When was 4-sp trans driven speedo gear installed ?

[Wayne M.]

For C2 to C3 and beyond, I will submit that the appropriate driven gear with the correct colored plastic/tooth count was installed on the engine dress line, at St. Louis. It would not have been feasible to stock 3 different M21 trannys (as an example); for low numerical economy axles (3.08, 3.36, 3.55), and two for the performance-lite axles (3.70 and 4.11). Only 4.56 was available in the all-out set-up, so that particular driven gear [steel] could either be installed at Muncie or St.Louis.

Stocking only 4-speeds with 3 different trans drive gears would simplify inventory. The Chevy parts catalogs (P&A30) [ie. service] seem to show only 3 versions available for M21 and M22, although they make proper selection much more difficult than necessary.

M20 close ratios are simpler, as no 3.70-up axles were factory-available.

So what do the experts say ?

[Douglas L.]

Re: When was 4-sp trans driven speedo gear installed ?

I'm by no means a expert but seems logical and makes sense when you look at the trans part#s in the 68 AIM. In the AIM if you look at M21s specifically, the 2 part #s are 3915090 with L79 and 3915091 for L36. Only available ratios for L79 M21 was 3.70 and 4.11 while the L36 M21 could be had with anything from 3.08 up to 3.70. Stands to reason the 3915091 came from Muncie with the small drive gear while the 3915090 came with the large drive gear. Also makes the fact that the M21 in my 3.70 equipped L36 car has a 3915090 tag make alot more sense.

[Joe L.]

Re: When was 4-sp trans driven speedo gear installed ?

For C2 to C3 and beyond, I will submit that the appropriate driven gear with the correct colored plastic/tooth count was installed on the engine dress line, at St. Louis. It would not have been feasible to stock 3 different M21 trannys (as an example); for low numerical economy axles (3.08, 3.36, 3.55), and two for the performance-lite axles (3.70 and 4.11). Only 4.56 was available in the all-out set-up, so that particular driven gear [steel] could either be installed at Muncie or St.Louis.

Stocking only 4-speeds with 3 different trans drive gears would simplify inventory. The Chevy parts catalogs (P&A30) [ie. service] seem to show only 3 versions available for M21 and M22, although they make proper selection much more difficult than necessary.

M20 close ratios are simpler, as no 3.70-up axles were factory-available.

So what do the experts say ?

Wayne------

Through 1970, I think the driven gear was installed at Muncie. There were more Muncie transmission assemblies used in PRODUCTION than were available in SERVICE.

I'm far away from home right now and can't check this but, as I recall, there are no transmission driven gears shown in the AIM through 1970. So, if they're not shown in the AIM, then they were not installed at St. Louis.

After 1970, things changed. The 71-74 Muncie had only one available DRIVE gear. Only the 18-22 (silver) driven gears could be used with this drive gear.

[Douglas L.]

Re: When was 4-sp trans driven speedo gear installed ?

So, if they're not shown in the AIM, then they were not installed at St. Louis.

Seems as if they would have had to been installed at St. Louis.
I could be wrong here but as I understand it it, the transmissions were sent from Muncie w/o any designation in regards to a specific car. And if, according to the AIM(in 68 at least) in production there were only 4 different parts #s for 4-speeds 3915088-M20, 3915090-M21(L79), 3919091-M21(L36) and 3915893?-M22, then theres no way Muncie could have shipped them with the correct driven gears for every application given the variety or available differential gear ratios with just those 4 part numbers.

I think Wayne's theory makes alot of sense especially since one of the M21 production part #s is designated specifically for L79 use. If you got a L79 with a M21 the only gear ratios available were 3.70 and 4.11 and when you look at the drive gears, the only driven gears that will work with the small drive gear are for 3.70-4.11 ratio. That means that L79/M21 would be the ONLY M21 application that HAS to have the small drive gear installed in the transmission while all other M21 applications(BBs) may need a small or large drive gear depending on the intended cars differential ratio.

[John H.]

Re: When was 4-sp trans driven speedo gear installed ?

There's no need to speculate - the DRIVEN gear was installed on the Engine Dress Line at St. Louis (and at every other Chevrolet assembly plant); Muncie had no clue which car any given transmission was going to end up in, much less the detail of the dealer order for that car that specified the rear axle ratio (every ratio had its own DRIVEN gear). All Muncie knew (and all they needed to know) was that there were two "ranges" of axle ratios that called for two different DRIVE gears (plus the very low-volume 6-tooth gear).

[Joe L.]

