1967 M21 close ratio transmission questions

[Roy S.]

I think Joe might be able to answer these questions with his extensive GM parts knowledge, but anybody that knows the facts I would appreciate hearing from. AIM and Noland Adams Book offer conflicting data, don't have a parts book old enough to shed light on it.

1. Could you buy a 3.70 differential gear M21 close ratio L79 - Noland says no page 409 G81 table
2. AIM says yes UPC M20, M21, M22 A1 part identification 1
3. Does the part identification tag on the transmission, specify speedometer drive gear or something else.
4. Were not the only internal differences in a close ratio transmission the speedometer drive gear.
5. What did the part identification tag specify if not speedometer drive gear

All this is asked because of complete confusion on my part the aim says 3880853 is M21 L79 . should the aim not say M21 (3.70 differential) incidentally according to Noland their were no L79 M21 3.70 differential cars built.
Does anybody have the GM explanation for 3880853, 3880855, etc.,

[Leif A.]

Re: 1967 M21 close ratio transmission questions

I think Joe might be able to answer these questions with his extensive GM parts knowledge, but anybody that knows the facts I would appreciate hearing from. AIM and Noland Adams Book offer conflicting data, don't have a parts book old enough to shed light on it.

1. Could you buy a 3.70 differential gear M21 close ratio L79 - Noland says no page 409 G81 table
2. AIM says yes UPC M20, M21, M22 A1 part identification 1
3. Does the part identification tag on the transmission, specify speedometer drive gear or something else.
4. Were not the only internal differences in a close ratio transmission the speedometer drive gear.
5. What did the part identification tag specify if not speedometer drive gear

All this is asked because of complete confusion on my part the aim says 3880853 is M21 L79 . should the aim not say M21 (3.70 differential) incidentally according to Noland their were no L79 M21 3.70 differential cars built.
Does anybody have the GM explanation for 3880853, 3880855, etc.,

Roy,
Then I own a documented (tank sticker) unicorn. The L79, as you know, could NOT be ordered with an automatic so it had to have a manual. My car has the stock "AS" code rear end (3.70 non posi) which was the default rear gear with the M21 and L79. Engine and transmission are documented original to the car.

[Roy S.]

Re: 1967 M21 close ratio transmission questions

Roy,
Then I own a documented (tank sticker) unicorn. The L79, as you know, could NOT be ordered with an automatic so it had to have a manual. My car has the stock "AS" code rear end (3.70 non posi) which was the default rear gear with the M21 and L79. Engine and transmission are documented original to the car.

Leif, I have no doubt that your car is not one of none, my resaon for the question is why would this tag not be the same for the clsoe ratio L79 3.70 and close ratio L71 3.70. in my review tryig to figure all of this out I realized Nolnd's book showed not available and at the same time the aim does not show a L71 with the same transmission and gear.

[Roy S.]

Re: 1967 M21 close ratio transmission questions

Guys here is part of the answer, confusion even in the AIM M21 section translated to vendors beeing part of the issue.


The 4-speed section of the 67 AIM (M20, M21, M22) sheet A shows the 3880853 as being used with the L79 engine and the 3880855 with the L71 engine. However, the G81 Positraction page, sheet A1, shows the 3880853 as being used with L36 & L71 with 3.55,3.70 or 4.11 diff ratio. No mention of any L79 with 2.20 4-speed. Obviously, there is some confusion/error in the AIM as there's no mention of an L79/M21 from the M21 section with any diff ratios. I believe the 3880853 would be correct for the L79, L36 & L71 with 3.55/3.70/4.11 diff. I think the 3880853 is simply a number that specifies a close ratio transmission with speedometer drive gear which functions with the 3.55/3.70 or 4.11 gear ratios and has no partiality to engine size 327 or 427.

[Patrick B.]

[Leif A.]

Re: 1967 M21 close ratio transmission questions

Guys here is part of the answer, confusion even in the AIM M21 section translated to vendors beeing part of the issue.


The 4-speed section of the 67 AIM (M20, M21, M22) sheet A shows the 3880853 as being used with the L79 engine and the 3880855 with the L71 engine. However, the G81 Positraction page, sheet A1, shows the 3880853 as being used with L36 & L71 with 3.55,3.70 or 4.11 diff ratio. No mention of any L79 with 2.20 4-speed. Obviously, there is some confusion/error in the AIM as there's no mention of an L79/M21 from the M21 section with any diff ratios. I believe the 3880853 would be correct for the L79, L36 & L71 with 3.55/3.70/4.11 diff. I think the 3880853 is simply a number that specifies a close ratio transmission with speedometer drive gear which functions with the 3.55/3.70 or 4.11 gear ratios and has no partiality to engine size 327 or 427.

Tag on my M21, L79, 3.70

[Joe L.]

