Early C2 Diff Covers

[Wayne M.]

Further to Pete's thread below (to identify diff he has -- needs an early unit), here's some photo data on what I've found. The autoload shot is of a "303" cover off a (cast) 3818753-N carrier CD_1_3_64, cast J-18-3. The fine thread plug is at the top, and matches the tap in the cover; the plug below (NPT type) is from a late '65 with a "375 cover. I cleaned the grime off the paint dab just a few hours ago; first time it's seen daylight since 1968.

The following link is of the inside of a "303" cover from my early '65 #014xx, also a 3831753-N cast carrier G-4-4, AP_9-17-64, clearly showing the coarse thread.


The last shot (click on title under the auto-load pic) is of the exterior of the same '65 cover, showing the "303" number as well as the casting mould "GM 3".

Note that NONE of these covers have dates next to the fill plug. IMO, NO covers prior to '68 or '69 (these would be "375" covers) had cast dates. Let's slay this dragon once and for all. As time passes, more and more C2 cars are popping up with "375" dated covers ("donated" from diff's of a decade later).

[Harry Sadlock]

Re: Early C2 Diff Covers

Wayne, I've been scratching my head on this for about a year now. I have a 303 cover that is a GM 3 that takes the fine thread. I have a 303 cover that is a GM 4 that takes the course thread???

Thoughts...

Harry

[Alan Drake]

Re: Early C2 Diff Covers

Did a little work and confirmed I have a GM 6 cover with a thread of 20 (assume that's fine).
Also noticed that in the area that Wayne has the "303" mine is B14 - I assume that is a date of Feb 1, 1964. I have confirmed the cover pn as 3830303. The carrier is marked CE 3 10 64, not sure where other numbers are on carrier. The car is a late Mar 64 which I've owned since Aug 67 with

[Harry Sadlock]

Re: Early C2 Diff Covers

On the inside of the 3830303 covers are.

The one with the GM 3 cast on the outside has 303 cast on the inside with a 3 stamped above the 303.

The one with the GM 4 cast on the outside has 303 cast on the inside with a 4 stamped above the 303.

Other than the casting of 3830303, GM (3 or 4), the 303 on the inside and the stamp of (3 or 4), there are no other numbers on the cover.

Since yours is a GM 6 with fine thread, is it possible that fine thread or course thread was just a random tap based on what machine did the tapping?

Harry

[Alan Drake]

Re: Early C2 Diff Covers

Thanks Harry,

Missed the fact the 303 cast was on the INSIDE and will not be looking for it soon.

I second your idea that the fine v course was due to what was there on the factory floor.

[Wayne M.]

Chev Service News -- Leaking Corvette Rear Axles

Found it -- Sept-Oct 1965 issue

"use correct fill plugs -- mixing 1964 rear axle fill plugs (straight thread with gasket) and 1965 rear axle fill plugs (dry seal with no gasket) will create leakage. Both 1965 positraction and standard rear axles use the dry seal threaded plugs with a flange. The plug flange does not require a gasket. This flange is required only as a retainer for a tag. The tag identifies "positraction lube only" and is not to be used with a gasket to seal the axle fill hole. For proper sealing, torque at 20 ft.lbs. Correct axle fill plugs are called out in group 5.400 of the Parts and Accessories Catalogue."

So (to me) it appears that the fine thread (20/inch) ended with 1964 model year, and the (coarser) taper thread began at S.O.P. 1965. From the single example of my Sept 17 assembled pumpkin, I have to conclude that GM exhausted the "303" covers, but made sure they had the new plug design.

Any other early '65's out there ?