71 LT1 Rear Differential Decoding

[Mark Chance]

All,

I'm considering purchasing a numbers matching 71 LT1. Three quick questions:

1) Did the 71 LT1 come with just a few rear axle gearing combos, or did it come with the full range (3.08, 3.36, 3.55, 3.70, 4.11, and 4.56)?

2) I'm trying to decode the rear differential code. I only have a partial - the mechanic is taking a picture for me. It starts out AW (posi 3.08) and has 343 2 for the fourth thru seventh digits. My books tell me this means it was made by the second shift on the 343rd day of 1971. My mechanic, however, believes this means it was made in 1972 and doesn't match the rest of the car. AW, by the way, was only used in 1971 (CAW in 1970 and not used at all in 1972). Does anyone have any insight into this?

3) Was "2" a standard shift code for the differential? My books just have 1 (first shift), D (day shift), and N (night shift). Anyone know if 2 is valid.

Thanks for any help.
Mark

[Chuck S.]

Re: 71 LT1 Rear Differential Decoding

Mark, see Warren #12139 post about 3-4 threads below for complete axle availability in 71. My opinion is that the AW (3.08:1) is not original if the car was born as an LT-1.

The 70-72 TIM&JG indicates that AW differentials, while not stated as available in 72, have been observed on original 72 cars. From the judging standpoint, I don't believe the differential stamp is judged, but I could be wrong.

Since the differential is not uniquely assigned to the car, it would be of less concern to me than other components that are unique to the VIN number such as the engine and transmission. In other words, that information (non-original diff) might affect the price negotiations, but would not be a deal killer for me...I would, however, be sure the car started life as an LT-1.

[Peter M.]

Re: 71 LT1 Rear Differential Decoding

Mark,

Don't quote me but I think it is close to this - ordering LT1 option came with 3:70 and M21. IF you wanted to stick with M21 but wanted optional performance ratio, you got 4:11's. If you wanted economy ratio with M21, you got 3:55.

If you wanted LT1 but opted for M20, you got 3:55. If you wanted perform ratio with M20 you got 3:70. If you wanted economy ratio, you got 3:36.

4:56 was only available as an optional ratio if you had a ZR1.

This is how I understand it but there are guys definitely on this board who know the real deal. I am sure they will respond.

[Mark Chance]

Re: 71 LT1 Rear Differential Decoding

Chuck, Peter,

Thanks for the info - I'm going to do some more digging.

Mark