Rear differential question

[RodnRudy]

I put a '71 12 bolt rear in my 68 mainly for the 5 lug but it also had been changed from a 3-73 to a 3-07 which is what I was looking for. I've driven it about 3500 miles and the rear is real clunky when I put it in gear. Idle is set around 500 RPM. I'm thinking that the ring and pinion is incorrect for the carrier, three verses four. Now for my question. How do I tell if I have a three or a four series carrier in the rear? I'm suspecting that it has a four series carrier and who ever set it up couldn't set the back lash correctly. Either way it looks like I'm going to have to find a shop that can set it up correctly.

[Mike C]

I'd start by checking out the u-joint... That's usually where you find a clunk. A 3 versus 4 series the teeth wouldn't hardly touch. Usually when a diff is bad it whines like a teenager.

[RodnRudy]

The truck has a new drive shaft and joints but I'll check them again. The reason I suspected the wrong carrier is that there's a lot of play at the yoke. Not in and out play but rotational play

[Mike C]

Sometimes the pinion nut can loosen itself. Or the bearing is bad. But the wrong size carrier just won't work unless some clown has just shimmed it like crazy to get it close (I had a Jeep Dana 30 I bought used that a SHOP did this to!)

[Spf79]

I'm going through a rear end overhaul right now, and I don't think you can bolt a three ring onto a 4 carrier and get it to work? The pinion gear size is drastic between the 4:11 & 3:07. I'm probably wrong... I'm not a mechanic .
Also, I think the 3:07 & 3:73 stock carriers are the same, I went to 4:11's and had to go with a new carrier. If I went with the 3:73 I could have used the same carrier. The 3:73 gear ratio seems to be the top end of the 3 carrier, or maybe 3:93?

Question? Have you pulled the cover and checked your oil to see if it is a mess with shavings? Once you have the cover off I would pop your axles and spiders out and look at the wear. Once you have it apart you can hold the pinion still and then rock the carrier up and down with a 1/2" wrench to see how bad it is. I have had a couple 10 bolts give me almost a 1/2 rotation of the driveline when going from drive to reverse... such a distinct sound. Lol. Definitely a sign of wear. You mentioned that your pinion felt good, so your carrier bearings may be shot? You might get lucky with just replacing the carrier bearings? You can pull the carrier given your gears look good and then have the bearings replaced, just keep track of your shims and put it back together the way it came apart. Fingers crossed of course.

I have a 3 & 4 series set of gears and carriers on the bench right now if you want measurements or pics. I hope this is helpful. Watch some YouTube of overhauls, it will make you feel more comfortable doing the work. Good luck buddy!

I would give you my 3 carrier, but I had to destroy it to get the spider gears out since the pin that holds the spider gear shaft was broken. I was not pleased. Lol.

[RodnRudy]

I replaced the axle bearings and seals after I got the rear so I had the rear cover off. Everything looked good to me but I'm no expert at rear differentials. I'm going to find some one local that can check and repair mine if it needs it. Thanks for the replies every one.

[davepl]

I forgot which direction (big carrier - small gear, or the opposite) but isn't there a setup where you can run a spacer with the "wrong" carrier? I'm told it's not the hot ticket but possible. It's also another possible failure point to be looking at if so...

[Mike C]

The carrier break on a truck is 3.73 and higher numerically. The 3.07-3.42-3.55 uses a different carrier, the "3 series". There is a third carrier that is for 2 series gears.

You can use a spacer on the 2 series and run 3 series gears or on the 3 series and run 4 series gears. They also make "thick" ring gears so you can put 3.73 and numerically higher gears on the 3 series carrier.

I have had pinion nuts back off and cause excessive play. After the u-joint, this is what I would check next. Set parking brake. Remove driveshaft. Using a 1 1/8" socket and half inch ratchet handle, tighten the nut as tight as you can. If it does tighten up, I would order a new pinion nut. Once the new nut arrived, I would pull the yoke, replace the seal, and tighten the nut down as tight as you could with the ratchet handle again. The set parking brake holds everything so you can put some torque on the nut.

[RodnRudy]

I talked to a shop this afternoon and they'll get me in and check things out in a couple of weeks. Drag racing is fast approaching and they're busy as hell. He did mention the pinion nut coming loose but I just replaced it and tightened it back down to where it was. Thanks again every one that responded to my questions.

[davepl]

Originally Posted by Mike C

The carrier break on a truck is 3.73 and higher numerically.

That's only for one particular kind of axle. For example my truck came with a Dana60 and the carrier break is at 4.10, not 3.73

So, Dana 60: 4.10 & Down / 4.56 & Up

The GM Truck 12-bolt has breaks at: 2.73 & Down / 2.76 to 3.42 / 3.73 & Up

The GM Auto 12-bolt has breaks at: 2.73 & Down / 2.76 to 3.42 / 3.73 & Up

[Mike C]

That's true, but this was about 12 bolt trucks, so I didn't see why we would bring other diffs up? I guess I didn't specify 12 bolt but it seemed clear at the time. And you are right the 3 series break is 2.73 and up. The 2 series is less than 2.73.

[RodnRudy]

So you guys are saying that my carrier is correct for both 3/07 and 3/73 ring and pinion?

[davepl]

I didn't mean to get overly pedantic but you said "For our trucks", not "For our 12-bolt trucks", and didn't want someone scanning the thread later to get incomplete info. You're 100% right for 12-bolt trucks, I just wanted to be precise for later reference.

[bryanw1968]

3 series stops at 3.42. Smallest 4 series is 3.73. If the crush sleeve inside the yoke gives out it will leak and whine.