rocket69

Hey guys,

I gotta swap the guts out of my rear end but need to know more about it to see if certain options are available.

We found the numbers:
40 222 7
CFD8
AB (or A8... can't tell)
328

Is this a 2.56 or a 3.08? Our research is failing us...

dan1s

Year and make?

dan1s

Identification for GM Differentials

Stamped Codes:

The two letter code is approx. 3/8" in height and is stamped into the axle tube on the inboard, passenger side (toward passenger compartment). Sometimes they are also found outboard on the passenger side (towards rear of car). There are usually more than two letters or numbers, because other manufacturing codes were stamped there.

It's normal to have to wire brush the heck out of the area before you will be able to see it, but it will be there. Typically it is covered/rusted over.

What you are looking for exactly is the two or three adjacent letters, preceded by one or more numbers, of the code that is stamped (NOT RAISED) into the axle tube.

The letters themselves can be very fine and very faint. Further, there can be flaws in the stamping dies or the application of same. What may look like one letter is actually another, so look carefully.

I recently measured the distance the code was from the right side backing plate which was 13". This one happened to be on the outboard side of the axle.

ALSO TRY THIS:

Determining Gear Ratio

You will have to put the rear up on jackstands, put the transmission in neutral. Make sure that the car is quite stable, get under the car, and TURN THE DRIVESHAFT. Count the number of turns of the driveshaft it takes to turn one wheel one time. If you turn a wheel, instead of turning the driveshaft, you will only turn the other wheel (not the driveshaft).

It also works better (more accurate) if you turn the driveshaft enough times to turn one wheel ten times, and count the number of times you turn the driveshaft. This is because it may be difficult to tell if the driveshaft goes around, say, 2.56 or 2.78 times for one turn of a tire. But if you turn the driveshaft enough times to turn a wheel ten times, you will have 25.6, or 27.8, or 41.1 (or whatever) turns of the driveshaft. Then just divide by ten to get the ratio of the rear.

Note also that there are two different ring gear carriers, one for 3-series ratio gears and one for 4-series gears. The difference is the dimension from the ring gear mounting flange to the pinion centerline, due to the larger diameter pinion required for the lower numerical gear ratios. A set of 4-series gears can be installed in the 3-series carrier with a spacer and longer bolts (not desireable due to reduced strength), but the reverse is not true.

Formulas
These formulas are for giving you a good idea of what to expect not 100% accurate, but pretty close. Some things you need to remember is that if your tire is 25" standing still it will be taller at speed. How much depends on tire construction, rubber compound, inflaltion. It will be shorter on acceleration depending on engine power, wheel spin. Then you have tranny slippage and probably other small variables.

To figure rpm: 168 * gear ratio * miles per hour / tire radius
To figure miles per hour: tire radius / 168 * rpm / gear ratio
To figure gear ratio: tire radius * rpm / 168 / miles per hour
To figure tire radius: 168 * miles per hour * gear ratio / rpm

rocket69

How deep are those stamped? We can't find anything and wonder if they can disappear... (we're in WA state..)

Actually knowing the current ratio isn't that important. What's important is what it originally had in it, because I'm attempting to maybe swap in a 3.90, and the kit I'm looking at only accepts 3.08 rear ends, or 3.23's, or anything bigger than 3.42... or some numbers like that...

Are there any other ways to find this out than the code? Such as cracking it open? If so, what exactly am I looking for?

J-(Chicago)

If you take the rear cover off, your ratio should be stamped on the ring gear (the big one)

don71

You'll need to know the 3 digit number cast into the carrier. But this could of been changed also.

#671 was used for 3.08-3.23 with 28 spline axles. If you use the newer richmond style gears, this is the carrier you need. If you have used gears or maybe NOS ones, you could follow these:

#588 was for 3.42 and up with 28 splines: this one if using factory 3.90

#673 was for 2.56 and 2.78 with 28 slines

#672 was for 3.42 and up with 31 splines

I could be wrong, but this is how I understand it.

dan1s

rocket69

Is this a chevy rear or "O" axle rear? What's the year and make of the car? Alittle more info would help.

rocket69

It's a '69 442, I'm pretty sure it's an 'O'.

dan1s

Here's the info that I found about a 69' 442

1969:
Model Ratio Teeth Open Anti-spin
Type "O" differential
except Vista Cruiser 2.56• 41-16 R2, R8 R3, R9
2.56¤ 41-16 R6 R7
2.78¤ 39-14 S2 S3
3.08¤ 40-13 S4 S5
3.23¤ 42-13 S6 S7
3.42¤ 41-12 S9
2.78• 39-14 SA SB
3.08• 40-13 SC SD
3.23• 42-13 SE SF
3.91¤ 43-11 T3
3.42• 41-12 TM
3.81• 43-11 TO
4.33• 39-9 TY
Type "C" differential
2.78• 39-14 RA RB
3.08• 40-13 RC RD
3.23• 42-13 RE RF
3.07¤ 43-14 T4 T5
3.31¤ 43-13 T6 T7
3.55• 39-11 TL
3.73• 41-11 TN
2.56¤ 41-16 TR TH
2.73¤ 41-15 TT TJ