66 Muncie differences 3880853/3880855

[Ronald L.]

Searched the archives...


66 Muncie - AIM calls out two RPO M21 numbers 3880853 for L79
and 3880855 L72 & L36

Anyone know what the difference is other than the metal tag?

[Joe L.]

Re: 66 Muncie differences 3880853/3880855

Searched the archives...


66 Muncie - AIM calls out two RPO M21 numbers 3880853 for L79
and 3880855 L72 & L36

Anyone know what the difference is other than the metal tag?

Ronald------


The difference was the installed speedometer gears. The 3880853 used the GM #3708145 DRIVE gear and GM #3860345 DRIVEN gear. The 3880855 used the GM #3708144 DRIVE gear and either the GM #3860343 or GM #3860344 DRIVEN gear (I can't confirm which).

[Ronald L.]

Re: 66 Muncie differences 3880853/3880855

Thanks Joe I knew you'd know! and it had to be real subtle at that.

[Ralph P.]

Re: 66 Muncie differences 3880853/3880855

The latest "Restorer" shows an original 425 Coupe ( page 37 ) with a 3880853 M21 tag. I know of at least one other 425 with this tag . How often do you see this on known original cars ?

[R N.]

Re: 66 Muncie differences 3880853/3880855

My '66 425HP still has it's original 3880853 tag. I understood that this tag number was also for 390/425 HP with M21 with 3.70:1 and 4.11:1 rear.

[Ralph P.]

Re: 66 Muncie differences 3880853/3880855

Thanks, Kurt....

The confusion came from Noland's book.. ( page 363 ) , which seems to say thay only the 3880855 was specified with the 425 .

[Wayne M.]

Re: 66 Muncie differences 3880853/3880855

Ronald------


The difference was the installed speedometer gears. The 3880853 used the GM #3708145 DRIVE gear and GM #3860345 DRIVEN gear. The 3880855 used the GM #3708144 DRIVE gear and either the GM #3860343 or GM #3860344 DRIVEN gear (I can't confirm which).

Ronald, Joe -- Specifically referring to the 1966 L79 close ratio with the 3880853 tag, according to the power team combinations chart, it could have been factory delivered with either the '345' [green, 22 tooth, for 3.70 final drives], or 3860347 [yellow, 24 tooth, for 4.11 drives].

But for BB motors, the '853' tag could have been found on close ratio trans for 3.08 (3860342, natural color 19-tooth) final drive (L36), or a '343' [blue, 20 tooth, for 3.36 final drives -- L36 or L72]; also a '344' [red, 21 tooth, for 3.55 final drives -- on L72]. this, in addition to the green and yellow gears mentioned above.

Maybe it's the fault of the AIM or Noland , but there's a lot of confusion, with many owners trying to identify the motor by the trans tag. Some of the major repro catalogs repeat this logic, to a certain extent.

And we haven't even talked about wide ratios yet .

[R N.]

Re: 66 Muncie differences 3880853/3880855

Thanks, Kurt....

The confusion came from Noland's book.. ( page 363 ) , which seems to say thay only the 3880855 was specified with the 425 .

That would be true when the car came with the standard rear ratio, which I believe was 3.55:1

[Joe L.]

Re: 66 Muncie differences 3880853/3880855

Ronald, Joe -- Specifically referring to the 1966 L79 close ratio with the 3880853 tag, according to the power team combinations chart, it could have been factory delivered with either the '345' [green, 22 tooth, for 3.70 final drives], or 3860347 [yellow, 24 tooth, for 4.11 drives].

But for BB motors, the '853' tag could have been found on close ratio trans for 3.08 (3860342, natural color 19-tooth) final drive (L36), or a '343' [blue, 20 tooth, for 3.36 final drives -- L36 or L72]; also a '344' [red, 21 tooth, for 3.55 final drives -- on L72]. this, in addition to the green and yellow gears mentioned above.

Maybe it's the fault of the AIM or Noland , but there's a lot of confusion, with many owners trying to identify the motor by the trans tag. Some of the major repro catalogs repeat this logic, to a certain extent.

And we haven't even talked about wide ratios yet .

Wayne-----


I don't understand how the same part number transmission could have been used with different rear end ratios, especially with respect to the same part number being used for 3.70/4.11 and 3.08/3.36/3.55.

Theoretically, the same part number could have been used for EITHER 3.70 or 4.11 and the same part number could have been used for 3.08, 3.36, and 3.55 IF the driven gear was installed at the assembly line.

However, for the same part number to have worked for ALL of the above ratios would have required 2 different speedometer DRIVE gears. Those could not have been installed or changed on the assembly line. It seems inconceivable to me that 1 part number would have been used for an M-21 which would have been applicable to all available ratios.

