My Dart has had a noticeable clunk intermittently when changing gears depending on rpm & throttle position...

A few weeks ago I had the pleasure of swapping out the rear springs on her, and took the opportunity to disconnect the rear U joint from the axal. Rotating the yoke left and right by hand causes a pronounced clunk as it moves through the slack. There is only a slight whine from this rear end at road speed.

Is this unit in need of replacement, shimming, baring replacement, or what. I have never disassembled, or repaired a rear end before, so I'm not up to speed as to how much lash is ecceptable with a 8 3/4" Sure Grip, or clunking...

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

I am not a profesional at rear-ends, however I have rebuilt quite a few. The clunk is probably a variety of things, ranging from excessive backlash, to worn bearings on the pinion or differential case. Is the 8.75" a removable carrier type? If so, I would yank that bad boy, grab a dial indicator w/magnetic base, and start checking backlash.

If it is integral carrier type, then pull the cover and do the same. Factory service manuals will give specific backlash measurements, when I did my 7.25" just recently, the spec was .003" to .009" I set mine at .006" since the gears were worn. This causes a little bit of whine, and will proabably continue until the gears contour themselves to eachother.

I am sure someone here who has done a fair share of these guys can point you the right direction for specifications.

When you have your hand on the yoke, can you move it sideways or up and down? In/out? If there is ANY play in it, there is no pre-load on the pinion bearings, which can easily be the source of the clunk you hear. If that is the case, you need new bearings, and possibly cones.

If you take the cover off/remove the carrier, and the differential case moves at all, then the bearings are probably toast, and possibly the cones on that one too.

Let us know what you find out. Might not hurt to roll a contact pattern and see what you come out with.


~THOR~

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Certified Auto Appraiser - RevItUp Classic Appraisals
Too young to be slow, too old for high speed.

You got me, how dose one know the difference between removable carrier vs integral? There is no cover on the rear facing surface of the pumpkin.

I don't recall any vertical or east west movement of the yoke, just rotational play. I should recheck this.

I just did a quick read through the FM section on Rear Axle. A bit confusing without a unit sitting in front of one on a bench. It will require several additional readings for me to grasp what is being said.

In a nut shell, one needs to pull both axles, drain the oil, drop the drive shaft, than remove the carrier before any detective work can begin.

Would it be true that the carrier assembly is most likely where ye old clunk is coming from?

If there is no vertical or horizontal play in the yoke the shimming of the barings & gears is suspect?

There is a god, spell check is back!!!

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

The 8¾" is a removable-carrier type rear axle assembly.

The "Chuk!" noise when engaging Reverse or Drive from Park or Neutral is very common in RWD Mopars. It's not <i>good</i> or desirable, and if there's excessive lash in your ring-pinion mesh you ought to correct it, but it also does not indicate your rear axle is about to grenade.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks Dan

Looks as if I'm going to be learning how to shim a ring gear this winter. I have been on line for several hours looking into the 8 3/4" rear end. There are three renditions: 741; 742; and 489 series. I quess I need to pull my carrier to ID what I have first.

I also noticed there are tons of 3.55 and up ratio pinion gears out there, but almost no gears sets from 2.76 to 3.23 available. Presently I have 3.55's, and would not mind something a bit higher if I'm going to pull this thing apart. Having an engine cranking almost 3000 rpm at speed on the open road is ruff on the rigging, and annoying to listen to.

Please correct me if wrong here:

compared with a 3.55:1 ratio,
2.76 gear would reduce rpm by 23% so 3000 rpm is reduced by 690 rpm
2.93 18% 540 rpm
3.23 9% 270 rpm

With a 2.76, wouldn't passing gear become more useful at 60 mph and up, and fuel economy improve?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

It's an 8¾", not an 8-3/8". (corrected above)

It is not necessary to pull the centre chunk to determine which carrier you have. It is almost certainly a 741, but you can look at the casting number on the side of the casting itself; the last three digits of the 7-digit casting number will tell the tale.


Low (numerically high) gearsets are easy to find because that what the drag racers prefer, and they're the ones most commonly spending money and time on rear axle work. Street ratios are harder to find, but not impossible. Available ratios included 2.76, 2.93, and 3.23.

3.55 is generally too low (numerically high) for the street. Optimal ratio selection depends on tire size and engine/transmission configuration. Don't pick at random; you'll wind up disappointed. Post the details of this car including specific info on how the engine is "dressed" and configured, what transmission you have, and what tire size you run (and if you're planning on changing tire size, mention what size you're planning to use), and you'll get some ratio suggestions.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yeah I know it's a 8.75, just dyslexic laced bad typing...

The car is an auto show weekend put-put sun out kind of ride that has to cover 100 miles one way to most shows. Presently she has cliford intake & headers, Holley 4 bl, with a mild cam riding on P20-70-14 tires. A904 equipped with a god awful reverse manual valve body that is getting returned to normal operating condition. A mild porting & gasket matching project, and some other head work will take place this winter.