Re: When was 4-sp trans driven speedo gear installed ?

There's no need to speculate - the DRIVEN gear was installed on the Engine Dress Line at St. Louis (and at every other Chevrolet assembly plant); Muncie had no clue which car any given transmission was going to end up in, much less the detail of the dealer order for that car that specified the rear axle ratio (every ratio had its own DRIVEN gear). All Muncie knew (and all they needed to know) was that there were two "ranges" of axle ratios that called for two different DRIVE gears (plus the very low-volume 6-tooth gear).

John------

I don't understand why this is then not shown in the AIM?

[Wayne M.]

Re: When was 4-sp trans driven speedo gear installed ?

John------

I don't understand why this is then not shown in the AIM?

Joe, John -- my error in not looking first in the AIM ('65 AIM, UPC, sheet 4, view B, M20 M22). Item 6, 3860356 Speedo Driven Gear Asm. (includes the bullet, the plastic driven gear, the thingy that holds the bullet in place. Production (3-sp ?) retaining screw is (re)used.

I guess the reason this part is not found in the base 3-speed drawings (UPC 7, sheets B2,B3) is because (in '65, at least) there was only one axle ratio (3.36) for this trans. Therefore no need to refer to a "chart" to determine which plastic gear.

[Joe R.]

Re: When was 4-sp trans driven speedo gear installed ?

John------

I don't understand why this is then not shown in the AIM?

Also, how did the line people know what DRIVE gear was installed. It could have been a 6,7,8,9 tooth thru the years. Was the transmission marked in some way besides the input having grooves?????

JR

[Mike E.]

Re: When was 4-sp trans driven speedo gear installed ?

There is a reference to the speedo gear on the '65 AIM.

I don't fully understand what it saying.





Mike

[Joe L.]

Re: When was 4-sp trans driven speedo gear installed ?

There is a reference to the speedo gear on the '65 AIM.

I don't fully understand what it saying.





Mike

This indicates that the installation of the driven gear IS covered in the AIM. I'm out-of-town and could not check the AIM's myself. So, this confirms everything.

[Michael M.]

Re: When was 4-sp trans driven speedo gear installed ?

Also, how did the line people know what DRIVE gear was installed. It could have been a 6,7,8,9 tooth thru the years. Was the transmission marked in some way besides the input having grooves?????

JR

I know that at the Fremont Plant, the build paper (we called it a manifest) called out the transmission code and the driven speedo gear. I'm sure St. Louis had something similar. The manifest took all the options (like gear ratio's) into consideration so there were no decisions made by the operator. We used to say, if all else fails, read the manifest!

[Wayne M.]

Re: When was 4-sp trans driven speedo gear installed ?

Hey Bill W. --- did you notice the right column in Mike Eby's '65 AIM sheet 4, "M20 M22" ? [His copy is much better than mine]. It says that on 23 April, they added trannys # 3879986, ..9987, & ...9988. These are the M22's which show up in Colvin's appendix Muncie plant ledger notes, that show a total of 30 total of these three part #s shipped to C.O. (central office ?) in the time period 19 April to 7 August, 1965. The three trans part #s covered the three drive gear groups: 3.08 to 3.55; 3.70 and 4.11; and the 4.56.

[John H.]

Re: When was 4-sp trans driven speedo gear installed ?

Open the '65 A.I.M. to UPC 12, sheet C3, View "B", item #9 (Speedo driven gear, refer to 3813578 chart). Speedo components were always released in UPC 12. This sheet is the "base car" with a 3-speed, but it doesn't matter in terms of the speedo DRIVEN gear - both 3-speed Saginaw and and 4-speed Muncie transmissions used the same family of DRIVEN gears (which were shown on the 3813578 chart, and were called out on every unit's broadcast copy and later on the manifest).

[Mike E.]

Re: When was 4-sp trans driven speedo gear installed ?

Open the '65 A.I.M. to UPC 12, sheet C3, View "B", item #9 (Speedo driven gear, refer to 3813578 chart). Speedo components were always released in UPC 12. This sheet is the "base car" with a 3-speed, but it doesn't matter in terms of the speedo DRIVEN gear - both 3-speed Saginaw and and 4-speed Muncie transmissions used the same family of DRIVEN gears (which were shown on the 3813578 chart, and were called out on every unit's broadcast copy and later on the manifest).

Here's the page John is referring to:

Quick question are there any Copyright Attorney around that might know if I'm violating any laws by posting these pages? I don't want to get NCRS or myself in any trouble?



Mike