8.75 Axle - 741 / 742 / 489

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marianamaya

marianamaya

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Guys,

Can someone give me some background on why there were 3 different 3rd members offered on the 8.75" axle? What are the differences between each one, and what were the determining factors that put them into different vehicles?

Long term, I will swap out my Slant 6 with a 340. I will swap my 7.25" for an 8.75". I may have found a 742 open axle. If I get this, will I need a new prop?

Long Term, I will also want to make it a sure-grip axle. I believe that all I have to do is replace the open center section with a sure-grip center section, or replace the open differential with a sure-grip differntial using the current open gear set. An I missing anything? I do not want to get in over my head any more than I have to.

Thanks,

Pete
 
Unless you are absolutely killing the thing with huge TQ and dead hooking, all of the carriers work fine.

Don't buy into the 741 is weak BS...

Get a carrier with a gear ratio you want, put a SG (most like the clutch style) in it and go. There are some that like the tru-trac stuff too.

You should measure your driveshaft to make sure it's correct length. Going from a 7.25 to 8.75 should require a shorter driveshaft.
 
crackedback said:
Unless you are absolutely killing the thing with huge TQ and dead hooking, all of the carriers work fine.

Don't buy into the 741 is weak BS...

Get a carrier with a gear ratio you want, put a SG (most like the clutch style) in it and go. There are some that like the tru-trac stuff too.

You should measure your driveshaft to make sure it's correct length. Going from a 7.25 to 8.75 should require a shorter driveshaft.
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x2
 
Mopar used the 741 in lighter weight lower HP applications and 742's and 489's were reserved for heavier higher HP cars. Like Rob said the 741 is plenty strong unless your hooking hard and racing a lot. But if the 742 you found is the gear ratio you want might as well go for it and add a sure grip. Either sure grip is a bolt right in. Just measure the R&P backlash before you disassemble it and when you install the sure grip set it back to where it was.
 
fishy68 said:
Mopar used the 741 in lighter weight lower HP applications and 742's and 489's were reserved for heavier higher HP cars. Like Rob said the 741 is plenty strong unless your hooking hard and racing a lot. But if the 742 you found is the gear ratio you want might as well go for it and add a sure grip. Either sure grip is a bolt right in. Just measure the R&P backlash before you disassemble it and when you install the sure grip set it back to where it was.
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Yes and no, most if not all 383 A bodys even with a 4 speed came with the 741 case.
 
Yes, you'll need to shorten the drive shaft.

Like others have said, don't discount the 741 even though is has the smallest diameter pinion. It is still bigger than the ubiquitous Ford 9".
 
if down the road you want to swap ring and pinion, 742 and 489 use different gear sets, not the same pinion. suregrips will interchange.
 
w2360Duster said:
Yes and no, most if not all 383 A bodys even with a 4 speed came with the 741 case.
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That's true. But an A-body is a light body Mopar. And the 383, while being a great running engine wasn't a real high horsepower engine. If I'm not mistaken nearly every 8-3/4 A-body left the factory with a 741 chunk, even the 69 440 cars which were high horsepower. I guess you'd have to call it the "A-body rule", LOL...
 
Anybody have a comparison picture between an open carrier and a sure-grip carrier? I'm looking into one that is said to be a sure grip, but would like to verify.
 
fishy68 said:
That's true. But an A-body is a light body Mopar. And the 383, while being a great running engine wasn't a real high horsepower engine. If I'm not mistaken nearly every 8-3/4 A-body left the factory with a 741 chunk, even the 69 440 cars which were high horsepower. I guess you'd have to call it the "A-body rule", LOL...
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You are correct-M codes used 741's. Any 8 3/4 is a great center and capable of taking a lot of abuse.
 
d5667 said:
You are correct-M codes used 741's. Any 8 3/4 is a great center and capable of taking a lot of abuse.
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I respectfully disagree. The proof is my 69 A56 4 speed 'cuda 340 that has the original rear end and it's definitely a 489 case w/ factory 3.91 ratio. (built 6-13) This car has HD everything including front and rear torque boxes and a 15/16" diameter front sway bar. I think it was the combination of the gear ratio and 4 speed that triggered the extra HD stuff. Everything I ever read says the last model year for the 742 case was 1968. I don't have one in front of me but the 69 FSM probably does not have the 742 info in it.

All the 440 A-bodies were automatics. If Chrysler followed suit like on the Hemi and 440 B-bodies, it kinda makes sense. The standard rear in a Hemi or 440 car with an automatic trans was an 8-3/4. 4 speed Hemis and 440's automatically got the Dana.
 
KosmicKuda said:
I respectfully disagree. The proof is my 69 A56 4 speed 'cuda 340 that has the original rear end and it's definitely a 489 case w/ factory 3.91 ratio. (built 6-13) This car has HD everything including front and rear torque boxes and a 15/16" diameter front sway bar. I think it was the combination of the gear ratio and 4 speed that triggered the extra HD stuff. Everything I ever read says the last model year for the 742 case was 1968. I don't have one in front of me but the 69 FSM probably does not have the 742 info in it.

