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I spent too much time searching other posts before just posting one myself.
Could you all help de-coding my rear?
My car is a 1969 L-71.... November build.
Casting mark looks like D219 ... maybe their is another 1 after the 9?
The dial.... I don't know how to read those.
Another casting marks looks like CON1
Another one says 10 GM
But I really want to de-code the stamping: This gets weird... The stamp is pretty clear. 2CFA 10 28 6 9 W ..... BUT no you are not seeing double. It looks like there was a weak stamp, and then it was stamped again. BUT not exactly the same. It looks like under the F there is a weak stamped A. Under the A, there looks to be a weak stamp of something I can't make it out. Everything else seems the same . So the weak stamp looks like 2CA? 10 28 6 9 W
Any help decoding all that would be great. I attached pics
Lets assume nobody is trying to fake something, I know the history of the car to know that's not the case.
Housing casting date of D 21 9 decodes as April 21, 1969
Assembly code is second shift (2) 3.70 to 1 HD ratio (CFA) October (10) 28 (28) 1969 (69) Warren (positraction source).
This is a late build 1969 rear as evidenced by the three character (CFA) ratio code…. You have a November or December 1969 build Corvette, assuming the differential is original to the car.
Thank you. what do you make of the what appears to be a double stamp thing?
Not sure…
It looks like it could be CAO which is the code for the standard 3.70 to 1 ratio positraction rear. CFA is the heavy duty version of the 3.70 to 1 rear. Difference is that the COA version used u bolts to retain the rear axle u joints where the CFA version used a cap an bolts.
There is a pretty big date spread between the housing casting date and the assembly date. Is it possible that the rear was built as a standard posi rear and then changed at a later date? Possibly… Would explain the overstamp, but from the appearance, it appears that other numbers are overstamped as well. You say you know the car well… Is it possible that someone replaced the rear at some point? Possibly screwed up restamping the rear and used the wrong code?
Is there a doc with these codes?
Yes, but very few that would identify these late three character codes as correct for late 1969. The NCRS TIM&JG is one that would.
Things I know:
Car was built Nov 1969 Like between the 7th and 11th, so the stamping date on the housing is accurate. I also believe the casting is accurate given things getting shelved for the strike and can explain the double stamp.
What is there is NOT heavy duty because it has the u-bolts on the driveshaft u-joints.
The car was built as an L-71 with M-21, so only one code would be used for the 427(454) with a non-heavy-duty posi using the 1970 codes... that is the CFA 3.70 (CFA was not heavy duty in 1970 according to what I am reading). The rest of the 1970 codes don't align because it is either for small block or, auto, or heavy duty.
Now in 1969 the only options for the 427 are heavy duty including the FA 3.70. In 1969 the 3.70 non-heavy duty posi was the AO. And was originally stamped AO. But the AO would have been an incorrect stamp since in 1969 codes the AO was only for the small blocks and they switched to the 1970 rear options and codes around August, so it needed to get re-stamped to correctly reflect it being a 1970 option, but still having the correct build date for a November built 1969 car.
So I think the 2CFA is stamped over AO and for that reason.
All that said, I have documentation from the last owner who said its 3.55...... and what I wanted to find out is if it was ordered a 3.55 or if he changed the inside to 3.55 for his special purpose. I was hoping the re-stamp would prove the former, but according to all this, it seems to be the ladder. The 3.55 does not align to this car's options (non-heavy duty posi)nor does the look of the original stamp.
Originally Posted by Rowdy Rat
Not sure…
It looks like it could be CAO which is the code for the standard 3.70 to 1 ratio positraction rear. CFA is the heavy duty version of the 3.70 to 1 rear. Difference is that the COA version used u bolts to retain the rear axle u joints where the CFA version used a cap an bolts.
There is a pretty big date spread between the housing casting date and the assembly date. Is it possible that the rear was built as a standard posi rear and then changed at a later date? Possibly… Would explain the overstamp, but from the appearance, it appears that other numbers are overstamped as well. You say you know the car well… Is it possible that someone replaced the rear at some point? Possibly screwed up restamping the rear and used the wrong code?
Yes, but very few that would identify these late three character codes as correct for late 1969. The NCRS TIM&JG is one that would.
Part of the ‘heavy duty” composition of these HD differentials was the cap/bolt stub axles. The fact that it you have the standard duty u-bolt style stub axles, an AO (or perhaps CAO) code on the differential, and a really big spread on the casting date vs. assembly date has me wondering if the differential may have been replaced (and along with that, restamped). I’m really not sure what to make of it.
The rear codes switched from two to three character in late September/early October 1969. Having said that, there were still two character code differentials showing up in November 1969 for the less commonly used ratios.
Except for the 10, the whole string looks double-stamped – with only the axle code being changed. I've got no explanation for the long casting to assembly date, but I suspect it's authentic GM.
