M20 or M21

[Jeff B.]

Am trying to authenticate the Muncie 4-Spd in my 67 - 427-390 A/C coupe, #8902, built F23 (Jan 23).
The case number is 3884865. Date stamp is P7S23, and vin stamp 7S108902. However, the metal tag # is 3880853. This tag number is referenced to a 327-350 engine with an M21. The rear end is stamped 12/29/66 AZ, indicating a 3.55. I understand that a 3.55 should have an M20. Did the big block Muncie trans use a different tag # than the small blocks?
I bought this car in 1978 and it seems to be totally unmodified in any way, yet the trans metal tag # is confusing.
Any thoughts?
Jeff

[Mark F.]

Re: M20 or M21

Am trying to authenticate the Muncie 4-Spd in my 67 - 427-390 A/C coupe, #8902, built F23 (Jan 23). The case number is 3884865. Date stamp is P7S23, and vin stamp 7S108902. However, the metal tag # is 3880853. This tag number is referenced to a 327-350 engine with an M21. The rear end is stamped 12/29/66 AZ, indicating a 3.55. I understand that a 3.55 should have an M20. Did the big block Muncie trans use a different tag # than the small blocks? I bought this car in 1978 and it seems to be totally unmodified in any way, yet the trans metal tag # is confusing. Any thoughts? Jeff

Jeff,

The chart below is from the Hobby Kit the GM Heritage center offers called the Model Specifications Book with 390 HP circled.

According to it, I believe the 350 HP could only come with a wide ratio trans (M20) if the diff was 3.55 in a Corvette (have no idea about other Chevy applications and that trans)

And, I also believe your 390 HP only came in a close ratio trans (M21) if the diff was 3.55.

That being said, this chart looks goofy to me because it doesn't have 4.11 diffs w/ 390 & 400 HP BBs, which I thought were available...

The publication date on this was September 1966, so maybe things changed as the model year progressed (dunno ?)

Transmissions Power Teams and Diff ratios 1967_001.jpg

[Ray K.]

Re: M20 or M21

Jeff,

Seems to me that the number #3884865 you are calling the case number is actually the side shifter cover with the correct number being #3884685.
The main case number should be #3885010. You should read the well prepared 4 speed transmission article by Joe Tripoli in the latest issue of the Corvette Restorer magazine (Volume 50 #3 ). His information identifies your transmission as a close ratio 2.20 which is also confirmed by the metal tag with the #3880853. My P & A catalog ( 1967 ) also supports the tag #3880853 as being as a close ratio transmission.

Hope this helps,

Ray

[Duke W.]

Re: M20 or M21

It's easy to differentiate the WR from CR by the following method.

1. Observe vehicle speed at 3000 RPM in third gear.

2. Observe revs at the above speed in fourth gear.

3. If 2000-2100 the trans is WR and CR if 2300-2400.

I have no doubt that the powertrain chart is accurate. A 4.11 axle would be ridiculously short for the torquey 390/400 HP 427s and neither was considered a "racing engine".

The AMA specs were an "engineering document" and maintained throughout the model year. Many years clearly show revisions. I've see a few data points in some of the AMA specs that I suspect are errors, but they are pretty rare compared to the errors I've seen in service manuals and P & A catalogs.

Duke

[Gary S.]

Re: M20 or M21

Am trying to authenticate the Muncie 4-Spd in my 67 - 427-390 A/C coupe, #8902, built F23 (Jan 23).
The case number is 3884865. Date stamp is P7S23, and vin stamp 7S108902. However, the metal tag # is 3880853. This tag number is referenced to a 327-350 engine with an M21. The rear end is stamped 12/29/66 AZ, indicating a 3.55. I understand that a 3.55 should have an M20. Did the big block Muncie trans use a different tag # than the small blocks?
I bought this car in 1978 and it seems to be totally unmodified in any way, yet the trans metal tag # is confusing.
Any thoughts?
Jeff

Jeff, FWIW, my L36 390hp '67 was built with an M21 and 3.55 rear gear.

I don't remember seeing any depiction of a midyear L36 that was built with a 4.11, but I've probably only seen a few dozen.

Gary

[Mark F.]