Actually, I think there were more part numbers used than even are listed in the AIM.

[R N.]

Re: 66 Muncie differences 3880853/3880855

Ronald, Joe -- Specifically referring to the 1966 L79 close ratio with the 3880853 tag, according to the power team combinations chart, it could have been factory delivered with either the '345' [green, 22 tooth, for 3.70 final drives], or 3860347 [yellow, 24 tooth, for 4.11 drives].

But for BB motors, the '853' tag could have been found on close ratio trans for 3.08 (3860342, natural color 19-tooth) final drive (L36), or a '343' [blue, 20 tooth, for 3.36 final drives -- L36 or L72]; also a '344' [red, 21 tooth, for 3.55 final drives -- on L72]. this, in addition to the green and yellow gears mentioned above.

Maybe it's the fault of the AIM or Noland , but there's a lot of confusion, with many owners trying to identify the motor by the trans tag. Some of the major repro catalogs repeat this logic, to a certain extent.

And we haven't even talked about wide ratios yet .

Hi Wayne,

Here is a quote for you dated 2006:

Muncie trans tag

wayne midkiff <Send E-Mail> -- Tuesday, 10 October 2006, at 6:11 p.m.

Rex -- 3880853 is the GM part # for a close ratio Muncie for big and small blocks for 1966 and 1967 model years, with final drive ratios of 3.70 or 4.11.
Wide ratios had a different #, as did close ratios with 3.08 through 3.55 final differential ratios. An other trans # was required for 4.56 and up ratios, the only difference being the speedo sender gear (on the output shaft).

[Ronald L.]

Re: 66 Muncie differences 3880853/3880855

I will go back and see if in the AIM there could be a conflict, I tend to agree that a different axle has to have a different speedo gear, in fact, the 4.11 should too.

My L36 has the 3.7 in it and it does not appear it has ever been apart in its 49k history. We'll see if we can get under it this weekend and verify the speedo gear color but I did verify the speed via radar and it was within 1mph.

[Ronald L.]

Re: 66 Muncie differences 3880853/3880855

OK so there is some potential conflict in the AIM.
P345 the positraction page has that a posi axle/usage chart and it also has a trans/usage chart.

On that page L36 & L72 are linked up with the 3880853

It is ONLY on the 4-speed pages - un numbered after F389 - that the 3880855 is charted to the L36 and L72.

[Wayne M.]

Re: 66 Muncie differences 3880853/3880855

....I don't understand how the same part number transmission could have been used with different rear end ratios, especially with respect to the same part number being used for 3.70/4.11 and 3.08/3.36/3.55.

Joe -- Mea culpa; in my second paragraph, 2 errors. On the first line I should have said '855' tag (not 853). And, near the end of the same paragraph, I should have reiterated (from the first paragraph) that the '853' tag was for the green and yellow driven gears.

My head was hurting....

.. Actually, I think there were more part numbers used than even are listed in the AIM.

By that, do you mean those for the M22 ? I believe '66 [I don't have that AIM] used the same tags as in '65, as the countershaft had the same 1 inch shaft. Tags 3879987, 86 and 88, respecively for 3.08 to 3.55, 3.70 & 4.11, and 4.56 final drives.

[Joe L.]

Re: 66 Muncie differences 3880853/3880855

By that, do you mean those for the M22 ? I believe '66 [I don't have that AIM] used the same tags as in '65, as the countershaft had the same 1 inch shaft. Tags 3879987, 86 and 88, respecively for 3.08 to 3.55, 3.70 & 4.11, and 4.56 final drives.

Wayne-----


Actually, I think that some of these transmissions were supplied with an INSTALLED driven gear for a specific ratio rear end. All of those part numbers might not be in the AIM, though.

[Ronald L.]

Re: 66 Muncie differences 3880853/3880855

With AIM in front:

Pg 345
trans ext ratio usage
3880853 3.7& 4.11 RPO M21 with L36 & L72
3880857 4.56 RPO M21 with L36 & L72
3884695 4.56 RPO M22 with L88
3884693 3.7 4.11 3.55 RPO M22 with L88*** added 11-26-65
3890563 3.55 RPO M20 with L79 *** added 11-26-65

On the un numbered page following F389 UPC M20M21M22:

3870354 RPO M20
3880853 RPO M21 WITH L79
3880855 RPO M21 WITH L72 & L36
3878449 RPO M21 WITH L79
3879987 RPO M22 WITH L88 ***WAS L72, 3879986 RPO M22 RMVD

Yes - there were quite a few part numbers.