDH) made this bearing specifically for this purpose. Their number was 41307B. It is unique to the Muncie and was used until the very end of production in all main cases with the exception of the 3831704 case.
1964 and 1965
By 1964 the 3851325 case appeared. Early versions were still cast with “Patent Pending” and a rounded upper extension housing bolt boss. By late 1964 the 3851325 case was cast with GM Patent Number 3088336. The 1965 case with the same casting number has a squared-off upper extension housing boss.
The left case is a 1963 T-10. The bearing bore is smaller than the Muncie case on the right. The 3831704 case used the same bore as the 1963 T10.
The 3839609 main case was used during 1963 and 1964. It’s identical to the 3831704, except it had a bigger front bearing bore. Most of these castings look very rough. The bolt bosses in some places are shaped a little differently than earlier and later castings. It may have been from a different foundry. These surface from time to time but are rare.
Corvette production determined how Muncie design evolved. By late 1965, the 396 Turbo Jet engine, rated at 425 hp, was introduced. A very small number of 396 engines also found their way into the 1965 Impala. I learned from speaking to many builders that the rare 3864848 case seems to be in a few Corvettes and full-size GM cars with big-block engines. The 3864848 case with the last 848 numbers milled off is in some full-size cars, such as the Impala. This usually indicates that the gears used in the milled cases might be of a different steel alloy. The case appears to be identical to a 3851325 case so perhaps it is made of a different aluminum alloy. It appears that more milled cases are in circulation compared to unmilled cases. They all seem to come from big-block cars.
The 3864848 case, with a December 17, 1964, build date, was probably for an early 1965 car. These cases were run in conjunction with the 3851325 case. This might have been a different alloy because its appearance is identical to the 3851325 case and it was to be used strictly with the new big-block 396 engine. (Photo Courtesy Ed Hartnett)
1966 and 1967
The 1966 main cases saw some additional design changes. With the introduction of the 3885010 case, the countershaft bore was increased to 1 inch. The case countershaft bore usually stretched out so the increase in shaft diameter spread the counter-shaft load over a wider area.
These two milled 3864 cases have March 1965 assembly dates. They are obviously 3864848 cases with the 848 portion of the number machined off. (Coincidentally, March 1965 seems to be a time when the 396 engine started to migrate into more vehicles, such as Z16 Chevelles, Corvettes, and full-size Impalas.) These transmissions probably were assembled with special alloy gears, because the case casting is identical to the 3851325 as well. Usually when an external modification is made to the case, such as milling, it means a special internal mod. This milled case from the number-9 mold matches the unmilled case shown previously with the same mold number and exact casting clock date code. What you can see is that before March 1965 these cases were not milled. (Photo Courtesy Ed Hartnett)
This 3851325 case, manufactured May 1964, shows a patent number that’s identical to the PAT. PEND. case. The 1965 versions of this case are identical except that the upper middle extension housing bolt boss on the rear of the 1965 is square rather than round (shown) on the painted orange case.
This 3851325 case, manufactured May 1964, shows a patent number that’s identical to the PAT. PEND. case. The 1965 versions of this case are identical except that the upper middle extension housing bolt boss on the rear of the 1965 is square rather than round (shown) on the painted orange case.
This case was used to the end of 1967. Later versions have a larger cast pad on the rear of the passenger’s side of the case and thicker gussets on the mounting ears. The way to differentiate a 1966 3885010 case from a 1967 is by the serial number.
The early “3851325 PAT. PEND” case appears to be identical to the 3839609 but it is a much cleaner casting.
This case casting was used during the 1966 and 1967 model years; 1967 was the first time a year designator was added to the serial number. This one has a January 19, 1967, date code.
This is a 1967 3885010 case dated June 2, 1967. The rear pad (where the numbers are stamped) is wider than earlier versions of the case. The mounting ears also have thicker gussets than earlier cases. This casting design evolved into the 1968-and-up castings.
1968 and 1970
The 3925660 cases were used primarily in 1968 to 1970. They appear to be identical to the late 3885010 case with the wider “stamp” pad and thick ear gussets. Some spots also have more material and the case has a thicker look to it. Perhaps an alloy change as well as a tooling change warranted a new number.
By the start of 1969 these cases and all future cases had the drain plug boss drilled and threaded. The 3925661 casting seemed to surface by the last quarter of 1969. Stamped serial numbers for all 1969 model year cases also included the year designator.
1970 to 1974
The last Muncie case was the 3925661. The “661” is considered the strongest factory GM case. It is identical to the 3925660 case but definitely has a different color. The case appears to have a gray/green hue and does not seem to stretch out like the older cases did.
This 1968 3925660 main case has a P8R04 assembly date and was cast in a number-2 mold. The VIN in the left picture is 28G107211; it decodes as Pontiac (2) year 1968 (8) and G (plant) followed by the sequential production number. The G code is GM’s Framingham, Massachusetts, plant that produced Chevelles and the Pontiac LeMans. Therefore, this is a LeMans case.
Here is a rare 3925661 casting from a 1969 Camaro. The assembly date of P9P16A translates to September 16, 1969. The “A” stands for M20. Notice the odd casting clock above the fill plug and that it was drilled for a drain plug. This was mold pattern number 1.
It was used primarily with the 26-spline input shaft and larger 32-spline output-shaft models, so it appears that they were looking to optimize strength everywhere possible since the 454 engine was introduced in 1970. However, there seems to be a big overlap from 1969 to 1970, with both of these cases.
This is a 3925661 case from a 1974 Corvette M20 transmission. It has an August 7, 1974, build date. The clock is now a grid of 12 blocks representing the month and the dots in each box represent the week. This one was cast in the fourth week of January 1974 in mold number 15.
This is a 1970 M20 case. The VIN is hard to read. This is mold number 1. Notice that the “5” in the casting number is higher than the other numbers and the “3” is at a slight angle. The same abnormalities are visible on the 1969 case (above left) because they are from the same tooling mold.
Written by Paul Cangialosi and Posted with Permission of CarTechBooks
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