Re: M20 or M21

These are Noland's counts for '67...
PS - they don't add up to 22,940

ECL	G81 - Positraction Rear Axle	Count	Tranny
BA	3.08 ratio with Base Engine and Manual Transmission	820	3-Spd or M20
JA	3.08 ratio with Base Engine and Manual Transmission or L36	1,161	M20 or M21
DA	3.08 ratio with L88 and M22	0	Rock Crusher
CA	3.36 ratio with Base Engine or L79 w/ M20 my bad, M20 is Wide>	5,952	WIDE Ratio
KA	3.36 ratio with L36 or L71	2,235	Close Ratio
LA	3.36 ratio with L88 and M22	2	Rock Crusher
EA	3.55 ratio with L79	749	Wide Ratio
MA	3.55 ratio with L36 or L71 with M21	1,635	Close Ratio
XA	3.55 ratio with L36 and M35	23	Powerglide
VA	3.55 ratio with L88 and M22	3	Rock Crusher
FA	3.70 ratio with L79 and M20	2,597	Wide Ratio
NA	3.70 ratio with L36 or L71 with M21	3,025	Close Ratio
YA	3.70 ratio with L36 and M35	36	Powerglide
SA	3.70 ratio with L88 and M22	4	Rock Crusher
PA	4.11 ratio with L71 and M21	1,572	Close Ratio
GA	4.11 ratio with L79 and M21	483	Close Ratio
TA	4.11 ratio with L88 and M22	6	Rock Crusher
UA	4.56 ratio with L88 and M22	5	Rock Crusher

[Leif A.]

Re: M20 or M21

Mark,
Noland doesn't list the "AS" ECL that I have in my June '67 built L79, M21 with 3.70 open diff.

[Mark F.]

Re: M20 or M21

Mark, Noland doesn't list the "AS" ECL that I have in my June '67 built L79, M21 with 3.70 open diff.

Hi Leif,

I agree, but...

If you look at Table C-4 in the latest TIMJG on page 208 you'll see the ratio code "AS" is listed, but it is in the "Open non-posi" category.

The data Noland has for what I posted is only for G81 posi's...yours w/o posi would not appear in his data for that OPTION (G81) IOW, yours is a base diff - not posi, so he apparently didn't have counts broken out like that, but I'll go back and look to see if it's listed somewhere else under the Base discussions.

PS - His counts fall short of the 22,940 total '67 production, so if his numbers are right, it would appear there were ~2,600 open diffs made in '67.

does this all this make sense ?

[Leif A.]

Re: M20 or M21

Mark,
It sure does...

[Jeff B.]

Re: M20 or M21

Thanks to all for your insights. I did read the excellent article by Joe Tripoli, but could not determine for sure if 3.55 ratio Corvettes could be had with an M21. I also wonder that if (in 1967) you went to a dealer to custom option your Corvette, could you choose your 4 Speed trans as an M20 or M21? Or did you simply indicate your wanted the "4 speed" option, and St. Louis picked an M20 or M21 based on the rear end ration you chose?

[Duke W.]

Re: M20 or M21

As you can see in the post #2 chart both the 390 and 400 horsepower 427s could be order with either WR or CR 4-speeds or PG.

Whether a customer special order or dealer stock order the first thing selected once an engine selection was made was the transmission type. Then you either went with the standard axle ratio or any of the optional ratios that you see in the chart, and all required Positraction, which was a mandatory option for all big blocks engine regardless or transmission type as you can see from the ** footnote.

Duke

[Jack M.]

Re: M20 or M21

Thanks to all for your insights. I did read the excellent article by Joe Tripoli, but could not determine for sure if 3.55 ratio Corvettes could be had with an M21. I also wonder that if (in 1967) you went to a dealer to custom option your Corvette, could you choose your 4 Speed trans as an M20 or M21? Or did you simply indicate your wanted the "4 speed" option, and St. Louis picked an M20 or M21 based on the rear end ration you chose?

FYI- Perhaps these 1967 Corvette ordering excerpts might help, and be easier to read...
As overall, the dealer and customer had many options, to customize their order (based on listed availability).
And if something changed during the year, the Regional Zone would likely kick-back the order, to the initial dealer.

This image is part of the 1967 Corvette Dealer Order Form (dated 8-66):
1967-Corvette-Ordering-Procedure.jpg

And is image comes from the early 1967 Finger-Tip Facts (dealer only album):
1967-Finger-Tip-Facts-Power-Teams.jpg

[Gary B.]

Re: M20 or M21

In Noland’s chart I don’t find L79, with 3.36 posi, and wide ratio, M20. But in the last chart that Jack posted, that combination is there. My ‘66 has that combination. Did that combination disappear for ‘67? Or is Nolan’s chart incorrect?

Gary

[Gary C.]

Re: M20 or M21

Gary,

Depends on which data Noland used. Sometimes he used preliminary marketing data, which could and did change throughout the model year.

Noland's books are good "guidelines", but they're not the "ends all be all".

Gary
....

[Jack M.]

Re: M20 or M21

It appears Noland listed OPTIONAL G81 rear ends... as also mentioned in the 1967 Production Trends.
Speculating there would be no individual ECL's for STANDARD equipment, so those axle ratios were omitted from his chart.

This image lists the various 1967 Rear Axle Codes (not the ECL code):
1967-Rear-Axle-Codes.jpg

NOTES observed in Spec Manuals:
1967 "AQ" and "AR" codes & ratios possibly have been used early in the 1967 model-year, or not at all.