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I believe the 3839606 casting indicates a 63-64 Muncie. Do you have a pic of the input shaft? Going off memory here...not always a good idea, an early Muncie with no 'Rings' cut into the input shaft means a
wide ratio while a single 'Ring' meant a close ratio. There are quite a few members with far more knowledge about how to decipher a Muncie than me and no doubt they'll respond shortly.
Mike T - Prescott AZ
Weren’t all early Muncies coded M20, wide and close ratio? Out of the car, mark the input and output shafts, put it in first gear, and rotate the input until the output makes one complete turn and count how many turns were required of the input 2-1/2 (WR) or 2-1/4 (CR). Probably easier to count the input turns for 2 or 3 complete turns of the output to get a little larger distinction between the two.
Keep in mind the Muncie's were better and stronger than the T-10's. If you have a modified or larger engine than the 283 you are better off.
T-10's weren't upgraded until years later when they came out with the SUPER T-!0
Unless you are going the NCRS judging rout , you are better off with what you have , although the engine you have would not hurt the T-10 as they were designed for each other .
Unless you are going the NCRS judging rout , you are better off with what you have , although the engine you have would not hurt the T-10 as they were designed for each other .
Under spirited straight-line driving, the T-10 will eventually give up the ghost relative to a Muncie. None of these boxes are repair proof if there are constantly pounded, street light to street light.
In road race form, however, the T-10 does very well. Shelby's Cobras did well, winning multiple USRRC and SCCA Championships with this box, as well as Le Mans, the World's Manufacturers Cup, and setting 21 land speed records at Bonneville with CSX 2287, the first and only Coupe designed by Pete Brock.
Last edited by Dan Hampton; May 22, 2024 at 11:07 PM.
The WCFB is a good, simple, dependable small 4bl carb. The 2x4 versions are about 385cfm each (770 total). Not quit as good as a professionally dialed in Holley or Q-jet(be careful when you bad mouth a Q-jet).
But a trick Holley just does not have the magic image as a well built 2x4 setup. And the early 2x4 intake bolts right up to any first generation small block and performs great.
I have a 57-61 style 2x4 setup on a 389 in the 51 Chevy with an M20 and 3.08 posi. Excellent combo!
Hello
i have a casting number of 3839606
How do I tell if M20 or M21?
thank you
john
The following is NOT, repeat, NOT from me. This is research/observation by Alan Colvin in his book CHEVROLET by the Numbers 1960-64.
OK, I will try to cover some things that have been asked.
In the early years of Muncie production there were a few oddball main cases.
The "normal" and most common 63 case is the 3831704. And as a performance Muncie version, it is the least desirable.
Another early 64(Aug 63) oddball (VERY rare) main case is 3839606. I have never personally seen a 606 case, BUT, I would guess it had the 64-later larger front bearing and bearing retainer as used in all 64-74 Muncie main cases.
The MOST COMMON, and best known 64-65 Main cases (VERY common) are 3851325. I have a few.
An oddball 65 case is 3864848. I have never seen one, and have no clue how many were made.
The 010 cases were 66-67. The 660 cases were 68-69 (MAYBE 70). The 661 cases were 71(70?)-74
The OPTION CODE for a 63-65 4sp was M20-------------THERE WAS NO M21 OR M22option code. The M20 installed in any car was either referred to as a close ratio or wide ratio (and still is by some folks today).
For the 63-65 cars with an M20 Muncie, the determination as to whether it was a close(2.20 1st) or wide(2.56 1st) ratio, WAS DETERMINED AT THE FACTORY, DEPENDING ON ENGINE OPTION AND REAR AXLE RATIO. NOT, repeat, NOT THE CUSTOMER.
The M20-M21-M22 option codes started with the 66 models. YES, YES, YES, In 1965 there were a VERY few M22 Muncies(heavy duty close ratio) installed in cars with very hi-perf engines. Probably the biggest quantity were the 425hp/396 Corvettes. But the M22 was supposedly available for full size Chevys with the 396 and 365hp-375hp(FI) small block Corvettes.
I'm interested in this kind of information-------------------BUT I DO NOT OBSESS OVER IT!
INPUT SHAFTS
This can be confusing. Typically, IF THE MUNCIE IS OUT OF THE CAR, the ratio of a Muncie can be determined by observing the input shaft-----------------SORTA.
The 63-65 WIDE ratio had NO grooves around the input splines.
ALLLLLLLLLLLLLLLLLL 63-74 CLOSE ratio Muncies, M21, had one groove around the input splines.
ALLLLLLLLLLLLLLLLLLL 66-74 WIDE ratio Muncies,M20, had 2 grooves around the input splines.
ALLLLLLLLLLLLLLLLLL 66-74 M22 Muncies had NO grooves around the input splines. THUS, an unscrupulous, or unknowledgeable person (sometimes, the same person) would claim that the Muncie he was selling was an M22. This is important----------------THE ONLY SURE FIRE WAY TO TELL IF A MUNCIE IS AN M22, IS TO REMOVE THE SIDE COVER AND OBSERVE THE ANGLE OF THE GEAR TEETH. PERIOD!!!!!!!
IF, IF, IF a Muncie has been rebuilt, and one of the replacement parts was a new input shaft, MOST NEW INPUTS DID NOT HAVE ANY GROOVES AROUND THE INPUT SPLINES!!!!!!!!!! Thus, if a Muncie was rebuilt with a no-groove input, there is no way to know if it is an M20 or M21 version. So, mark the input shaft, mark the tail shaft, turn the input and count how many turns results in a full turn of the tail shaft. About 2 1/4 turns for a close ratio, about 2 1/2 turns for a wide ratio.
ALLLLLLLLLLLLLLLLLL OF THE ABOVE IS FOR FACTORY BUILT MUNCIES.
NOW THERE ARE AFTER MARKET GEAR SETS OF DIFFERENT RATIOS FOR THE MUNCIE, WHICH MAKES IT A DIFFERENT BALL GAME.
Am I a Muncie expert? ABSOLUTELY not!!!!! But, I have been rebuilding them for 50+yrs and have picked up a few things about them.
For sure, the 606 848 main cases are quite rare. Over the years, I have sorta known of them, but never had one on the workbench. I am not a big Muncie rebuilder because I do not want to be covered up. I always have plenty to do without being covered up with a specialty.
Since I have never had a 606 or 848 on the workbench, I am not aware of any specific or different features about them as compared to the 325-010-660-661 cases.
SOOOOOOOOOOO, if anything and every thing that fits inside a 325 case, then you should be fine.
Now, with that said, the 63-65 Muncies are considered to be less desirable because the smaller 7/8in cluster shaft. I have had several 325 cases bored out to accept a 1in shaft. To convert a 325 case to a 1in requires using the 66-later type input and cluster gears. Everything else fits. BUT, the 63-65 and 66-74 synchronizer hubs and brass rings are different. One thing I do is upgrade the 63-65 syncros to the 66-74 type.
