C2/C3 Rear End Differential Housings

[jim34671]

Despite having a common casting number of 3899143, were there any significant differences or "beefier" units that were physically apparent with heavy duty or hi-perf applications?..especially 68-69 vintage?
If so,are these visible?
Thx,
Jim

[Joe L.]

Re: C2/C3 Rear End Differential Housings

Jim-----

Absolutely no differences that I know of with any 3899143. This casting was machined into only one finished carrier housing which was known as GM #3917861. This piece was used from 1967 through 1979 without any changes. It is the best of the 63-79 housings although it is not really significantly stronger or significantly better than the 63-66 versions. The 3917861 will also SERVICE all 63-66 applications providing that the snubber bracket bolts are both changed to the through-style bolts.

GM never offerred a "HD" carrier housing for 63-79 Corvettes, either in PRODUCTION or SERVICE.

[Joe L.]

Addendum

By the way, the limited strength of the 63-79 Corvette differential assembly, including the carrier case, as well as the limited strength of the rest of the rear drive assembly (u-joint, half shafts, and rear bearings) is one of the major factors in limiting the amount of horsepower and torque that are practical for said Corvettes.

I've heard a lot of talk by "kids" (especially on other discussion forums) about all sorts of "one-upsmanship" regarding horsepower. Huge big blocks, blowers, nitrous oxide, you-name-it. The fact of the matter is, unless one is willing to discard the IRS and Corvette differential assembly and go to a strong solid axle (like, for instance, a "built" Ford 9"), all of this stuff is just nonsense.

Some folks, mostly "juveniles" (of whatever age) think that you can never have too much power. However, you ABSOLUTELY CAN have too much horsepower and torque in a 63-79 Corvette chassis. There's really no practical way to beef up the stock Corvette rear drive to a degree that would make it compatible with a high power engine, either. The 63-79 Corvette chassis was designed around small block power. With the big block, it can be quite marginal. With a big block producing the kind of power levels that can be had with today's technology and monster displacements, it's really out of the question.

Lots of "kids" will learn the hard way, though. They always have and they always will.

[jim34671]

Re: C2/C3 Rear End Differential Housings

Understood Joe,but I've noticed in various publications that you come across both "standard" positraction and "heavy-duty" positraction for both 427 and 454 of 68-70 vintage.Is this the axle yolk types fitted or perhaps 4 series carriers instead of 3 series.? I thought all big block cars came with the stronger yolks.Just curious.
Thx

[Craig S.]

Re: C2/C3 Rear End Differential Housings

Joe - one other item I noted when building up a 66 rear end versus a 67, and this is something Charlie Smith mentioned in his class, is that the internal width from one side of the carrier to the other is cut wider in the later 67 and up units, to allow using the wider cast iron shims. I think a typical shim width on each side to preload the bearings is in the 0.235" range on the later units, while the 66 back they are around 0.072 - 0.095 or so. My 66 unit uses the steel thin shims versus the wider cast iron style as in the 67. I believe this was more for fast assembly in the factory, with the iron shims. From a service perspective, I believe GM started offering a wide spacer that was used in conjunction with the steel shims in the early midyear era to come up with a service pack that got you to the 0.245" range. They frown on reuse of the cast iron factory shims in the overhaul manuals, their worry is you may crack the cast iron tapping it in, obviously not an issue with the steel shims.....Craig

[Joe L.]

Re: C2/C3 Rear End Differential Housings

Jim----

The "standard" and "heavy duty" refers only to the type of yoke axles which were originally fitted to the differential unit. The u-bolt type are the "standard". The cap type are the "heavy duty". Prior to about 1972, the u-bolt type was used for all small blocks and the cap type for all big blocks. Beginning in about 1972, the cap type started to be used for small blocks, too, and was used intermittently thereafter for small blocks.

Other than the yoke axles, the differential units were the same. Of course, there were differences with respect to gear ratios, but that had no real affect on the "standard" or "heavy duty" issue.

[Ralph A.]

Re: C2/C3 Rear End Differential Housings

Joe, unless I am reading the judgiong manul wrong (page 108 for 70 to 72) all Mark IV, Lt1 and M40 automatics were cap type HD. I have a 71 with M40 and it has caps. Until I read this page I was told caps were not available on SB 71's.

My .02

[Jim T.]

Re: C2/C3 Rear End Differential Housings

Joe and Ralph, I have the M40 turbo 400 in my original owner 70 and it came with the cap style u-joint and is a 3:08 ratio unit.