1970 Rear End Bang 454 automatic

[Jeff C.]

Wondering what the source might be for a loud bang/clunk I'll hear when turning that is coming from the rear. It is loud and it seems to happen after it's warmed up. I say that because I don't remember it until a few miles into a drive. It's real noticeable at slow speeds and happens when I turn. I think there's a slide shift occuring at the same time as the bang too. Could it be as simple as a needed fluid flush in the differential? I only drove the car about 50 miles y.t.d. and wonder if that could play into it too? Your comments are always appreciated.

[John H.]

Re: 1970 Rear End Bang 454 automatic

Jeff -

Sounds like Posi clutches chattering. Suck out the old fluid, add 2 bottles of GM Posi additive, fill with GM diff lube or 75W90/80W90 gear lube, and do some low-speed figure 8's in a parking lot to work the lube into the plates.

If that doesn't help, it may be time for a Posi rebuild.

[Terry B.]

Re: 1970 Rear End Bang 454 automatic

I had the same problem with my '68. Did what John has recommended and have not had any problem for over 5 years and counting....knock on wood. It maybe time for new fluid now that I think of it. How long can you go between service?

[Jeff C.]

Re: 1970 Rear End Bang 454 automatic

Thanks John. You say suck out, it won't drain on it's own?

[Gary R.]

Re: 1970 Rear End Bang 454 automatic

Try the GM and only GM additive. Do not use any of the many super duper oils and additives on the market, you don't need them. You can use GM oil but I never do and I have rebuild many of these. I use Lucas 85-140 gear oil or you can use 90 wt gear oil.
There is no drain unless someone tapped one, this is also something I always do but it's not "correct" sure does make it easier to pm the diff though. You'll have to get a suction gun and pull it out the fill hole.
If 1 bottle of additive doesn't work then try a 2nd but that's it, if it still hammers around corners you iehter live with it or drop the diff out.
If you do drop it out be aware there are a few ways to rebuild the posi,some better then others. The condition of the posi case and clutches are also important.
If you hear a clicking sound,you better drop it soon as this usually means the spiders are going or the cross shaft hole is opened. Left unchecked could cause the diff to lock up and you will then grenade the whole unit.
There is no internal difference between the BB and SB units. The only diff is the BB used the "HD" yokes with caps, some SB auto's did as well and I think all used them by 75.
Here is a stock posi unit, I would not rebuild one like this but many do and the problems continue. A tuned posi will not have the springs, still have posi,but no hammering. You can see the date code and tooth count along with the GM stamped in the side of the RG. This is a common 336 diff, 1973?

Also look at the front snubber,many times the pinion seal leaks on it and breaks it down over time. As a result the diff will hammer starting and stopping. You may have that condition as well.

[Richard R.]

Re: 1970 Rear End Bang 454 automatic

Jeff your 70 corvette has no drain plug. Something I have never understood. Hope this helps

[Tom H.]

Re: 1970 Rear End Bang 454 automatic

Thanks John. You say suck out, it won't drain on it's own?

Nope. Fill plug is up high.

[Patrick H.]

Re: 1970 Rear End Bang 454 automatic

U-joints can also give you a heck of a Bang noise.

Try the fluid change first, and look closely at the u-joints while you're under the car.

Patrick