71-72 AX vs. LR differential codes

[Mike E.]

Both codes (AX and LR) are used in 1971 and 1972. Both are 3:36 positraction. Why 2 codes, both years, for the same 3:36 units?

[Joe L.]

Re: 71-72 AX vs. LR differential codes

Both codes (AX and LR) are used in 1971 and 1972. Both are 3:36 positraction. Why 2 codes, both years, for the same 3:36 units?

Mike------


The "AX" unit is equipped with cap-type yoke axles. The "LR" unit is equipped with u-bolt type yoke axles.

[Mike E.]

Re: 71-72 AX vs. LR differential codes

Thanks, Joe. Why would that be the case with 3:36's only?

[Joe L.]

Re: 71-72 AX vs. LR differential codes

Thanks, Joe. Why would that be the case with 3:36's only?

Mike------


3.36:1 was the standard ratio for the standard engine with standard transmission. So, I think that GM probably believed that the u-bolt axles were a little cheaper, adequate for the standard configuration, and consistent with the "economy-level" Corvette. All other ratios used the cap type axles. I suppose that GM figured that supplying the cap type axles for all these, while costing a bit more, was worth the benefit of greatly reducing the number of different carrier assemblies that the St. Louis had to keep in inventory. For example, for 1970 there were 18 different carrier assemblies. For 1971 there were 7 and for 1972 there were only 5.

[Mike E.]

Re: 71-72 AX vs. LR differential codes

So the often-shared "wisdom" that you can check your rear end to see if you originally had an LT1 or LS5 by looking at caps vs. u-bolts would only apply to a 3.36 rear end? A base motor optioned with a 3.70, e.g., would have caps?

[Joe L.]

Re: 71-72 AX vs. LR differential codes

So the often-shared "wisdom" that you can check your rear end to see if you originally had an LT1 or LS5 by looking at caps vs. u-bolts would only apply to a 3.36 rear end? A base motor optioned with a 3.70, e.g., would have caps?

Mike------

After 1969, the presence or absence of cap-type yoke axles is not a reliable indicator of original engine option. For 1965-69 the cap-type axles were used only for big blocks. After 1969 they were also used for some small blocks.

Base engines never had the option of a 3.70:1 axle ratio. A 3.36:1 was standard with manual transmission and a 3.08:1 was optional. As far as I know, from 1971 onward if any optional ratio was ordered, including 3.08:1, for any engine the cap-type axles were used. Big blocks continued to always be equipped with the cap type axles and most, if not all, LT-1's received the cap-type axles regardless of rear ratio.

Beginning for 1970 if any optional axle ratio was ordered there was a charge for that. I presume the extra cost of the cap-type axles was figured into that cost or figured into the cost of any engine option which also included the cap-type axles.

[Gary R.]

Re: 71-72 AX vs. LR differential codes

I agree with Joe, unless the car is known to be 100% original you could have a mix match sitting under there. My original base motor TH400 had the "HD" axles in it.

I personally think too much is made about the yoke styles. I have beat up stock yoke without breakage and typically with abuse other areas are going to fail before an axle u-bolt breaks. The internals were weak in some areas and those were used in all applications.

[Terry M.]

Re: 71-72 AX vs. LR differential codes

I agree with Joe, unless the car is known to be 100% original you could have a mix match sitting under there. My original base motor TH400 had the "HD" axles in it.

I personally think too much is made about the yoke styles. I have beat up stock yoke without breakage and typically with abuse other areas are going to fail before an axle u-bolt breaks. The internals were weak in some areas and those were used in all applications.

Gary

I have seen a lot of early (1968-1972) TH400 small blocks with the HD axles. Enough to the point I am almost ready to say: "TH400 = HD rear axles

[Patrick H.]

Re: 71-72 AX vs. LR differential codes

Gary

I have seen a lot of early (1968-1972) TH400 small blocks with the HD axles. Enough to the point I am almost ready to say: "TH400 = HD rear axles

Agreed. I have seen the same.

[Mike E.]

