Muncie M20 ID Tag 3890563 Mystery..HELP

**Michael H.** · September 21, 2008, 09:53 AM

Re: Muncie M20 ID Tag 3890563 Mystery..HELP

Originally posted by Thomas Ervin (42996)

I am doing a frame off on my 66 Roadster with the following powertrain:
327-350, M20 4-Speed and 3:55 gear. This is a very original all numbers matching car down to the horns.
On close inspection of the transmission I noted the the ID Tag #3890563 doesn't seem to be correct according to the experts. The transmission has the correct dates and matching serial number clearly shown.
This car is a late June build #124341 and I know this M20 came in the car from the factory. I defer to the experts on this list to solve my mystery.
Thanks, Tom #42996

Your 3890563 IS the correct transmission for your car. Unfortunately, that number is not listed in the M20/M21 section of the AIM.
The number was added some time after start of production and didn't ever make it to the transmission section of the AIM.
The number IS shown in another section of the 66 AIM though. I'll dig it out and post in an hour or two........

**Joe L.** · September 21, 2008, 02:43 PM

Re: Muncie M20 ID Tag 3890563 Mystery..HELP

Originally posted by Thomas Ervin (42996)

I am doing a frame off on my 66 Roadster with the following powertrain:
327-350, M20 4-Speed and 3:55 gear. This is a very original all numbers matching car down to the horns.
On close inspection of the transmission I noted the the ID Tag #3890563 doesn't seem to be correct according to the experts. The transmission has the correct dates and matching serial number clearly shown.
This car is a late June build #124341 and I know this M20 came in the car from the factory. I defer to the experts on this list to solve my mystery.
Thanks, Tom #42996

Tom-----

I concur with Michael. The GM #3890563 is the correct transmission for your car. GM #3890563 is a wide ratio (M-20) Muncie used for 1966-67 Corvettes with 3.55:1 rear gear. I'm not sure that this transmission was used for early 1966 L-79 with M-20 and 3.55 but it was definitely used for later 1966 with L-79, M-20, and 3.55:1 as well as other applications.

**Wayne M.** · September 21, 2008, 04:43 PM

Re: Muncie M20 ID Tag 3890563 Mystery..HELP

Originally posted by Michael Hanson (4067)

Your 3890563 IS the correct transmission for your car. Unfortunately, that number is not listed in the M20/M21 section of the AIM.
The number was added some time after start of production and didn't ever make it to the transmission section of the AIM.
The number IS shown in another section of the 66 AIM though. I'll dig it out and post in an hour or two........

Time's up

; found it in the '66 AIM UPC G81, Sheet A1. In fact it was probably you that posted a jpeg of this on the NCRS or VH site a few years ago (which I copied for my files, as I don't have a '66 AIM).

Can't make out the date the addition was made (looks like 7-16-65) but maybe someone with a clearer copy can report. AIM table indicates this part # addition was made for '66 L79 w/M20 and 3.55 posi ONLY.

Why, you may ask

. Here's my theory. The 7.75 tires used from 1965 through 1967 MY are not all the same rolling radius (lower profiles, etc). There were probably complaints of high speedo readings on cars equipped with 3.55 (red 21 tooth driven gear, that requires the 3708144 speedo drive gear).

This was corrected in 1967 (maybe earlier) with a Chev Dealer Service Info Bulletin (March 23/67) that calls for the green 22 tooth plastic gear and the corresponding 3708145 drive gear for Corvettes (and other passenger cars) with 7.75 x 15 rubbber ane 3.55 rears. The 3.70 diff still called for the same speedo gear, so one or the other of these would give speedo error of up to 5%.

The '66 and '67 Power Train charts show that L79 was the only engine where you could get 3.55's as an optional gear ratio combined with a wide ratio trans. The above switch to a 22-tooth green gear [the crossover small dia. grey 22-tooth gear #3987922 wasn't available in the late '60s] meant that Muncie now had to assemble a wide ratio with an 8-tooth steel sending gear that was traditionally reserved for close ratio trans with the 3.70 and 4.11 differentials. No other engines, incl big block, offered 3.55 (or higher numerical) rear gears with wide ratio.

Enter the weirdo 3890563.

Comments invited; would also be interested to know if the '67 AIM is same as the '66, in this respect.

**Michael H.** · September 21, 2008, 05:01 PM

Re: Muncie M20 ID Tag 3890563 Mystery..HELP

Originally posted by Wayne Midkiff (3437)

Time's up

; found it in the '66 AIM UPC G81, Sheet A1.

Yup, you win the prize. The 3890563 transmission was added on AIM date 11-26-65.
I remember the discussion a few years ago about this. That discussion probably had something to do with L72 though.
I think the reason the 3890563 wasn't shown on the M20/21 sheet is probably because that sheet wasn't updated at the same time that the G81 sheet was.

Wonder what the prize was??

**Thomas E.** · September 21, 2008, 05:15 PM

Re: Muncie M20 ID Tag 3890563 Mystery..HELP

Thanks everybody!!! I thought for awhile the ID tag got switched during assembly. All my research failed to come up with even the history on this ID tag number.
Tom #42996