Muncie 4 Speed dated after assembly date on an LT-1?

[Mick D.]

Hi folks,

Am new here, but I did try to identify and date my Muncie using 6th edition TIMJG and it doesn't make sense.

From the pictures I have taken, I believe it to read C11S116326 with a date code of P1M13B (Tag is 3981710WY)

This would give me an 13th of August 1971 transmission which has the correct partial VIN, that is on a car that was built 5th of May 1971, and registered on the 28th of May 1971 in South Carolina.

Am I reading the numbers correctly? (I know its a little hard to read)

Thanks in advance,

Mick

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[Mark F.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Hi folks, Am new here, but I did try to identify and date my Muncie using 6th edition TIMJG and it doesn't make sense. From the pictures I have taken, I believe it to read C11S116326 with a date code of P1M13B (Tag is 3981710WY)...This would give me an 13th of August 1971 transmission which has the correct partial VIN, that is on a car that was built 5th of May 1971, and registered on the 28th of May 1971 in South Carolina. Am I reading the numbers correctly? (I know its a little hard to read) Thanks in advance, Mick

Hi Mick,

Looks to me as though you have read the numbers correctly - and yes, (agree) the P1M13B date code does not appear TFP for a May 1971 build date.

You may want to try to determine the CASTING dates of the the main case; extension housing and side cover to see if they line up more closely with your build date.

The following link gives some advice (see italics below) about Muncie casting dates...

https://www.motortrend.com/how-to/1607-how-to-identify-a-muncie-four-speed-build-date-codes-vins-and-input-shaft/

One circle is the Date Marker and the other circle is referred to as the Status Marker.

One half of the Date Marker circle will have a month designator for when the part was cast. The month is determined by numbers 1 through 12, which correlate with the months of the year. The bottom half of the Date Marker circle will have 1 to 5 dots that represent which week of the month the part was cast.

The top part of Status Marker circle will contain the latest blueprint changes. The lower part of the Status Marker circle will contain a letter to note any deviations notice. It can be acceptable for the Status Marker circle to be blank. The Status Marker information was used by the Muncie plant to help track a problem part.

[Mick D.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Hi Mark,

I'll try to get some better pictures of the other numbers, thanks for the advice!

[Duke W.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Look for the VIN derivative stamp. Does it match? If blank the trans may is probably a warranty replacement.

Duke

[Mick D.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Hi Duke,

Yes, from what I can see, the VIN Derivative stamp matches the car. See below, C11S116326, the car sequence number is 16326

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[Mick D.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Hi Mark,

I did quite a bit of research on the casting clock for the M21 transmission, but really could not figure out what mine actually is dated. Even articles that mention the 12 box grid, don't seem to have one quite like mine.

As you can see below, it has dots in several months. This is the best contrast shot I could get of it.

casting clock.jpg

Hi Mick,

Looks to me as though you have read the numbers correctly - and yes, (agree) the P1M13B date code does not appear TFP for a May 1971 build date.

You may want to try to determine the CASTING dates of the the main case; extension housing and side cover to see if they line up more closely with your build date.

The following link gives some advice (see italics below) about Muncie casting dates...

https://www.motortrend.com/how-to/1607-how-to-identify-a-muncie-four-speed-build-date-codes-vins-and-input-shaft/

One circle is the Date Marker and the other circle is referred to as the Status Marker.

One half of the Date Marker circle will have a month designator for when the part was cast. The month is determined by numbers 1 through 12, which correlate with the months of the year. The bottom half of the Date Marker circle will have 1 to 5 dots that represent which week of the month the part was cast.

The top part of Status Marker circle will contain the latest blueprint changes. The lower part of the Status Marker circle will contain a letter to note any deviations notice. It can be acceptable for the Status Marker circle to be blank. The Status Marker information was used by the Muncie plant to help track a problem part.

[David H.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Look for the VIN derivative stamp. Does it match? If blank the trans may is probably a warranty replacement.

Duke

Mick

Replacement case (no VIN stamp) with partial VIN stamp made during an earlier restoration?

Dave

[Patrick B.]

[David H.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

The “1” in the P number means model year 1971 not calendar year. Assuming M is August, the build date is August 13, 1970. This is very early for a May 1971 car, but certainly not impossible.

Patrick

Believe year in GM castings refers to calendar year - not model year. Believe this casting is from 1971.

John Hinckley article originally from Corvette Enthusiast.

cast_codes.pdf (camaros.org)

Dave

[Mark F.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Patrick Believe year in GM castings refers to calendar year - not model year. Believe this casting is from 1971. John Hinckley article originally from Corvette Enthusiast. cast_codes.pdf (camaros.org) Dave

Hi Dave,

I believe the "P" stamping is the assembly stamp for the tranny - not the raw aluminum casting date of the case it is stamped into.

long weblink, but here's what it says in case the link blows up on you...

1967-1974 production codes will start with the letter P and have five numbers, for example: P9D09.

• P represents the manufacturing plant, Muncie.
• 9 represents the model year.
• D represents the month code when the unit was assembled, April.
• 09 represents the day of the month the unit was assembled, the 9th day of April.

https://www.google.com/search?q=corv...client=gws-wiz

[David H.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Mark

John Hinckley's article is specific about calendar year on GM castings.

Wonder source of Google's information?

Dave

[Mick D.]

[Jim D.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Mark's info above is correct. The assembly stamp is model year not calendar year. I think there's some confusion between the casting date and assembly stamp.

[Mark F.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

Jim - I agree with what you are saying.

Dave,

Maybe this analogy will explain my thoughts a little better (I think John's article is talking exclusively about casting plants - not subcomponent assembly plants, right?):

The transmission case casting date is molded in at the casting plant - "x" days, weeks, or months before that casting is assembled at the transmission assembly plant. The "P" number is stamped (not cast) into the case when assembly takes place.

The same goes for the engine block casting date, which was molded in at the casting plant - "x" days, weeks, or months before that casting was assembled into an operating engine at the engine assembly plant. The engine assembly plant places their assembly stamp on the engine pad when assembly takes place days, or weeks after the block is cast. as in "T"ddmmXX"

Does my comparison make sense, or have I misread John's article?

[Mick D.]

Re: Muncie 4 Speed dated after assembly date on an LT-1?

First, thanks everyone for trying to help a newbie, what a great group of folks.

I'm starting to lose track ... are we potentially saying that the assembly stamp P1M13B is by Model Year (1970), whereas the casting date (which I posted a 'dark' picture of above) is by Calendar Year? I still can't understand the casting date grid below. Here are two new pictures.

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