Help!!! Did I wreck my diff??

[Fm3 a66]

Did I wreck my diff???
Ok folks I need some help! I'm not a dodge guy but my father is and I'm helping him out with his 1970 340 cuda convertible. It's been sitting for about 2 years and we just got it on the road but noticed it had a differential leak in storage. We put about 200miles on it with no problems but figured we better check how low it is?  So i put it up on the hoist and I check the rear end fluid level and it was pretty low! So I grabbed a litre of straight 80/90 gear oil that I use in my old Chevy truck! (Not thinking this cars got what I now believe to be a sure grip setup?) anyway I filled it up and went for a drive! I made it about 2 blocks before the rear end started clunking and banging then it just seized up!!! I pressed on the gas and it wouldn't move, I put it in reverse and it broke loose and then I put it back in first and started going back home at about 5mph but then it happened again! So I jacked it up and the rear wheels turn free when driveshaft is off? I put the drive shaft back on jack the rear up and put it into gear to let it roll and it turns a few revolutions then starts shaking and seizing up again??? Did I mess things up with the wrong fluid? Help I'm stuck???

[anlauto]

Your rear end sounds messed up, but it's not because of the 80W90...

[Bullitt-]

Installed a used cone style posi in my car & I couldn't move in the driveway without it snapping & popping.
Added some GM posi lube , think it's called friction modifier, ran it on jack stands a bit n the issue went away. 

[Fm3 a66]

So maybe I should drain the fluid and put some different fluid with additive and try it? I've got nothing to lose but a litre of fluid I guess???

[Fm3 a66]

Oh I'm also trying to find out info on this diff. Can anyone tell me what it is off these #s
Here's a pic

[TelisSE440]

Oh I'm also trying to find out info on this diff. Can anyone tell me what it is off these #s
Here's a pic

Your differential has the number 2070741 (the known 741 case), which means it's a 8 3/4 axle which uses a 1 3/8 stem pinion. This was used mostly for low to medium horsepower applications. Not the best but still a very good piece.

[burdar]

Not the best but still a very good piece.

According to DrDiff, the 741 is the strongest "factory" 8 3/4 rear.

http://www.moparts.org/Tech/Archive/axle/1.html

[RzeroB]

It's a goner ... hopeless ... kaput. Your Dad should just get rid of it as it isn't worth the trouble trying to fix. I can be there Saturday with a flatbed to take it off his hands

Just kidding (but I sure wouldn't mind taking it off his hands )

Interesting that it turns freely with the driveshaft removed. Would almost think that if the Sure-Grip was the culprit that it would make no difference if the driveshaft was in it or not. Being that it's a '70 it probably has the Borg-Warner cone type Sure-Grip in it. Maybe it's stuck from lack of use while being in storage? Maybe all it needs is a drain and refill of the fluid? Sounds like you have access to a lift, and with a lift draining and refilling that puppy is relatively easy.

[Chryco Psycho]

Maybe I am wrong but that looks like 2070341 , could be from the late 50s or early 60s .
could it be the sprag in the trans that has the problem if the diff turns freely with the driveshaft out ?

[Fm3 a66]

RzeroB thanks for the advice and the offer but I know my dad will not part with his baby! He's almost 70 but he loves cruising in this car! I just need to get it back on the road for him!

Chryco Psycho
Yes I'm pretty sure it is a 2070341 and it does have a big X cast into it above the numbers but I just can't find any info on that #?  Everything looks pretty darn factory under there but were not the original owners so who knows what could have been done?  I guess I will tear it down this weekend to see what's going on?

[mopar jack]

Cone type sure grips need a friction modifier. You can find Sta lube Equa- torque at most auto part stores, it's only 4 onces so you won't need to drain much fluid. Put it in and take the car out and do a couple of figure 8's and that should   loosen it up.

[RzeroB]

Maybe I am wrong but that looks like 2070341 , could be from the late 50s or early 60s .

I agree, it does look like a "3"41, but as far as I know, the 8.75" chunks carried only one of three numbers - 741, 742, or 489. I think even the really old ones with the keyed axle hubs probably sported a "741" on the chunk too!

RzeroB thanks for the advice and the offer but I know my dad will not part with his baby! He's almost 70 but he loves cruising in this car! I just need to get it back on the road for him!

This axle problem sounds reeeaaalllyyy bad! I don't think Dad should go through the frustration of trying to get it fixed. Probably better to break it off clean and just get it gone and out of his sight.

I can still be there Saturday with the flatbed.

[mopar jack]

I believe there is another number for the 1/2 ton truck diffs.

[Chryco Psycho]

For sure there are other casting #s , I have come across them before & never found any info inthe internet on other #s
 Looking again both 7s seem to have a miscast on them so they could both either be 3 or 7 so it likely is a 741 . If the diff still turns easily I would look further up the driveline , could be in the trans

[1 Wild R/T]

The rarely seen old carrier number is 657..  Thats a 741 which as mentioned initially is the weakest of the three typically seen 8.75 axles... The 742 is the heavy duty piece... But the 741 will still take a ton of abuse.. And adding lube didn't cause this..... Could be in the trans as mentioned... Could be in the brakes... It could be the pinion nut has backed off allowing the pinion to slide back & hit the carrier... If thats the case you'll be able to feel side to side movement of the differential yoke... What ever it is your gonna find it... But don't worry about the gear oil, the wrong oil is a hell of a lot better than what it had before... No oil....

BTW looks like your dad has a very nice Cuda...

If it truly does have a Sure Grip (Dodge slang for Posi) it does need friction modifier but that won't lock the rear end up... It will make popping & scraping noises mainly when turning corners but it won't lock up the axle...