Help me decode my diff casting #s please
01-30-2008, 04:50 PM
#1
Burning Brakes
Thread Starter
Help me decode my diff casting #s please
Can anybody please help me decode the casting numbers in the diff of my '76 please? It's not NCRS but I like to learn about it.
The rear cover has the following idents:
GM 9
3871375
A2 96
Do they tell me anything?
I just counted the ring gear teeth... There are 32. Does that tell me the axle ratio somehow?
The rear cover has the following idents:
GM 9
3871375
A2 96
Do they tell me anything?
I just counted the ring gear teeth... There are 32. Does that tell me the axle ratio somehow?
01-30-2008, 05:50 PM
#3
Drifting
Member Since: Oct 2007
Location: Collinsville MS
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St. Jude Donor '08
Count the number of teeth on the ring gear and divide the by the teeth on the pinion.
Should be a hand stamp on the bottom of the diff right next to the cover. Looking for two letter combination. May have to scrape off road gunk to see it. It is not raised like the other numbers on the casting.
OA = 3:08
LR OD = 3:36
OB = 3.55
OC = 3:70
Should be a hand stamp on the bottom of the diff right next to the cover. Looking for two letter combination. May have to scrape off road gunk to see it. It is not raised like the other numbers on the casting.
OA = 3:08
LR OD = 3:36
OB = 3.55
OC = 3:70
01-30-2008, 06:08 PM
#4
Burning Brakes
Thread Starter
Thanks for the replies. I wondered whether the casing numbers identified a manufacturing date or anything.
My '76 is an L82 according to the VIN so the 3.55 ratio looks likely. I'll be surprised (but pleased) if it's the original differential but I'll look tomorrow to see if I can see some stamping on the bottom of the housing. (11pm here in the UK.) I've only just drained the oil so I haven't seen the pinion gear yet.
My '76 is an L82 according to the VIN so the 3.55 ratio looks likely. I'll be surprised (but pleased) if it's the original differential but I'll look tomorrow to see if I can see some stamping on the bottom of the housing. (11pm here in the UK.) I've only just drained the oil so I haven't seen the pinion gear yet.
01-30-2008, 06:13 PM
#5
tooth count for factory available 63-79 GM ring and pinions
ratio / ring / pinion
2.73 / 41 / 15
2.92 / 41 / 14*
3.08 / 37 / 12
3.36 / 37 / 11
3.55 / 32 / 9
3.70 / 37 / 10
3.90 / 39 / 10*
4.11 / 37 / 9
4.56 / 41 / 9
4.88 / 39 / 8*
5.14 / 36 / 7*
* over the counter only, not factory installed
Ben, this is the L82 axle ratio info I have
L82 M20 3.55 all
L82 M21 3.55 std, 3.70* opt
L82 M40 3.36 std, 3.55 opt
*3.70 without A/C only
ratio / ring / pinion
2.73 / 41 / 15
2.92 / 41 / 14*
3.08 / 37 / 12
3.36 / 37 / 11
3.55 / 32 / 9
3.70 / 37 / 10
3.90 / 39 / 10*
4.11 / 37 / 9
4.56 / 41 / 9
4.88 / 39 / 8*
5.14 / 36 / 7*
* over the counter only, not factory installed
Ben, this is the L82 axle ratio info I have
L82 M20 3.55 all
L82 M21 3.55 std, 3.70* opt
L82 M40 3.36 std, 3.55 opt
*3.70 without A/C only
01-30-2008, 07:30 PM
#7
M20 Borg Warner Wide Ratio (ratios 2.64, 1.75, 1.34, 1.00 )
M21 Borg Warner Close Ratio (ratios 2.43, 1.61, 1.23, 1.00)
M40 GM Turbo-Hydramatic 400 (ratios 2.48, 1.48, 1.00)
With the L82, you could have the M20, M21 or M40 (L48 was M20 or M40 only). There might still be a tag on your side cover.
If it's still there, will say something like AS*-T10 and then a letter; W for 2.64 low (M20) and S for 2.43 low (M21).
M21 Borg Warner Close Ratio (ratios 2.43, 1.61, 1.23, 1.00)
M40 GM Turbo-Hydramatic 400 (ratios 2.48, 1.48, 1.00)
With the L82, you could have the M20, M21 or M40 (L48 was M20 or M40 only). There might still be a tag on your side cover.
If it's still there, will say something like AS*-T10 and then a letter; W for 2.64 low (M20) and S for 2.43 low (M21).