1967 Munice Transmission ID Tags

[Judson Laws]

I have a '67 coupe & '67 roadster both are 327/350hp cars with M20's. What do the stamped metal tags on the front lower side cover bolt designate? The one on the coupe is #3870354 & the roadster is #3890563. I was told it says they are M20's one with 2.54:1 first gear & the other with 3.55:1 first gear?? I looked in Corvette by the Numbers book at the transmission specs & gear ratio specs & it doesn't list such. Can anyone clarify this for me?? Thanks, Judson Laws wjlaws@alltel.net

One other question. Did M21's have a drain plug in '67 or did just M22's have a drain plug? All M20's didn't have a drain plug, right?

[William C.]

Re: 1967 Munice Transmission ID Tags

In '67, all M-20 trans had 2.54 low gear and no drain plug. The tag designated the speedo drive match to a specific rear axle ratio.

[Wayne M.]

Re: 1967 Munice Transmission ID Tags

Your 3870354 tag should be OK for 1967 (or 1966) 300hp or 350hp wide ratio cars with 3.08 thru 3.55 differential gears. I get this from Long Island Corvette's section 32-item 59, but it is not gospel as it fails to take into account what Bill C. refers to (ie. you could have an L79 with wide or close ratio. There are many more tag #s than are shown in the LI catalog, to cover all possibilities.

This gets quite complicated as the tag numbers were changing rapidly; if you have an Assembly Instruction Manual (AIM) look at the tables in the pages for G81 and M20-21. [I don't have one to examine]

But your second tag # 3890563 has me baffled, as it's a number associated with transmissions past the 1967 model year. Not to say it's wrong, but maybe some combination of trans ratio and final drive ratio (like close ratio with numerical low rear) that I can't find data on. Does the maincase have the cast part # 3885010 ?

[Barbara S.]

Question

Are all 4 spd 327/350 hp cars for 67 equipped with M21's? If not, please explain when the M20 vs M21 was used for 350 hp cars in 67.

Thanks!
Tony

[Judson Laws]

Re: 1967 Munice Transmission ID Tags

Yes, the maincase does have the casting number 3885010, that's on both transmission in both cars. Both are matching numbers to the cars. The coupe is a early car built November 25 1966 & the roadster a late car built March 13 1967. The coupe is currently a AK code 3.36 non-posi, but it's not original to the car. The roadster was originally a AN 3.55 posi, but it currently has a AO 3.70 posi, I still have the AN 3.55 posi. I'm baffled too about the 3890563 tag. Corvette Central has the 3870354, but not the 3890563.

[Joe L.]

Re: Question

Tony-----

No, the 1967 L-79 could be ordered either with the M-20 or M-21. When the M-20 was ordered, a 3.36:1 rear axle was included with a 3.55:1 optional. With the M-21, a 3.70:1 rear axle ratio was standard with a 4.11:1 optional.

[Joe L.]

Re: 1967 Munice Transmission ID Tags

Judson------

The GM #3870354 is a wide ratio (2.52:1) Muncie used for the 1966 and 1967 model years and in conjunction with the standard (3.36:1) rear axle.

The GM #3890563 is a wide ratio (2.52:1) Muncie used for the 1967 model year and in conjunction with the optional 3.55:1 rear axle ratio.

The transmissions are identical except for the ORIGINALLY installed speedometer DRIVEN gear. The GM #3870354 had a GM #3860343, 20 tooth BLUE gear installed; the GM #3890563 had a GM #3860345, 22 tooth GREEN gear installed.

By the way, both of the above transmission part numbers were not, at all, unique to Corvettes; they were used pretty much across the Chevrolet line.

[Barbara S.]

Re: Question

Thanks Joe.

[Judson Laws]

Re: 1967 Munice Transmission ID Tags

Joe, Speaking of speedo gears. The one taged #3870354 has a 20 tooth steel driven gear. I thought these were suppose to be plastic. My speedo is off & I tried to change it, but plastic gears won't go in it. Is the drive gear that drives the driven gear different for a steel gear? I saw some were that there were two different steel drive gears, one for 3.08 to 3.55 ratio & another for 3.70 to 4.11, is this correct? Do you know anyone repoing the trans ID tags? I can find the 3870354 in corvette catalogs, but not the 3890563. Did M21's have a drain plug in '67? Thanks.

[Joe L.]