Re: 1967 M21 close ratio transmission questions

I think Joe might be able to answer these questions with his extensive GM parts knowledge, but anybody that knows the facts I would appreciate hearing from. AIM and Noland Adams Book offer conflicting data, don't have a parts book old enough to shed light on it.

1. Could you buy a 3.70 differential gear M21 close ratio L79 - Noland says no page 409 G81 table
2. AIM says yes UPC M20, M21, M22 A1 part identification 1
3. Does the part identification tag on the transmission, specify speedometer drive gear or something else.
4. Were not the only internal differences in a close ratio transmission the speedometer drive gear.
5. What did the part identification tag specify if not speedometer drive gear

All this is asked because of complete confusion on my part the aim says 3880853 is M21 L79 . should the aim not say M21 (3.70 differential) incidentally according to Noland their were no L79 M21 3.70 differential cars built.
Does anybody have the GM explanation for 3880853, 3880855, etc.,

Roy------

You could definitely buy a 1967 Corvette with L-79, M-21, and 3.70:1 rear axle ratio. The part number for the transmission, PRODUCTION or SERVICE, was GM #3880853. The AIM is correct.

The identification tag which includes only the GM part number for the transmission assembly does not specify the installed drive gear but, by part number, defines it. For example, there were two different transmission part numbers for M-21 transmissions. The above-referenced part number included drive gear GM #3708145. The other M-21, GM #3880855, was used for applications with 3.08 or 3.36 rear ratios. As far as 1967 applications go, this applied only to L-36, L-68, and L-71. The GM #3880855 transmission assembly had installed a GM #3708144 speedometer drive gear.

The only difference between M-21 transmission assemblies for the 1967 model year was the installed speedometer DRIVE gear. No other difference.

[David B.]

Re: 1967 M21 close ratio transmission questions

In answer to questions 3,4,& 5 What did part ID tag specify? Chevrolet on a regular basis produced "Transmission & Extension Assembly Charts" which (in this case) listed all Muncie 4 speed transmissions built by part number (re: metallic tag).
The part # shown on the tag (as explained by Lucia) defined what was inside that particular transmission. The parts listed on these assembly charts give much more then the ratio of 1st, 2nd, 3rd, 4th & reverse. They list the clutch gear part #, the counter gear part# & the speedometer drive gear part#. In addition it specifies by part number the correct: lever - trans 1-2 shifter, lever - trans 3-4 shifter, & lever - trans reverse shifter. How many times has someone complained about not being able to adjust the shifter levers? Probably using the wrong levers --- Other information given includes bolts, lock washers etc.
Needless to say -- that little tag is far more important then most people realize.

[Wayne M.]

Re: 1967 M21 close ratio transmission questions

The 1967 dealer order form seems to confirm the availability.

[James G.]

Re: 1967 M21 close ratio transmission questions

Muncie M20	Ratio	Speed 1	RPM	Speed 2	RPM
1st	2.52	30mph	3211	50 mph	5352
2nd	1.88	50 mph	3993	75 mph	5989
3rd	1.46	75 mph	4651	105 mph	6511
4th	1.00	105 mph	4460	130 mph	5522
Cruise		70 mph	2973

WIDE RATIO WITH 3.36 POSI

[James G.]

Re: 1967 M21 close ratio transmission questions

3.36 CLOSE RATIO
Muncie M21 / M22	Ratio	Speed 1	RPM	Speed 2
1st	2.20	30mph	2803	50 mph	4672
2nd	1.64	50 mph	3483	75 mph	5224
3rd	1.28	75 mph	4078	105 mph	5709
4th	1.00	105 mph	4460	130 mph	5522

[Edward D.]

Re: 1967 M21 close ratio transmission questions

The only difference between an M20 (wide ratio) and M21 (close ratio) is the input gear size and its meshing gear on the cluster counter gear. All other gears (1,2,3) on the mainshaft are identical. An M20 input gear has 21 teeth; an M21 has 26 teeth. Corresponding cluster gear has 25 teeth for M20 and 27 teeth for an M21. All of the gear ratios (except 1:1 4th gear) are changed by this difference in input gear diameter. There is no actual gear for 4th, the trans mainshaft is simply locked to the input gear OD so it spins at the same rpm as the crankshaft. The case and tail shaft housings are the same as well. Any muncie M20 or M21 can be converted from one to the other by swapping out the input shaft gear and the cluster gear. M22s are different, all of the gears are straight cut instead of helical cut, ratios are the same as M21. M21s generally came with 3.55 to 4.11 axles and had a speedometer worm gear with an OD of 1.76 to use the pencil gears with more teeth. M20s generally came with 3.08 to 3.55 axles and used a worm gear with a 1.84 OD for the pencil gears with fewer teeth. As Joe mentions above some M21s (different assy p/n) could be had with with the 1.84 speedo gear and M20s could be gotten with the 1.76 gear used for the 3.55 to 4.11. When rebuilding a transmission, you have to know what axle ratio is going to be used to ensure you use the right speedo worm gear, regardless of whether it is an M20 or M21. Otherwise you could end up with a pencil gear that doesn't quite mesh with the worm gear or interferes with it. Besides the axle ratio, the tire size (diameter) has an impact on appropriate pencil gear for the application. In general, M20s tend to work best with axle ratios of 3.08 to 3.55 and M21s work best with 3.55 to 4.11+. You are keeping the engine in its optimal power band for a given speed by changing the gear reduction. Remember that the 3.08 means the driveshaft turns 3 times for every single wheel revolution while the 4.11 turns more than 4 times for each wheel revolution. The off the line power advantage in the 3.55 to 4.11 is due to torque multiplication. So far as the highway is concerned between M20 and M21, only the axle ratio makes a difference to engine rpms as the trans is 1:1 on both.

