Anyone know of a source (GM or otherwise) for the speedo cable gear that is held into the tailhousing of a Muncie M21? (they mount on the outside of the case and go into the case to mate up with a plastic gear inside). I know they have different units depending on the gear ratio, etc. Maybe someone has a link to a site that tells which one to get. I will check George N's webiste now. Thanks
Patton, I have an old scan of the original GM decode sheet. It is a very poor copy. I will try to attach it. It shows all Buick trans and speedo combinations. To start off I believe the 1970 Buick 4-speeds used an 8 tooth speedo driver gear. You should use that in a speedo program with your tire size & rear end ratio to determine your driven gear # teeth. I can Email you a Excel program if you send me your Email address. GM Part # 3460341 18 Teeth Brown 2.93 rear with 3-speed GM Part # 3460342 19 Teeth Natural 3.23 with tall tires GM Part # 3460343 20 Teeth Blue 3.23 all other tires GM Part # 3460344 21 Teeth Red 3.42 all GM Part # 3470351 17 Teeth Purple 3.64 tall tires + yellow adapter .785 GM Part # 3460341 18 Teeth Brown 3.64 with yellow adapter .785 UPDATE: It appears that the V8Buick attachment program makes it not readable, if anyone wants an Email of the better copy I have- just let me know & provide your Email.
I had to get one of these drives for my M20 about a year ago and I just ordered it from the local GM dealer.Took a few days to get it in and it cost around 15.00 though.You just need to know what color you need.They are actually the same in the saginaw 3 and 4 speeds too as they were pretty standard for many years in GM cars I believe.........Bruce.
It would be a broad statement to say that all M20 (wide ratio Muncies) used 8 tooth speedo drive gears, & is def not the case with M21's & M22's. There were also 7 & 6 tooth speedo drive gears, & they often were used in M21's & M22's. The GM 6 tooth drive gear has been disco'ed for a loooong time. Maybe a repro out by now, if so I'd like a dozen p The '64 GTO when ordered with 3.90 gearing, received an M20 vs the M21, only avail in a Corvette for that year. I'd bet that a factory 3.90 equipped '64 GTO is one of the few exceptions to any 8 tooth M20 "rule". Will do some digging around in my old MPC's when I get home & post back. Got Posi? :3gears: Roger
I think so, and I'm not clear on which gear they are asking about, the drive gear or the driven gear. There are two 19 tooth gears listed.
I think so, and I'm not clear on which gear they are asking about, the drive gear or the driven gear. There are two 19 tooth gears listed. http://www.tciauto.com/Products/components/speedometer_gears.asp
Can someone help out a poor non-gearhead like me that happens to own a bone stock non-AC numbers matching 70 Stage 1 4 speed with it's 3.64 rear? All I know is that with stock size tires my speedo is reading way too fast..like 60 when I'm only doing 50. I'm certain it has the wrong gear. So which of these do I need? The 18 Brown? And what does "yellow adapter .785 refer to? GM Part # 3470351 17 Teeth Purple 3.64 tall tires + yellow adapter .785 GM Part # 3460341 18 Teeth Brown 3.64 with yellow adapter .785
Joe, Several pieces of added info are needed. By numbers matching, am assuming the original M21 is still in place & it has it's original speedo drive gear on the shaft?? Mention that, as have torn down quite a few M21's over the last 15 years that have been previously rebuilt, usually out of multiple parts Muncies, & in several, the 6 or 7 tooth drive gear had been robbed & replaced with an 8 tooth gear. This throws everything off, the prescribed driven gear will not work, instead one will have to plug & chug with a small range of driven gears combined with aftermarket external adaptors. Adaptors... Speedo adaptors were often fitted between the speedo cable & transmission on musclecars with really low gear ratio's & "short" tires. For example, G70-14's & 4.33's always required an external adaptor on RAII & RAIV GTO's. 4.10's or 4.56's & F70-14's required an external adaptor in Chevyland. Later in the '70's, different geared adptors were often used in 3/4 & 1 ton pickup applications. Some were even used on really mundane GM offerings with a 5spd :grin: With 3.64's & G60-15's on 15x7's, & that tire/wheel combo being slightly taller than G70-14's on 14x6's, I'm somewhat put back that an adaptor was even used, however, your info notes one. Is there an external adaptor in place between the trans & the speedo cable on your car?? If not, & many times this is the case, the original adaptor is MIA, & you will need to be on a search for the correct one. This can be a real PITA While there are many different styles of these adaptors, the two styles I'm most aquainted with as being original useage on GM musclecar/supercar era vehicles are the L shaped & slight offset Stewart Warner styles. Will try & put a link up with pics of about a dozen different styles. Have gone though the MIA speedo adaptor scenario many times in the past. Got Posi? :3gears: Roger
The most important piece of information that is needed is the number of teeth on the DRIVE gear. It is located on the output shaft. If you pull the DRIVEN gear sleeve out, you should be able to slowly turn the driveshaft and count the teeth. All that is needed after that is the rear gear ratio, and the rear tire size. Then it is a simple mathematical calculation.
Guys, This is the ORIGINAL M21 transmission with the stamped vin number matching the vin of the car and vin stamped on the SS motor as originally delivered. The tires and wheels are 15' and G eqivalant as originally delivered. Rear is 3:64 as originally delivered. The only work done on this gearbox was when pulled (by John Csordas) and resealed when I bought the car. When I told John the speedo was reading faster then the actual speed (or this was the worlds fastest bone stock Stage 1 car :3gears: ), he pulled out a plastic gear that was inserted into the the side of the transmission (I forget the color) and said it was most likely the wrong one. Perhapes sometimes during its life this car has a different tire and wheel set up? (it was a small angled gear about 1" in diameter) So is this an adaptor or the speedo gear drive? Thanks for replying.
Will need to know what color driven gear is in the M21 now, then should be able to calculate. Got Posi? :3gears: Roger
Not to muddy the waters but here are the 6, 7, & 8 tooth drive gear part numbers for all small output shaft Muncie 4spds. The 6 & 7 tooth drive gears were also used in early T-10's. Grp. 4.343 3708144 GEAR (8 teeth) 9785023 GEAR (7 teeth) 3845079 GEAR (6 teeth) Got Posi? :3gears: Roger
Great info guys...that gets me where I need to be in determining which gear to buy. Thanks very much....this thread SHOULD BE A STICKY:TU:
