C2 rear end casting number

[Larry S.]

I have a 1966 M21 427/425 car built oct 28th 1965 ,I am trying to find out what the correct part number should be for the rear end housing assembely .

I understand the rear cover should be a part # 3871375 with a square head and should be non dated.THAT I HAVE ON THE CAR

From my aim manual if I read it correct I should have a part # 3880126 for the case witch is a 336 with L72 but what I have is a 3876476-N (what is this for)

The casting date of the case is is f145 june 14 of 1965?

The assembly date on the bottom is AU 10 12 65

Also I read in my judging manula that a norm between casting to assembly was 4 to 12 weeks if I am understanding my dates correct I am at 16 weeks

Thanks Again

[Joe L.]

Re: C2 rear end casting number

Larry----

You have the correct CASTING number for the differential housing. The "N" after the casting number denotes nodular iron, the material that the case is cast from.

4 to 12 weeks might be the "norm", but it's easily possible for the dates to fall outside that range. That's why NCRS and NCCB allow a 6 month "window". Your carrier assembly date is just right for the car and the "AU" codes denotes a 3.36:1 positraction unit originally used for 427s.

[Jack H.]

AGREE...

if you 'play a game' using the survey data sections from Noland Adam's Vol 2 restoration book tracking individual dated components from a given VIN survey car, you'll find the average is about what you quote. But, you'll also see the variance or standard deviation is DARN LARGE. So, I wholeheartedly agree with Joe Lucia--you're A-OK on dates!

[Larry S.]

Re: AGREE...

As always THANK YOU

Why do I have the wrong number in my aim manual.Is it because the number in the aim is actually an assembley number and the number I am looking at is just a case number?

If I am correct with the number being an assembley number how would the guys on the line know what to grab? where they in piles labled 336 w 427? or was there a tag on them?

Thanks Again .

Larry Savino

[Jack H.]

Expect the AIM....

to reveal the specific GM part numbers used for various components on the final assy line. Quite a few of these parts are made up of specific GM cast pieces where secondary operations (e.g. drill/tap, add specific internal components, Etc.) were performed at 'upstream' sub-assy facilities to generate that given part number. In these situations, the part typically shipped to the final line with the actual PN on a paper label/tag, on the part's in-transit protective packaging, Etc. and this is LONG GONE....

That's why you need a Judging Guide book in addition to the AIM. It tries to call out those identifying marks on various components which are 'telltale' to factory original authenticity.

Example 1:

Shock absorbers were installed with individual items (shock, mounting nut, washer, Etc.) called out separately. But, GM/Chevy service inventoried 'kits' whose box contained the shock itself along with known needed support components (nuts, washers, Etc.). So, the PN on the box if you happen across a real McCoy NOS shock at a swap meet WILL differ from the PN shown in the AIM as well as what's called out in the JG book....

Example 2:

Water pumps had a cast number on the housing as well as a legitimate PN for the completed assy. Pump housing were cast at Saginaw and shipped to Flint for final assy AS WELL AS at the Tonawanda foundry facility. You can have the same cast number on a SB waterpump housing for a given PN, but typically the Tonawanda cast number is incorrect for a SB Corvette (expect an originality deduction).... Also, some housings were drilled/tapped for external bypass feature with others not drilled/tapped. Size/style of impeller along with shaft length, hub size/style and press-fit position varied with target water pump final PN spec. But, many different PN's begin with the same basic housing cast number.
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Unfortunately, life isn't so simple as ONE 'correct' number.... But, given time and study, you'll get the hang of the numbers game!

[Wayne M.]

Re: AGREE...

Larry -- As Jack H. said, assy line had benefit of visible sticker codes to know what to pull from inventory. In the case of your early '66 diff, there would have been a white sticker w/red lettering "AU" on the rear cover flange face that bolts to the frame X-member. There also might have been stamping "AU" on the top rear flange of the carrier, where it mates with the cover. So even without seeing the bottom assy flange date and code, they knew what diff went with what chassis.

[John H.]

Re: AGREE...

The elusive "Broadcast Copy" (see my article "Numbers And The Missing Link - The Broadcast Copy" in the current issue of the "Restorer") had all the broadcast codes (or part number derivatives) printed on it, which appeared on the parts and containers on the line. If the Broadcast Copy had "AU" printed in the "Axle" box, they grabbed a differential with "AU" on it, etc. Hi-Lo drivers and line stockchasers used the full part number to store, track, and move inventory around, but the guys on the line used the Broadcast codes to select the parts.

[Jack H.]

Who printed these labels?

During disassembly of my '65 396, there was a really neat paper label on the rear axle. The code 'FC' was clear as a bell (396 with 4.56 gears) but it was written by hand.... Of course this axle code is NOT the same as the broadcast code/ECL for L78 with 4.56 rear end (that was QA). Says there were dual code sequences for certain items and people burning midnight oil making labels by hand!

[John H.]

Re: Who printed these labels?

Jack -

Some had sticky labels, some had ink stamps, some had tags, some had the Broadcast code hand-scrawled; about all they had in common was that all were visible to the line operator with the diff in the shipping rack. If not, and the operator had to walk around the other side of the rack to see the ID, someone was on the phone to Gear & Axle to bitch about it. The stamped codes weren't visible in the rack, as the diff was shipped in car position.

[Patrick H.]

Re: Who printed these labels?

My Dad's 72 had a nice label on top, about 1x2 inches, white with nice letters for the axle code. It was on the flat surface where the diff mounts to the crossmember, and hidden once installed.

I made a new one just like it, and installed it. Sure, no one sees it, but I know it's there.

Patrick