M20 or 21?
06-08-2014, 01:15 PM
#1
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M20 or 21?
(doubt that there is ANY chance of a M22, so I won't go there)
I have done more homework on my newly-acquired 1965 and here are the applicable as-built specifics:
L76 365hp, 4.11 posi, M2x case (correct with VIN and matching date code).
My guide book tells me that all M20s and M21s (also M22s) have the same case, with the same main case, extension housing, side cover numbers.
So, is there an easy way to tell if mine is a M20 or 21 short of doing a "count the revolutions" exercise?
I have done more homework on my newly-acquired 1965 and here are the applicable as-built specifics:
L76 365hp, 4.11 posi, M2x case (correct with VIN and matching date code).
My guide book tells me that all M20s and M21s (also M22s) have the same case, with the same main case, extension housing, side cover numbers.
So, is there an easy way to tell if mine is a M20 or 21 short of doing a "count the revolutions" exercise?
06-08-2014, 01:51 PM
#2
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Shift to 1st gear and bring car back up to the previous speed. Note the tachometer.
If tachometer indicates 2540 RPM, you have a wide ratio box.
If tachometer indicates 2200 RPM, you have a close ratio box.
Simple. Easy to remember.
Jim
06-08-2014, 01:59 PM
#3
Burning Brakes
As a slight variant, I ran mine to 2000 rpm in 4 th then 2000 rpm in 1st. Divide the 4th speed by 1st gear speed. The result is the ratio. If about 2.2 the close ratio; if about 2.6, then the wide ratio. A little more speed will help make the speed differentials more obvious. Any rpm can be used, just make both the same and divide
06-08-2014, 03:01 PM
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Thanks, guys!
06-08-2014, 03:13 PM
#5
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(doubt that there is ANY chance of a M22, so I won't go there)
I have done more homework on my newly-acquired 1965 and here are the applicable as-built specifics:
L76 365hp, 4.11 posi, M2x case (correct with VIN and matching date code).
My guide book tells me that all M20s and M21s (also M22s) have the same case, with the same main case, extension housing, side cover numbers.
So, is there an easy way to tell if mine is a M20 or 21 short of doing a "count the revolutions" exercise?
I have done more homework on my newly-acquired 1965 and here are the applicable as-built specifics:
L76 365hp, 4.11 posi, M2x case (correct with VIN and matching date code).
My guide book tells me that all M20s and M21s (also M22s) have the same case, with the same main case, extension housing, side cover numbers.
So, is there an easy way to tell if mine is a M20 or 21 short of doing a "count the revolutions" exercise?
And if the transmission "protests loudly" to you during this test, you may just have that elusive M22 (ie. the rock-crusher) .
But the chances are probably slim to none. 
Larry
06-08-2014, 03:34 PM
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I did find an old post during a lengthy search that stated all L76s were built with M21s. Not sure about the factuality of this statement, though.
06-08-2014, 03:39 PM
#7
Team Owner
Travel in 4th gear with engine turning exactly 1000 RPM. Note your speed.
Shift to 1st gear and bring car back up to the previous speed. Note the tachometer.
If tachometer indicates 2540 RPM, you have a wide ratio box.
If tachometer indicates 2200 RPM, you have a close ratio box.
Simple. Easy to remember.
Jim
Shift to 1st gear and bring car back up to the previous speed. Note the tachometer.
If tachometer indicates 2540 RPM, you have a wide ratio box.
If tachometer indicates 2200 RPM, you have a close ratio box.
Simple. Easy to remember.
Jim
06-08-2014, 04:07 PM
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This will explain the M20/M21 ratio applications:
06-08-2014, 04:20 PM
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Nope - that's a function of '65-up linkage setting - has nothing to do with the transmission itself. All '65-up 4-speeds were built with shift levers on the side cover with two holes for the linkage rods; standard setting was the hole closest to the end of the lever, and that's the way they were all built. The dealer or owner could change to the holes closest to the shifter shaft nuts, for the "short-throw" setup.
06-08-2014, 06:01 PM
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Nope - that's a function of '65-up linkage setting - has nothing to do with the transmission itself. All '65-up 4-speeds were built with shift levers on the side cover with two holes for the linkage rods; standard setting was the hole closest to the end of the lever, and that's the way they were all built. The dealer or owner could change to the holes closest to the shifter shaft nuts, for the "short-throw" setup.
06-08-2014, 06:17 PM
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My bride's road racer has an M22 and lightly muffled side exit exhaust.
In 1st gear, the M22 is louder.
Jim
06-09-2014, 09:26 AM
#13
Safety Car
FWIW, I converted my original M21 to an M20 by changing the main drive gear and the countergear during a rebuild. There is nothing visible on the outside of the trans to show the change when the trans is installed in the car. M21s are a little light on gearing for the street, especially if you have a low differential gear ratio like 3.08 or 3.36.
06-09-2014, 11:34 AM
#14
Burning Brakes
Nope - that's a function of '65-up linkage setting - has nothing to do with the transmission itself. All '65-up 4-speeds were built with shift levers on the side cover with two holes for the linkage rods; standard setting was the hole closest to the end of the lever, and that's the way they were all built. The dealer or owner could change to the holes closest to the shifter shaft nuts, for the "short-throw" setup.
John, you are the best!
06-09-2014, 12:03 PM
#15
Burning Brakes
Ok, just finished making the change and I am amazed at the difference. The short-throw is really short compared to the longer-throw. I think I am going to enjoy it!
Last edited by 64roadster; 06-09-2014 at 12:04 PM. Reason: Spelling
06-09-2014, 12:16 PM
#16
Le Mans Master