Re: 1967 Munice Transmission ID Tags

Judson-----

The 20 tooth steel driven gear was used only for M-21 or M-22 with 4.56:1 rear axle and was of GM #3860329. It was used in conjunction with a special speedometer DRIVE gear of GM #3845079. This gear had 6 teeth. It will not mate with ANY of the nylon driven gears. Most nylon driven gears mate with 8 tooth DRIVE gears, although there are 2 different OD gears.

GENERALLY (and, I wish to double-emphasize the term "generally"), DRIVE gears for 3.08:1 to 3.55:1 rear ratios used one drive gear and those for 3.70 to 4.11 used another. However, the 3.55:1 is the one that "can go either way" depending on other factors (like tire size, etc.). It's much more accurate to say that any application requiring an 18 tooth to 22 tooth (silver) DRIVEN gear requires one configuration DRIVE gear and any application requiring a 22 tooth (green) to 25 tooth DRIVEN gear requires another configuration DRIVE gear. Both DRIVE gears have the same 8 tooth count. However, the OD of each is different.

I do not know of a source for transmission tags but there probably is one. These would not be too difficult to reproduce.

M-21's did not have drain plugs for 1967; only M-22's had them. The first year for a drain plug on all Muncie variants was 1970.

Also, your "3870354" transmission was NEVER originally supplied with the steel DRIVEN gear. So, either someone has installed an incorrect tag on another transmission or someone's changed the speedometer gears in a transmission otherwise correctly identified as as 3870354.

[Judson Laws]

Re: 1967 Munice Transmission ID Tags

Joe, Thanks for all the information. The "3870354" transmission is the original matching numbers trans for my '67 coupe. Like you said someone must have changed the gears. Maybe when the trans got rebuilt before I got the car. They just put what ever they had. Maybe it's a M21 & I don't know it & the tag got swapped for the wrong one. I'm planning on putting a AN code 3.55 posi rear in the car. It currently has a non original AK non-posi rear now. The car was built November 25 1966 & the rear is dated 5 5 67. I will probably pull the tail housing on the trans & change the drive gear & put the right plastic gear in it.

[Wayne M.]

Here's the 4.56 set Joe's referring to

This used 6-tooth worm drive gear #3845079, 1.922" dia, 0.453" thk. The diameter of the NOS 20-tooth steel driven gear is 0.710". I don't think any plastic driven gears, which all have larger diameters, will fit against this 6-tooth driver. I have three 20-tooth steel driven gears, and they all have the yellow dab of paint.

But you seem to have found this out yourself; therefore we can conclude that you have this 6-tooth speedo drive gear on the output shaft of your transmission.

[Judson Laws]

Re: Here's the 4.56 set Joe's referring to

Wayne, That is the 20 tooth steel speedo driven gear I have. How are the worm drive gears installed on the shaft? Are they pressed on or bolted with a retainer plate? Do you know the dimensions/specifications of the two 8 tooth worm drive gears, one for 3.08 to 3.55 ratio & one for 3.55 to 4.11 ratio? Thanks.

[Wayne M.]

Dimensions of 8-tooth drive gears tomorrow AM *NM*

[Wayne M.]

Dimensions for drive and driven speedo gears

The pic shows two 8-tooth drive steel gears. These press on (interference fit) on the tail shaft [I believe in 1968 or 69 they were of plastic and held on the shaft with C-clips -- Joe L. would know the details].

The 8-tooth steel for 3.08 thru 3.55 rear end ratios inclusive is the larger diameter of the two @ 1.850", because it meshes with the smaller "family" of .788" diameter driven nylon gears (red at 21 teeth for 3.55; blue at 20 teeth for 3.36; brown at 18 teeth for 3.08 [all with 7.75 x 15 tires of 1965 and 1966]. The thickness of this steel gear is 0.610", which is the easiest way to tell it apart from the next steel gear. All steel gears have the same I.D. at 1.176".

Bottom 8-tooth steel gear is for 3.70 and 4.11 ratios, and is 1.765" dia and 0.531" thk. It meshes with the larger diameter family of nylon gears at 0.865"; green 22 tooth for 3.70; and yellow 24-tooth for 4.11. There are 2 other available gears in this family (shown in picture) The orange has 25 teeth and the black has 23 teeth. These 2 can be used to fine-tune speedo calibration between the 3.70 to 4.11 ratios.

There's another hybrid grey plastic 22-tooth [3.70 rear] small diameter gear which is designed to work with the larger drive gear (3.08 - 3.55). This was added after C2 production ended (as far as I can figure) to offer service flexibility.