I had the third member rebuilt about 6 years ago. I had a 323 ring and pinion and suregrip installed. I also replaced the u joints at the same time. Ever once and a while I would notice a slight thump when I accelerated. It didn't happen all time, and I could never narrow it down. So today we are out enjoying the day, and now I'm getting a louder more noticeable thump in all three gears. Brought it home to check it out. Fluid is a little low. Doesn't smell burnt but the pinion area is extremely hot to the touch. Any ideas of what to do next. 1967 dodge coronet 500. Automatic trans. Thanks Joe

in neutral I would wiggle the drivehshaft back & forth to check ring and pinion backlash. check ujoint bolts and trans mt bolts. Maybe check axle end play (doubtful on that tho). that'd get us started

Do you know if it is the original third member or maybe a later one? The reason I'm asking is the later model "489" (the last three numbers on the casting number under the drivers side of the pinion gear ) third members had crush sleeves on the pinion gear to set the pinon bearings preload and they can do cause problems with the pinon gear running hot if they are not set up correctly
Does the thump only occur going around corners while accelerating? If so maybe it is the posi causing that

In neutral I get a little movement when I wiggle the driveshaft. I am also hearing a clanking, but that could just be normal. I'm concerned about the heat issue though. What are your thoughts?

how hot and how much backlash movement. I ain't a rear end guru but I think the heat might be (way) more telling than the BL but the more info the better so the guys that know what they are doing can (further) advise

It's the original that came in the car. It's a 472 case. The thump occurs intermittently when decelerating. Most of the time it is with accelerating and turning corners. I drove about twenty miles home after a stop for dinner, and when I got it home, I checked it out. It was so hot you couldn't hold your hand to it in around the pinion area.

I'm no rear end expert but here's my $.02. First, when was the last time you added the sure grip additive to the diff.?? Second, a clunk going around corners can be worn clutches in the sure grip... due to lack of lubrication and no additive or possibly a broken spider gear.

My best guess on the heat problem is the fluid is warn out and just can't provide the lubrication needed. Not sure how much this helps though.

I haven't added additive or changed the fluid since I installed it. I think there is about 5000 miles on the unit. I appreciate any and all suggestions

First thing I'd check is the end play on the axles concerning the noise you hear.

As far as the heat is concerned, measure the temperature after 1/2 of highway driving with a contact type pyrometer (not infrared) and report back. It may be normal.

I'm going to say that the normal differential fluid temps(130F +) going down the road for 20 minutes + should be hotter than you can hold your hand to

The 1st thing I would check is to see if the pinion nut has backed off, put the rear end on jack stands with trans in neutral, turn the driveshaft slowly in one direction, then reverse the direction. What your looking for is how much the driveshaft moves before the tires change direction, should be almost immediately. If not, you need to find the source of the slop.

If that checks OK, the next thing I would do is set the car back on the ground and drive to the nearest fairly large blacktop parking lot. Once there, do a very slow tight figure 8 at least 7 or 8 times, then go for a ride and see if there is an improvement. If the car sits a lot, or isn't often turning corners, the sure grip lube gets squeezed out from between the clutches of the rear end. The figure 8s force the clutches to separate and allow the lube to get back between the clutches. The dry clutches make all kinds of noises. You may have to repeat the figure 8 thing a 2nd time, if there is no improvement after the 2nd time, the problem is elsewhere, its time to pull the center section and take a look. Gene

The heat sounds normal for a differential.

They get warmer than you think they would, check one after driving sometime.