There is a good chance a 300 hp small block may someday find its way under the hood if I can't get the 225 to crank out some more go.

Yet another old phart flash-back:

Years ago we had a new 1976, 400 lean burn 4bl Cordoba with a very high geared rear end, perhaps a 2.45:1 ratio. That car had 15 inch tires, and would not spin a wheel unless ice formed under it. However that baby could brake the national 55 mph speed limit in first gear, would shift out of second around 92 mph, bring the speedo needle back around to zero in third when in drive, and could deliver 19 to 20 mpg on the highway. I liked the long legs, and low rpm cruising with that high torque engine.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

With 225/auto and 205/70R14 tires I like 3.23s or 2.93s.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

For "mixed" driving, 50% around town and 50% in town freeways, I tend to use 3.23.
For mostly freeway with some around town, I like the 2.94 ratio
For "open road", cross country drives, the 2.76 is nice.

I tend to drive in the peak torque range of an engine, also called the RPM "sweet spot".
With 3.23 that is 60 - 70 mph...
with 2.94 that is 70 - 80 and with the 2.76, that is 80+ (depending on rear tire size)
DD

Dan, The axel is a 489, thanks for the help.

Doc,
We live in a county that half the size of the state of Connecticut, population about 30,000. and we have two operating red, yellow, green traffic lights in the whole place, which incidently just a third mile apart... Not a lot of stop & go.

The nearest mall & interstate highway is 100 miles away, and unlike other states in this great union, Maine's highest speed limit is 65 mph on the interstate, and 55 mph at best on major highways. Yup, we are slow.

I guess my point is with a higher geared rear end, passing gear could be used as needed to get into the torque band. With the 3.55's I have now, 80 mph is the point that the little 225 starts to run out of wind. I don't have a tachometer, so I don't know what she is turning at say 80, or 90.

Perhaps someone out there could give me a few red line speeds in second & third gear for a car with an A904 & 3.55 rear end.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Engine RPM at any given road speed can be calculated:

Tire diameter × π = tire circumference

(63360 ÷ tire circumference) × (rear axle ratio) = engine rpm @ 60 mph in 3rd gear.

(63360 ÷ tire circumference) × (rear axle ratio) × 1.45 = engine rpm @ 60 mph in 2nd "passing" gear.

Multiply either result by 0.83 to get engine RPM @ 50 mph, by 1.17 for 70 mph, by 1.33 for 80 mph, by 1.5 for 90 mph, and by (n ÷ 60) for n mph.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan,

Is tire dia. units in inches?

Something ain't right:
dia. =12.25"
C=12.25"X3.14=38.465"

(63360/38.465")X(3.55/1)=5847 rpm@60mph

She's not turning that fast @ 60mph. If I had to guess neerer to 3K.

Where did you get the constent 63360?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Yes, we're working in inches. I promise you your tires are not 12.25 inches in diameter! That flunks both the reality test and the logic test; how'd you get that number? Measuring accurately is hard, but you can approximate the tire diameter closely enough. Suppose you're using 205/70R14 tires. That size means the tires' cross section is 205mm wide at the tread, and 70% as tall as it is wide. So:

0.7 × 205 = 143.5mm

There are two such cross sections between the top of the tire and the bottom, so:

143.5 × 2 = 287mm

Convert to inches:

287 ÷ 25.4 = 11.3

Add the wheel diameter to get your tires' diameter:

11.3 + 14 = 25.3 inches

Plug 25.3 into the math given above and you'll get real and reasonable results.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan,

Sorry, one huge brain fart here, for some reason radius popped out. You are right, tire C is ruffly 24.5", not C/2. What a dope...

Plugging in the correct values, and turning the crank, I get 2923 rpm @ 60 mph. Ahhh, that's the ticket.

So what would a stock bottom end 225 red line be?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

H'mm. What size did you say your tires are? (there's a digit missing from the tire size stated in your previous post).

Redline's not really relevant, but 3krpm at 60 mph is going to sound/feel/ pretty "busy" and consume a fair amount of fuel. 3420rpm @ 70 mph even more so.

If you replaced that 3.55 centre chunk by a 2.93, you'd be running a much quieter, easier-on-the-pocketbook 2412rpm @ 60 mph, 2822rpm @ 70 mph.

If you replaced the 3.55 by a 2.76, your engine speeds would be 2272 @ 60 and 2658 @ 70.

You may want to scout around for a complete centre chunk with the ratio you want, rather than seeking to swap a different ring and pinion into your existing centre chunk. You do not need the extreme-duty 742 or 489 type; a #741 or #657 will be much more than adequate and these command lower prices. All 8¾" centre chunks fit all 8¾" rear axle assemblies, regardless of year or model. And that #489 3.55 centre chunk can easily be sold to cost-reduce the project.

(Oh yeah, two other things: Your tire d, not C, is 24.5" or thereabouts. And 63 360 is the number of inches in a mile.)

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.