Re: 71-72 AX vs. LR differential codes

Continuing with this differential conversation, but taking a bit of another tack.. talking 71 specifically, lest there be any difference between that and 72.
The original dealer's album (not always true gospel) indicates a standard, economy, and performance rear with each motor and tranny combo. 3.08 is the highest ratio, 3.70 the lowest. Even for LT1 and LS6. Yet 4.11 and 4.56 are listed among 71 axle codes in the JG. I assume those were extra cost options available to those in the know?
Also, according to the dealer album, you could only get a m20 with the standard motor (no m21) , and only a 3.36 or 3.08 as options. Correct? That makes some sense, because you certainly wouldn't enjoy a m21 with either of those rear ratios.

[Joe L.]

Re: 71-72 AX vs. LR differential codes

Continuing with this differential conversation, but taking a bit of another tack.. talking 71 specifically, lest there be any difference between that and 72.
The original dealer's album (not always true gospel) indicates a standard, economy, and performance rear with each motor and tranny combo. 3.08 is the highest ratio, 3.70 the lowest. Even for LT1 and LS6. Yet 4.11 and 4.56 are listed among 71 axle codes in the JG. I assume those were extra cost options available to those in the know?
Also, according to the dealer album, you could only get a m20 with the standard motor (no m21) , and only a 3.36 or 3.08 as options. Correct? That makes some sense, because you certainly wouldn't enjoy a m21 with either of those rear ratios.

Mike------

A 4.11:1 rear gear ratio was optionally available for 1971 with LT-1 and M-21 as well as for LS-6 with M-22. I don't know of any availability of a 4.56:1 for 1971.

The standard engine for 1971 was available only with M-20 or THM-400. The 3.36:1 ratio was standard with M-20 and 3.08:1 was the only optionally available ratio.

[Joe L.]

Re: 71-72 AX vs. LR differential codes

Gary

I have seen a lot of early (1968-1972) TH400 small blocks with the HD axles. Enough to the point I am almost ready to say: "TH400 = HD rear axles

Terry------


I could see this happening for 1971-72. In fact, I'd virtually guarantee it because I don't think there was any 3.08:1 assembly made with anything but the cap-type axles during those years. However, for 1968-70 it seems strange to me. During those years there existed 3.08:1 assemblies with u-bolt type axles. Why these would not have been used for THM-400 small blocks is a mystery to me. I can find nothing in GM information which indicates a cap-type axle differential was to be used with all THM-400 applications. I'd be interested to know what the differential code is for these cars.

Addendum:

I think I may have just figured this one out. For 1968-70, Corvettes with THM-400 used a differential input yoke designed for 1330 series u-joint. All other 1968-70 applications used a 1280/1310 series u-joint yoke. It may be that the was no 3.08:1 positraction (required for THM-400) differential assembly with the 1330 yoke that also had the u-bolt type axles. So, the unit with cap-type axles, otherwise used for big block applications with THM-400, had to be used for small blocks. If so, I believe that unit was coded "AW". Any extra cost for the cap-type axles could have been figured into the M-40 option cost.

[Mike E.]

Re: 71-72 AX vs. LR differential codes

Mike------

A 4.11:1 rear gear ratio was optionally available for 1971 with LT-1 and M-21 as well as for LS-6 with M-22. I don't know of any availability of a 4.56:1 for 1971.

The standard engine for 1971 was available only with M-20 or THM-400. The 3.36:1 ratio was standard with M-20 and 3.08:1 was the only optionally available ratio.

Joe,
I don't know if GM produced any, but I know we have a code for the 4.56 in the JG, and the info in the price and facts book indicated that GM intended to. Who knows? I'd love to see a real tank sticker with YE3 on it.

[Mike E.]

Re: 71-72 AX vs. LR differential codes

Here is a better scan of that page, hopefully.

[Dick W.]

Re: 71-72 AX vs. LR differential codes

Continuing with this differential conversation, but taking a bit of another tack.. talking 71 specifically, lest there be any difference between that and 72. The original dealer's album (not always true gospel) indicates a standard, economy, and performance rear with each motor and tranny combo. 3.08 is the highest ratio, 3.70 the lowest. Even for LT1 and LS6. Yet 4.11 and 4.56 are listed among 71 axle codes in the JG. I assume those were extra cost options available to those in the know?Also, according to the dealer album, you could only get a m20 with the standard motor (no m21) , and only a 3.36 or 3.08 as options. Correct? That makes some sense, because you certainly wouldn't enjoy a m21 with either of those rear ratios.

Mike, digging through the cobwebs in my head but some of the information in the JG's came from GM early on in the life cycle of the manuals. There are engine codes that are mystery codes also