All the 440 A-bodies were automatics. If Chrysler followed suit like on the Hemi and 440 B-bodies, it kinda makes sense. The standard rear in a Hemi or 440 car with an automatic trans was an 8-3/4. 4 speed Hemis and 440's automatically got the Dana.
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I believe D5667 was agreeing with what I said in my quote below and if you notice I did say "nearly every 8-3/4 A-body left the factory with a 741 chunk".. Nearly (not every) being the key word, LOL... That was the great thing about ma Mopar. You never know what to expect. Yours being an example that Mopar did put some 489's in a-bodies. Like you said it was probably because of all the other HD pieces. Then again they might have ran out of 741's that day and just wanted to fill the order. That happened also. Even though 68 was the last year slated for using the 742 case the 69 FSM does include information on the 742 case and I have seen a few early 69 axles with a 742. My guess is they were just using up the 742's they had left over from 68 and thus included info on them in the FSM.

BTW: can I assume your Cuda is a formula S since it has all the HD suspension goodies?

fishy68 said:
That's true. But an A-body is a light body Mopar. And the 383, while being a great running engine wasn't a real high horsepower engine. If I'm not mistaken nearly every 8-3/4 A-body left the factory with a 741 chunk, even the 69 440 cars which were high horsepower. I guess you'd have to call it the "A-body rule", LOL...
Click to expand...
 
adamsredlines said:
Anybody have a comparison picture between an open carrier and a sure-grip carrier? I'm looking into one that is said to be a sure grip, but would like to verify.
Click to expand...

Here's a link to a tech page showing the 2 types of sure grips Mopar used through 1974. Be aware that the first one is the cone type and they are not considered rebuildable, although if their not worn real bad you can machine the cones down to make them work again. But sometimes their completely worn out inside and their basically junk. Just tossed one like that a few weeks ago. If the one your looking at turns out to be a cone type don't pay much for it unless the seller proves it's not junk.

The one in the bottom picture is the clutch type and is rebuildable and much heavier duty (4 spider gears vs. 2 on a cone type). It is the sure grip of choice IMO...

http://www.moparts.org/Tech/Archive/axle/13.html

I can't find a good picture of an open differential. Here's a link to a page showing an exploded view with an open differential but it's not the easiest to picture since everything is disassembled

http://www.moparts.org/Tech/Archive/axle/9.html
 
Yep; you are correct Fishy,I certainly agree with you. I also know that these years were not the greatest quality control years.I have talked to a bunch of the line workers and I myself have worked in all the car plants in St Louis as a carpenter/Millwright.When you see these cars going down the line and the parts being put on you understand that one day this guy is a ditchdigger and today he is an electrical installer.Yep they had a build sheet but if the parts didn't match use what you got.
 
Shoot,, my Dart was on the showroom floor with 383 callouts on the hood,, it was, and still has original 340..

It musta been a Friday afternoon build.. lol
 
fishy68 said:
Here's a link to a tech page showing the 2 types of sure grips Mopar used through 1974. Be aware that the first one is the cone type and they are not considered rebuildable, although if their not worn real bad you can machine the cones down to make them work again. But sometimes their completely worn out inside and their basically junk. Just tossed one like that a few weeks ago. If the one your looking at turns out to be a cone type don't pay much for it unless the seller proves it's not junk.

The one in the bottom picture is the clutch type and is rebuildable and much heavier duty (4 spider gears vs. 2 on a cone type). It is the sure grip of choice IMO...

http://www.moparts.org/Tech/Archive/axle/13.html

I can't find a good picture of an open differential. Here's a link to a page showing an exploded view with an open differential but it's not the easiest to picture since everything is disassembled

http://www.moparts.org/Tech/Archive/axle/9.html
Click to expand...

Just wondering why you had to toss a cone style, I have had real good luck with them for the past 30 years, sometimes shimming and machining is needed, but not often.
I have run clutch styles also with minor issues.

Most of the time excessive wear is caused by the wrong fluid or additives which causes the cones/clutches to slip and basically overheat, wear and abuse does take it toll but any rear will eventually get tired.
 
sireland67 said:
Just wondering why you had to toss a cone style, I have had real good luck with them for the past 30 years, sometimes shimming and machining is needed, but not often.
I have run clutch styles also with minor issues.

Most of the time excessive wear is caused by the wrong fluid or additives which causes the cones/clutches to slip and basically overheat, wear and abuse does take it toll but any rear will eventually get tired.
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I've re-machined a few of them too with good success but this one was absolutely trashed. The housing where the cones rode had big grooves in it and the cross pin for the spider gears was wollered out real bad (I'm talking 1/4" gap, LOL). The spider gears and ring and pinion were fine. For that reason my guess is someone ran it with no limited slip additive for a long time. Not sure why the cross pin was wollered out but I've seen 3 of them that way.
 
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