[Roy S.]

Re: 1967 M21 close ratio transmission questions

My point is/was the 3880853 m21 close ratio transmission was utilized with any 3:55, 3.70 or 4.11 not just the L79 as the AIM M20, M21, M22 section implies.

The 3880855 was used with any 3.08 or 3.36 not just the L36 and L71 as the AIM M20, M21, M22 section implies.

The G81 section confirms the part the M20, M21,M22 section leaves out.

These two transmission numbers are not small block or big block specific. They are axle ratio specific only.

[Joe L.]

Re: 1967 M21 close ratio transmission questions

The only difference between an M20 (wide ratio) and M21 (close ratio) is the input gear size and its meshing gear on the cluster counter gear. All other gears (1,2,3) on the mainshaft are identical. An M20 input gear has 21 teeth; an M21 has 26 teeth. Corresponding cluster gear has 25 teeth for M20 and 27 teeth for an M21. All of the gear ratios (except 1:1 4th gear) are changed by this difference in input gear diameter. There is no actual gear for 4th, the trans mainshaft is simply locked to the input gear OD so it spins at the same rpm as the crankshaft. The case and tail shaft housings are the same as well. Any muncie M20 or M21 can be converted from one to the other by swapping out the input shaft gear and the cluster gear. M22s are different, all of the gears are straight cut instead of helical cut, ratios are the same as M21. M21s generally came with 3.55 to 4.11 axles and had a speedometer worm gear with an OD of 1.76 to use the pencil gears with more teeth. M20s generally came with 3.08 to 3.55 axles and used a worm gear with a 1.84 OD for the pencil gears with fewer teeth. As Joe mentions above some M21s (different assy p/n) could be had with with the 1.84 speedo gear and M20s could be gotten with the 1.76 gear used for the 3.55 to 4.11. When rebuilding a transmission, you have to know what axle ratio is going to be used to ensure you use the right speedo worm gear, regardless of whether it is an M20 or M21. Otherwise you could end up with a pencil gear that doesn't quite mesh with the worm gear or interferes with it. Besides the axle ratio, the tire size (diameter) has an impact on appropriate pencil gear for the application. In general, M20s tend to work best with axle ratios of 3.08 to 3.55 and M21s work best with 3.55 to 4.11+. You are keeping the engine in its optimal power band for a given speed by changing the gear reduction. Remember that the 3.08 means the driveshaft turns 3 times for every single wheel revolution while the 4.11 turns more than 4 times for each wheel revolution. The off the line power advantage in the 3.55 to 4.11 is due to torque multiplication. So far as the highway is concerned between M20 and M21, only the axle ratio makes a difference to engine rpms as the trans is 1:1 on both.

Edward-------


One minor point. Gears for the M-22 transmission were not straight cut although they are often described this way. They were actually just cut at a lower helix angle than gears for the other Muncie variants. If folks think they are noisy, they should see what they'd be like if the gears were actually straight cut.

[Joe L.]

Re: 1967 M21 close ratio transmission questions

My point is/was the 3880853 m21 close ratio transmission was utilized with any 3:55, 3.70 or 4.11 not just the L79 as the AIM M20, M21, M22 section implies.

The 3880855 was used with any 3.08 or 3.36 not just the L36 and L71 as the AIM M20, M21, M22 section implies.

The G81 section confirms the part the M20, M21,M22 section leaves out.

These two transmission numbers are not small block or big block specific. They are axle ratio specific only.

Roy------


Correct. The AIM might imply to some that the transmissions were unique to big blocks or small blocks, that's not, at all, how it was. The same transmissions were used for both big blocks and small blocks.

For example, an L-36 with with M-20 and 3.08 or 3.36:1 rear gear ratio used the same transmission as a base engine. Or, an L-71 with M-21 and 3.70:1 used the same transmission as an L-79 with that rear gear ratio.