diff ID

[John G.]

I'm in the process of changing out a diff assembly and would like to ID it. What's known is it's a 3:08 unit with spider gears. The numbers on the bottom flange are spread out and read, AW....W...357....E...1.
The numbers on the of the front carrier read 389974 (right side) and ON2 (left side). The side yoke casting number is 3872923. The yokes take caps and bolts instead of U-bolts. .. The diff unit is supposed to be out of a '72 year car. Can anyone confirm?

[Patrick B.]

Re: diff ID

AW is the 1971 code for a 3.08 rear. Probably out of a turbohydramatic 400 car.

[Edward J.]

Re: diff ID

John its a 308 ratio, and the automatic trans. had the heavy duty caps which bolt in. there should be a casting no. on the lower side drivers. generally a letter and some numbers witch is the date, the last digit is the year. and month is letters, A= Jan. etc.

[Gary R.]

Re: diff ID

John
I recommend that before you install it you pop the cover and take a look, now is the time to catch something. You can see if the gear is a GM 308 or aftermarket by looking on the side of the ring gear. It will be dated coded and have the # of teeth for the R&P, ie: 37 10 = 3:70 ratio. If it's an original diff then the posi clutches will be the much weaker "snowflakes" I have been preaching about for years. A cheap upgrade would be to install socket head cap screws in place of the hex heads in the bearing caps. It most likely won't make much difference unless you want to hammer it a little. You can mist some brake cleaner on the posi case and blow it off lightly with air to check for cracks at the window, side holes, or tear drop window. They do crack and then become a "time bomb".

Gary

[John G.]

Re: diff ID

The 3:08 diff is a unit I've had for some time to replace the 4:11 diff unit for some long distance out of state freeway traveling. The 4:11 diff appears to be a series 4 unit - much more beefier internals compared to the 3:08 with what looks like friction disks on the outboard ends of the larger spider gears. I remember seeing the R&P teeth # (12-37) on the ring gear. I didn't know about the date code. Interesting about possible cracks. I will definitely check for that. .. The unit appears to be otherwise in excellent condition.

Thanks guys for your replies.

[Gary R.]

Re: diff ID

John
There is no difference with the posi spiders other then the 65-70 used the weaker 10-18 and the 71-79 used the better 10-17 spiders. The clutches on the 65-70 were the better solids the 71-79 got the "snowflakes" in a weak attempt to eliminate posi chatter. All that did was weaken them and abusive driving caused them to break.
There is myth I heard for years that BB diff's were stronger when in fact the only difference was the code stamping on the housing. Given your attended usage, highway far from home, I would suggest you at least replace the bearings if the diff sat for a long time.

A local guy just went through the same deal, he installed a diff that sat for 20 years against advice only to have to remove it after the first drive do to noise and chatter. All I had to do with that one was replace the bearings and polish/tune the posi. works like new now with original R&P and side yokes.

Good luck with it, hope the case isn't cracked.

[Joe L.]

Re: diff ID

John

There is myth I heard for years that BB diff's were stronger when in fact the only difference was the code stamping on the housing.

Gary------


The reason for the difference in codes between big blocks and small blocks is because of the different stub axles (i.e. u-bolt versus cap). Otherwise, the differentials are the same. The differentials were, of course, delivered to St. Louis with the axles as part of the assembly. Sometime around 1971, if not earlier, the differentials with cap type stub axles began being used on some small blocks.

[Gary R.]

Re: diff ID

Hi Joe Yes that is correct on the axles. I over looked the axle differences because for most applications it won't make a difference. I have never seen one break at the joint. I have seen them twist and also fail when they were incorrectly "rebuilt". The only difference in the 3 & 4 series cases is the flange thickness to compensate for the RG thickness used.

[Jim T.]

Re: diff ID

Gary------


The reason for the difference in codes between big blocks and small blocks is because of the different stub axles (i.e. u-bolt versus cap). Otherwise, the differentials are the same. The differentials were, of course, delivered to St. Louis with the axles as part of the assembly. Sometime around 1971, if not earlier, the differentials with cap type stub axles began being used on some small blocks.

Joe, My original owner 1970 Corvette with turbo 400 350/300 built in July 1970 has the cap type and the rear gears are 3:08.

[Wayne M.]

Re: diff ID

... There is no difference with the posi spiders other then the 65-70 used the weaker 10-18 and the 71-79 used the better 10-17 spiders. .....

