1970 Z28 M21 stock speedo gears - 3:73 rear gear

Discussion in 'Original Drivetrain Topic' started by 70lt1z28, Nov 14, 2017.

  1. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Oct 3, 1999
    Beavercreek, Ohio, USA
    Changing out my speedo gears from my M21. I know they are not stock as I changed them out when I changed to a 4:10 rear gear many, many years ago. They are currently an 8 tooth "green" drive gear and a 25 tooth "orange" driven gear. Does anybody know what the stock gears were for a 3:73? I seem to remember the drive gear was steel? I also have a purple 10 tooth drive gear and a 22 tooth green driven gear.
    I also have the Stewart Warner external gear reducer box that has a 0.8241 internal ratio.
    I know the tire diameter makes a difference, but I will be going back to the stock tires eventually so whatever the stock gearing was is where I want to go.
    Thanks in advance.
     
  2. l16pilot

    l16pilot Veteran Member Lifetime Gold Member

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    Hurlock, MD, USA
    Try this Dave. Not the easiest to read and I really do not recall where I got the pic...or who wrote in the notes and added highlighter.

    100_1657.jpg
     
  3. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Beavercreek, Ohio, USA
    Thanks, Steve. I tried and even zoomed it in but could not fully read the part numbers. It LOOKS like it is a 14 tooth driven gear (3XX0347?) and a cross out on the driven gear teeth but maybe a 3924097 part number?
    One thing that confuses me is that for the close ratio the external gear reducer (adapter-Speedo) only seems to be used with the 4:10 gears (last line), but for the wide ratio M20 it is used with 3:55, 3:73 and 4:10, all with the Z's F60-15 tires. With the speedo gears being driven directly off the output shaft, what difference does it make if its wide or close ratio? The output shaft is directly coupled to the drive shaft so it doesn't matter what the gear reduction is in front of it, only the rear end gear and tire size behind it. Am I missing something here?
     
  4. l16pilot

    l16pilot Veteran Member Lifetime Gold Member

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    Dave - my image isn't much better, but I believe the M20 driven gear PN for 3.73:1 and 4.10:1 and F60-15 tires is 3987919 and the drive gear is 6261794...with the only difference being the reducer / adapter. The M21/M22 driven and drive gears are completely different PNs which I do not understand because I believe the output shaft diameter is the same and all are 1:1 in 4th gear. this chart covers Chevelle and Camaro for M21/M22, but the Chevelle M22 used the large output shaft in 1970 whereas the Camaro M22 used the same output shaft as the M20/M21...so maybe the bottom section really applies to just the large output shaft M22? Again, I do not recall who wrote the notes on the page, so i cannot account for accuracy....it's all I've got, but hopefully someone out there has more detail/insight to the question.

    As a side note, my unrestored 70Z with the M21/3.73 combo still has, I believe, the original speedo gear set in place. Next time I get it out and on the lift, (plans are to do so over Thanksgiving week), I can pull the speedo and check the gear colors.
     
  5. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Beavercreek, Ohio, USA
    Did some searching and did find the following on CRG:

    ’68-’70 Muncie 4-speeds used two different 8-tooth NYLON DRIVE
    gears, retained on the mainshaft with a spring steel clip:

    1.85” O.D. – GM #3924098 (later, #6261794); same “large” O.D.
    as #3708144, used the same range of 18-22-tooth “small”
    DRIVEN gears, and was light green in color.

    1.77” O.D. – GM #3924097 (later, #6261795); same “small” O.D.
    as #3708145, used the same range of 22-25-tooth “large”
    DRIVEN gears, and was medium blue in color.

    ’71-up 4-speeds had a larger mainshaft, and used only one DRIVE
    gear; 8-tooth, GM #3978758, the “large” size, 1.85” O.D., and used
    the 18-22-tooth range of “small” DRIVEN gears. There was no
    range of 22-tooth (green) to 25-tooth DRIVEN gears that would
    work with this DRIVE gear for 3.70 and higher numerical axle
    ratios, so an adapter gearbox was used for those axles along with
    the 18-22-tooth family of DRIVEN gears.

    The 3924097 gear corresponds to your chart, so maybe that is the right number to start with for that. I think maybe that is the one I already have. Based on these numbers, I'm not sure where on your page the 14 tooth fits in. It looks like that is too small for either of these drive gears.

    On your Z, does it have the external gear reducer box? I've seen both ways and mine didn't have one but I went ahead and got one anyway as folks I talked to said that EVERY 70Z was supposed to have one. Something about the fact that the F60-15 tires were brand new for 70 and they didn't have a internal gearing solution in place for it yet so they compensated with the box.

    Maybe at MCACN I'll crawl under a car and take it apart to find out! They wouldn't mind, would they?
     
  6. twozs

    twozs Veteran Member

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    Aug 5, 1999
    hopewell jct ny
    its in the assembly manual speedogear.jpg
     
  7. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Beavercreek, Ohio, USA
    My 1970 AIM references "See Chart" for speedo gear...but there is no chart!

    And, of course, I can't read yours.
     
  8. FlaJunkie

    FlaJunkie Veteran Member Lifetime Gold Member

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    Rockledge, Florida
    Long story but the last guy in Cocoa to touch my transmission put the gear inside the back of the tail when he reassembled the unit! What an imbecile!

    [​IMG]


    And here is a link to the discussions we had about speedo gears.
     
  9. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    Oct 3, 1999
    Beavercreek, Ohio, USA
    Brain fade...I looked through my Favorites and remembered this thread:
    http://nastyz28.com/threads/muncie-speedo-bullet-and-adapter-1970-z28.310766/page-3
    which spells it out.

    It looks like an 8 tooth 3924097 (3708145 steel) drive gear and a 20 tooth Blue 3987920 driven gear and the 0.8241 (also listed as a 0.8242) external gear box. This is for the stock F60-15 EDIT: 25.9 diameter tires.

    Still not sure exactly when the external box comes into play. The only thing it can be for is when the internal gears can't quite get it right. This shouldn't matter if its a M20. M21 or M22, only if there is a unique tire-rear gear combination. I can believe that when the F60 tires came on-board they didn't have the gears in place to work with them, but beyond that and why it seems to be on some original cars and not others is a mystery.
     
    Last edited: Nov 15, 2017
  10. twozs

    twozs Veteran Member

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    Aug 5, 1999
    hopewell jct ny
    well i guess i should have clarified .. mine is from the 73 AM and i felt it was just an example for the page number . i can read where it says 73 though . and if you think it applies still , all you have to do is ask and it can be zoomed in . 90% of the time on here people could care less what i post so if don't spend a lot of time on it .... last time i posted about sticker location in the AM and the OP told me it wasn't in there ... well it was and the page was the same as the other year .
     

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