I think the casting date spread is part of the reason for the restamp. I think it was stamped AO and then re-stamped CFA in flint to correct for 1970 inventory codes…
However, you said something I want to dig into more.
You said the stub axels determine heavy duty or not….. I thought the stub axels always had the cap and bolts…. I thought the pinion yoke having a ubolt or cap and bolts is what made the difference.
So I’m a little confused again…. Mine has the ubolt style on the pinion yoke, and the cap and bolts on the stub axels.
Does the pinion yoke play in this at all? Did they have cap and bolts or was the pinion yoke always ubolt?
Originally Posted by Rowdy Rat
Part of the ‘heavy duty” composition of these HD differentials was the cap/bolt stub axles. The fact that it you have the standard duty u-bolt style stub axles, an AO (or perhaps CAO) code on the differential, and a really big spread on the casting date vs. assembly date has me wondering if the differential may have been replaced (and along with that, restamped). I’m really not sure what to make of it.
The rear codes switched from two to three character in late September/early October 1969. Having said that, there were still two character code differentials showing up in November 1969 for the less commonly used ratios.
I misunderstood what you were getting at… The stub axles are what determine the “heavy duty” nature of the differential; the pinion yoke is always a u-bolt (at least for 1968-1972). If you have the cap/bolt style stub axles, then I would agree that what you have is probably OK and I’m less concerned about the date spread on the casting. All off the overstamps are an oddity, but I guess we’ll never know the reason for that.
In any case, it sounds like all is in order, if not a bit out of the norm.
And this all makes sense now. My understanding is that date spreads like we see here is not entirely uncommon due to shelving placement, the strike, other reason etc, so I think that is OK.
Regarding the double stamp, although it isn't a common occurrence; mis-stampings did happen. And when they did happen Flint would do re-stamp. In this case it would seem it was incorrectly AO for non-heavy duty 3.70, when it is in fact heavy duty based on the stub axel cap and bolts, so the correction stamp to FA. The three characters would align to stampings switching over to the 1970 codes around or about after August 1969.... Hence CFA and the date being Oct 10 1969.
Now, that we know it's 3.70 either way.... Do you know, does the 3.55 properly fit in 3.70 case? Is it the same case... my understanding it they are not the same case from the factory, but could you change a 3.70 to 3.55 without changing the case?
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Originally Posted by Rowdy Rat
I misunderstood what you were getting at… The stub axles are what determine the “heavy duty” nature of the differential; the pinion yoke is always a u-bolt (at least for 1968-1972). If you have the cap/bolt style stub axles, then I would agree that what you have is probably OK and I’m less concerned about the date spread on the casting. All off the overstamps are an oddity, but I guess we’ll never know the reason for that.
In any case, it sounds like all is in order, if not a bit out of the norm.
And this all makes sense now. My understanding is that date spreads like we see here is not entirely uncommon due to shelving placement, the strike, other reason etc, so I think that is OK.
Date spreads this large (six months) ARE uncommon… Typical difference between casting date(s) and assembly date for differentials is typically six to eight weeks. Difference between assembly date and vehicle completion date is typically two to four weeks (oddball ratios like 4.56 to 1 can be a lot longer). Can a date spread of six months occur? Yes. Is it common? No.
Regarding the double stamp, although it isn't a common occurrence; mis-stampings did happen. And when they did happen Flint would do re-stamp. In this case it would seem it was incorrectly AO for non-heavy duty 3.70, when it is in fact heavy duty based on the stub axel cap and bolts, so the correction stamp to FA. The three characters would align to stampings switching over to the 1970 codes around or about after August 1969.... Hence CFA and the date being Oct 10 1969.
This part of the question is one that I’m afraid we will never know. Why AO (or CAO) was changed to CFA when there are distinct differences between the two differentials I can’t explain. I also can’t explain the reason why most of the characters are overstamped. About the only thing that I can say is that assuming the differential is real, all of this nonsense happened at Warren (the differential plant). Outside of that, we’re just guessing. As far as the three character codes, it may have started in late September 1969, but more likely early October 1969… August is way too early. Also as mentioned, two character codes showed up intermittently during this time (October 1969 - December 1969) depending upon the ratio as old inventory was used.
Now, that we know it's 3.70 either way.... Do you know, does the 3.55 properly fit in 3.70 case? Is it the same case... my understanding it they are not the same case from the factory, but could you change a 3.70 to 3.55 without changing the case?
All Corvettes from 1968 to 1972 (and at least a similar case up to and including 1979) used the same main case. In addition, both the 3.55 to 1 and 3.70 to 1 ratio gear sets used the “three series” positraction carrier. There is little difference between these ratios outside of the gear sets (and perhaps the stub axles depending upon the application). It may be a 3.70 to 1 ratio “either way,” but it needs to be CFA due to the application (big block engine)… We’re assuming that AO (or CAO) was stamped incorrectly.
De-Code My Rear