Gary -- just to clarify your terminology for me; I assume that a 10-18 posi spider (gears) represents the two 10-tooth pinion gears that are held in the posi case by the differential pinion shaft; and these engage with the 18-tooth side gears with 17 internal splines (into which the output yokes [U-bolts or caps style] slide).

Question #1 -- are the 10-tooth gears the same GM / Eaton part # whether used on the '65-70 or the later posi units ? And if so, they must work with either the 17 or 18-tooth side gears, right ? [given shims selected to obtain correct range of tooth clearance].

Counting what's in my two '65s; I have a total of 7 posi differential units, covering 3.08 thru 4.56 final drive ratios, with a mix of 1st and 2nd design Eaton cases. On one unit, I noticed while checking the pinion gear-to side gear tooth clearance, that I have the newer 17-tooth side gear on one side and an 18-tooth on the other (those with the integral "skirt" that covers to the edge of the adjoining fixed clutch plate; my 17-tooth does not have this skirt). Give me your opinion on the workability of this mis-match. Given the typical sludge I found on everything, I have to assume that this posi case (eBay purchase ) was OK before.

Question # 2: I've forgotten the (real life) torque spec on the pinion shaft lock screw; the GM book says 20 ft-lbs. IMO, this would shear the head off a 1/4" standard thread bolt/screw.

[Gary R.]

Re: diff ID

Hi Wayne, Yes the 10 tooth are the pinion and the 18 or 17's are the side gears. I don't have the numbers on them since I buy the new 17's in a set in either 17 or 30 axle spline - std or heat treated. I never mix sets of new or old unless I am boring a worn case to use a 12 bolt cross shaft, then I will use new 10's with the original 17's and see how they tune. If they are not too worn they will be fine and certainly is a better option then using a new loaded Eaton-imo. I use #242 on the cross shaft bolt and hand torque then since they do feel like they will shear at 20 ft/lb. With the new plain head bolts that some sell I have sheared them. The best option is to mill the pad for deeper engagement of the bolt into the case. I do this on every posi I build because I have seen them break out the case.

[Joe L.]

Re: diff ID

Hi Wayne, Yes the 10 tooth are the pinion and the 18 or 17's are the side gears. I don't have the numbers on them since I buy the new 17's in a set in either 17 or 30 axle spline - std or heat treated. I never mix sets of new or old unless I am boring a worn case to use a 12 bolt cross shaft, then I will use new 10's with the original 17's and see how they tune. If they are not too worn they will be fine and certainly is a better option then using a new loaded Eaton-imo. I use #242 on the cross shaft bolt and hand torque then since they do feel like they will shear at 20 ft/lb. With the new plain head bolts that some sell I have sheared them. The best option is to mill the pad for deeper engagement of the bolt into the case. I do this on every posi I build because I have seen them break out the case.

Gary-----


The 1965-73 spider gear kit was GM #3869317 (aka Eaton ED 40334-6). The 1974-79 spider gear kit, which also replaced the 3869317 in October, 1976 for SERVICE, was GM #6260528 (aka Eaton ED 24304). The 6260528 was discontinued without supercession in February, 1999.

[John G.]

Re: diff ID

John
There is no difference with the posi spiders other then the 65-70 used the weaker 10-18 and the 71-79 used the better 10-17 spiders. The clutches on the 65-70 were the better solids the 71-79 got the "snowflakes" in a weak attempt to eliminate posi chatter. All that did was weaken them and abusive driving caused them to break.
There is myth I heard for years that BB diff's were stronger when in fact the only difference was the code stamping on the housing. Given your attended usage, highway far from home, I would suggest you at least replace the bearings if the diff sat for a long time.

A local guy just went through the same deal, he installed a diff that sat for 20 years against advice only to have to remove it after the first drive do to noise and chatter. All I had to do with that one was replace the bearings and polish/tune the posi. works like new now with original R&P and side yokes.

Good luck with it, hope the case isn't cracked.

Gary ..
I don't doubt replacing the bearings is a good idea. At this point the diff is installed, I'm thinking to just go with it. Seems for every step forward, I take two back. Looks like I will need a conversion u joint for the later pinion flange to early drive shaft hookup.

About the caps and bolts: I bought a set of caps and bolts for the side yolks. The caps have an arrow embossed into them. Not sure if the arrow is meant to point in or out when mounted up against the side yolk flange. The mounting holes in the caps being offset from center line, should the thicker side portion of the cap go in or out? .. Must be related to arrow orientation, sure.

[Gary R.]

Re: diff ID

Thanks Joe,
I knew you would have the numbers! I have to say I haven't looked at the part number on the kits, but they are Eaton clutches and most likely made in Mexico now.