1970 M21 speedo gears

[Terry L.]

What is the factory correct drive gear and driven gear for M21 with 3.55:1 and the 3.70:1 rear diff? I know this subject has been discussed before but I cant seem to find the info in the search. It appears the color of driven gear currently installed is green or blue.

Thanks

[Mike F.]

Re: 1970 M21 speedo gears

http://willcoxcorvette.com/instructions/1970.JPG

Found this,
Part 1: Speedometer Pencil Gear
L20T steel 3860329
NOTE: this ONE (1) gear used with 1.92" outside diameter circle gear.
L22T green nylon 3860345
L23T black nylon 3860346
L24T yellow nylon 3860347
L25T orange nylon 3860348
NOTE: these FOUR (4) gears used with 1.76" outside diameter circle gears.
L17T purple nylon 3987917
L18T brown nylon 3987918
L19T natural nylon 3987919
L20T blue nylon 3987920
L21T red nylon 3987921
L22T silver nylon 3987922
NOTE: these SIX (6) gears used with 1.84" outside diameter circle gears.



Part 2:

Drive Gear – Sometimes called “Circle” Gear and is located inside transmission tail housing on the main shaft. Transmission must be removed and some disassembly required to replace. There are three (3) steel drive gears two (2) large and one (1) small.

8 tooth (Large) 1.84” diameter (#3708144) – this is the most common and is for 27 tooth spline. --- For 32 tooth spline use (#3978758)

8 tooth (Small) 1.76” diameter (#3708145)

6 tooth (Large) 1.92” diameter (#3845079) – only works with 20 tooth driven gear (#3860329) which is for 4:56 rear gear.

Note 1: To check from outside use a cardboard gage through opening and measure width. 1.84” is .610” wide and 1.76” is .531 wide and both have 8 teeth

Note 2: "Large" drive gear is usually found with 3.08 to 3.70 axles and "Small" drive gear is usually found with 3.70 to 4.11 axles

[Paul O.]

Re: 1970 M21 speedo gears

Terry

From a 1971 and 1982 Chevy Part Manual the driven gear P/N is 3860345 for both a 3:55 and 3:70 with close ratio transmission.

[Terry L.]

Re: 1970 M21 speedo gears

Mike,

Thanks for the info. According to Wilcox's chart the 3.55 and 3.70 ratio diff with close ratio trans and F70/15 tires used the same 22T green driven gear. According to your info the 22T driven gear requires the smaller OD drive gear. Your note 2 suggest the larger OD drive gear for the rear diff ratio's of 3.08 to 3.70.
Could the M21 have the the 22T driven gear with the smaller OD drive gear with 3.55 or 3.70 rear diff.

[Michael W.]

Re: 1970 M21 speedo gears

There should be a chart in your AIM. The drive and driven gears tooth count/colours are the same for M20 and M21.

[Wayne M.]

Re: 1970 M21 speedo gears

There should be a chart in your AIM. The drive and driven gears tooth count/colours are the same for M20 and M21.

Hi Mike --- in the 1970 P&A30B (Corvette-only parts catalog, revision dated 1st of Jan 1970), they show different drive gears for close versus wide ratio, and then cover the numerically high(er) rear axles on the wide ratio via an external Stewart-Warner adapter, and the numerically low axles on the close ratio with a different S-W adapter. I think this was to rationalize transmission inventory.

Now to the OP's original query for M21; the catalog does show the green 22-tooth driven gear 3860345 for both 3.55 and 3.70 axles. External adapters are called out for only the 3.08 and 3.36 with M21. Should mention that all these are for F70x15 tires.

Same thing on the 1968 chart, so it appears they were willing to live with a (max) 5% error in speedo reading, hopefully split high on one ratio and low on the other.

Also, same for 1967 (ie. green plastic gear for both 3.55 and 3.70), with 7.75x15 rubber.

[Michael W.]

Re: 1970 M21 speedo gears

Wonder why a M21 with 3:36 would need a different combo than M20 with the same diff? Transmission output speed is identical.

[Joe L.]

Re: 1970 M21 speedo gears

What is the factory correct drive gear and driven gear for M21 with 3.55:1 and the 3.70:1 rear diff? I know this subject has been discussed before but I cant seem to find the info in the search. It appears the color of driven gear currently installed is green or blue.

Thanks

Terry------


It makes a big difference if it's green or blue. The blue gear is a 20 tooth gear of the "small OD series" and is compatible only with the "large" OD drive gear. The green gear is a 22 tooth gear of the "large OD" series and is only compatible with the "small" OD drive gear. So, correctly identifying the driven gear is necessary if you are to determine which series of driven gears are compatible with the installed drive gear.

[Cecil L.]

Re: 1970 M21 speedo gears

This is specifically for 69 but should be good for 70-72, I think. Good reference anyway.

[Terry L.]

Re: 1970 M21 speedo gears

Thanks for the info guys, Nice chart Cecil, It appears I have the correct 22T driven gear and drive gear. I wanted to make sure I had the correct gears before reassembling the transmission.

[Wayne M.]

Re: 1970 M21 speedo gears

Wonder why a M21 with 3:36 would need a different combo than M20 with the same diff? Transmission output speed is identical.

Mike -- you're correct that it wouldn't need a different combo, for the reason you say. But by selecting the correct ratio of Stewart-Warner external adapters on the less frequently demanded trans [close/wide] versus final drive ratios {per powertrain combination charts} and the appropriate colour of plastic driven gear, allowed St.Louis to reduce transmission inventory (ie. all wide ratios had the same internal drive gear, and all close ratios had the same (but different) drive gear).

Ditto in spades for the Muncie IN assy plant; only two types of 4-speed trannys to build, instead of four.

[Joe L.]

Re: 1970 M21 speedo gears

Cecil------


Things were a little different for 1970 as some 1970 applications used an adapter.

Things changed "big time" for 1971 and later. The 1971 and later Muncies (and Super T-10s, too) used only a "large" diameter DRIVE gear which was compatible only with the 18-21 tooth "small" OD DRIVEN gears as well as the new-for-1971 22 tooth "SILVER" DRIVEN gear. Rear gear ratios that couldn't use one of these DRIVEN gears as-is, used a specific speedometer adapter in combination with one of these 18-22 tooth DRIVEN gears.

By the way, while 1968-70 Muncies used a nylon DRIVE gear, 1971 and later went back to a steel DRIVE gear, although not the same as the ones used prior to 1968.

[Cecil L.]

Re: 1970 M21 speedo gears

Thanks Joe,
I have SB's for speedo gears for 67, 68 and 69 but don't yet have the 70, 71 and 72 in my collection. Still working on the library.