Differential decode FC 6 22 67

[Steven C.]

The guy I got this differential from said he thought was 4.56:1. Do the other numbers mean 6/22/67 mfg date?

Currently only has one side yoke, the HD type. Needs total rebuild, but seems like a desirable '67 diff with the 4-series carrier?

Steve

[Jack H.]

Re: Differential decode FC 6 22 67

The FC prefix denotes a 4.56:1 rear end ratio AND a 427 BB engine upstream. The date, 6/22/67 is obvious...rear end's assy date.

[Wayne M.]

Re: Differential decode FC 6 22 67

Yes, the "FC" indicates 4.56 posi for a big block. When you remove the rear cover, you can verify the ring and pinion GM part numbers, and the 9_41 tooth count stamped on each. The contemporary 4-series Eaton carrier should have cast (small end) EDB-30175, and just "175" on the ring gear end. You can also verify the carrier casting date (bottom edge under the driver side output yoke), which might pre-date the assy date by up to a few months.

I believe that this was probably an over-the-counter unit, as other than the L88 (hope I'm right), 4.56 gears were not factory-available.

Sounds like a desireable unit, as long as you drive around town with rare interstate bursts.

[Steven C.]

Re: Differential decode FC 6 22 67

Thanks, Jack and Wayne.

Any ideas about how it would perform behind a '69 L71 with CY automatic? Very limited driving, nice weather cruises.

I ask because my '69 was originally equipped with 3.08 HD posi, some previous owner swapped in a '68 3.70 small block posi.

I'm thinking the 4.56/435 hp/auto would be FUN streetlight to streetlight (not racing, just from stop to speed limit.

Steve

[Steven C.]

Re: Differential decode FC 6 22 67

Wayne,

Casting number 3899143, on passenger side, near input yoke...different than what you said?

The remaining side yoke is drilled smooth, not tapped -- not HD?

Date seems right F 7 7 (June 7, '67).

Is this a 4-series carrier, from mid-'67, with that casting number?

Thanks,

Steve

[Wayne M.]

3899143 is valid carrier casting # for all '67 s

And the June 7th date sounds correct. A non-HD yoke has the same smooth ID to accept a 3/8" slide-through fine thread U-bolt (strap). But you couldn't insert the U-bolt into the HD yoke, as the hole is smaller due to the threads for the 3/8" bolt. So one could drill out the threads in a HD yoke to enable use of the U-bolt, but not the other way around (ie. can't thread a non-HD to imitate a real one, unless larger bolts are used, and then it becomes an issue of strength, as there's not a lot of "meat" left.

P.S. I'll get to your posi case #'s soon.

[Steven C.]

Thanks, Wayne...a few more ??

Is there anything that differentiates (no pun intended) the 3-series from the 4-series carrier? I don't want to rely just on the stamped FC code for identification.

Also, at a glance how can I tell if it is a drilled side yoke vs. a non-HD?

I sincerely appreciate your knowledge on this topic.

I'm thinking I'll rebuild this one if it all checks out, and put it in my car instead of the (also) incorrect '68 3.08 non-HD with a leaking pinion seal.

Steve

[Steven C.]

Anyway to tell/guess/estimate '67 or '68

With the assembly date of June 22, '67; and if it was originally installed in a car (versus over the counter), would it likely have been in a '67 or a '68?

Steve

[Wayne M.]

Here's the "175" on a 4-series, and EDB 30117 on

... a series 3 case.

You can see the "175" cast on the case spider, just below the bearing cap. I've slipped a U-bolt on this non-HD yoke. You could never do that with a U-bolt through the hole on a HD yoke. Also, the 4-series flange (with bolts, that attaches to the ring gear) is thicker at the edge [0.46"] on a 4-series, than on the 3-series.

Second pic (click link) is from a 3-series (excuse the lint on the parts). Notice the postage stamp size window on the case; later C3 ? '69+ ? cases had a large window and instead of EDB 30117 or 30175, they had ED 32297 (3-series) or EDB 32298 (for 4-series).

[Steven C.]

Thanks, Wayne! It's 30117... *NM*

[Wayne M.]

Ring gear info on a 3-series, and HD yoke compare

On your ring gear, there's the GM part #, the teeth count 10 (pinion) and 37 (ring) giving a 3.70 ratio. Then there's the month and year. Note the thinner flange. Same data on the end of the pinion gear, but it's much harder to see when installed.

Click on following link for yoke comparison. You can see the threads in the HD yoke, and that the smooth holes are larger on the standard unit.


You say it's hard to turn; is there a driveline yoke at the front ? there's usually the last 3 digits of a casting # on the edge of it, facing forward. These diffs are like Muncie transmissions. Once they've been rebuilt, there can be sometimes parts from many "donors", so you have to do an autopsy.

That's good news that you have a 3-series, IMO. Much better with that automatic, and you have a choice of 4 ratios.

[Steven C.]

Thanks, Wayne: You taught me a lot! *NM*