742 vs 489 Case

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USCG CHARGER

USCG CHARGER

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I have a 742 case surgrip with 4.10 gears but was told it would not hold up to my expected HP levels and that I would need a 489 case....

I'd much rather use the 742 casing and jut put 4.30 gears in it and be done and maybe back brace the 8.75 too..... But I dont want to brake it only to find out it took my sprag with it...lol...

Have you guys ever witnessed a 742 failure? looking at the pictures the 742 pinion looks hulkier then the 489. I know I could go Dana but I'd like to save a few gand if I can get this 8.75 to hold up.
 
It's a matter of opinion which is stronger, really. I think the 742 may be the better unit,as there is no crush sleeve. If you 742 is a power lock I'd definitely stick with it.
 
I agree, the weakest part is right up front where the nut bolts it on. I think
both are about the same strength. There was a really go post on the web about
the tests done on all the units and very supprisingly the little 741 even did quite well. How much HP are you talking about, and it will be the torque that
would kill it, not the HP.
 
iv ran all three with no problems my car was 3000 lb. run 11.20s all the rears iv seen brake started in the water box you can't come out of water with weels spining untel thay hook . and not expect soomthing to brake.
 
Thanks fellas... I'm not sure about the tq but it will be a 416 stroker so i'd assume quite a bit.. it will be foot braked as well.
 
Who ever told you that, don't listen to them. I have also done burnouts leaving the water box and spinning them till they hook all my life. My 71 BB 6.40 scamp had probably 400 passes on it, no problem. Use good parts and they don't have to many problems breaking.
 
What spline is the carrier on the 742? I ask this bc when I buy 35 spline axles I should have to change any thing right?
 
:thumbup: Good to hear that you're keeping the 742 and saving yourself some $$$... It should do just fine.
 
So I guess the next question would be since i'm tryin to save my duckets.... and not buy a different carrier (35 spline to fit 35 spline axles) what can the 30 spline axles take?
 
USCG CHARGER said:
So I guess the next question would be since i'm tryin to save my duckets.... and not buy a different carrier (35 spline to fit 35 spline axles) what can the 30 spline axles take?
Click to expand...

You should run a good pair of 35 spline axles. The factory 30 spline won't take alot of abuse.
 
742 will work best, it is a stronger case.Even if you use a crush sleve eliminator, it is still stronger.
But back brace it.I blew a 489 apart (broke the caps off)within 10 passes ,so I went to a 742 which lasted for years. It was in a 3200 Duster, back half 14X32 tires.650 H.p. B.B. with Griner brake.
It packed the front wheels on every run, and ran 10:70's all day long.
The ticket is, run the street gears,not the pro gears.The street gears are softer, set them up tight. You will still need to check the ring and pinion every year, most likely after the 3rd year you will find chips off the ring gear. Even with the back brace.
The issue is, the car leaves, but the tires stay, this bends the housing, and pulls the ring away from the pinion.
30 spline will work, but 35 is better.
Hope this helps.
 
Talk about bring out your dead... A three year old thread. :D
 
I'll take the 489 over the rest.
What makes this really funny is that the OP is using a 9" Ford now so this thread is useless for him.
 
bajavan said:
742 will work best, it is a stronger case.Even if you use a crush sleve eliminator, it is still stronger.
But back brace it.I blew a 489 apart (broke the caps off)within 10 passes ,so I went to a 742 which lasted for years. It was in a 3200 Duster, back half 14X32 tires.650 H.p. B.B. with Griner brake.
It packed the front wheels on every run, and ran 10:70's all day long.
The ticket is, run the street gears,not the pro gears.The street gears are softer, set them up tight. You will still need to check the ring and pinion every year, most likely after the 3rd year you will find chips off the ring gear. Even with the back brace.
The issue is, the car leaves, but the tires stay, this bends the housing, and pulls the ring away from the pinion.
30 spline will work, but 35 is better.
Hope this helps.
Click to expand...
Wrong! street gears are harder. pro gears are made to give so the teeth dont break on hard launches.
 
daredevil said:
Wrong! street gears are harder. pro gears are made to give so the teeth dont break on hard launches.
Click to expand...

Ive always been under the impression to that Pro Gears are hardened and thats why people warm up their rearends in the pits because cold Pro Gears break since they are so brittle.
 
Stolen online.lol What is the difference between Pro Gears & Street Gears when ordering a rear end?

Pro gears are made from 9310 and then heat treated. It is a softer alloy than the 8620 Street gears. The softer 9310 alloy allows the gear to absorb higher impact loads that are generated in drag racing without developing cracks. A harder 8620 street gear could shatter under the same loads. As a side effect the Pro gears are not the best choice for street use as they will wear faster.
 
Same as a impact socket VS a chrome socket.
 
To All,
I may be responding to an old thread, but I am looking at it for the 1st time.Many other people still have questions too.
The info I added is NEW INFO.There are many NEW people coming to this thread daily.
As for the street gears verses the Pro Gears, The point I was making was not that the Pro Gears are soft, verses the hard street gears. I knew that (I typo'd) thanks for the correction.
The key point I was trying to make was several of us running 8 3/4" diffs, have better luck with the street gears drag racing (on the drag strip), then the Pro Gears.Why? I can't answer that. They just blow up less, and work better, time and time again. Also that the 742 case has held up much better then the 489.Something about the webbing and casting of the case. I.E. Blowing caps off the case.
We are all running over 600 H.P. with 14" X 32" tires on a ladder bar back half cars and a transbrake.The ticket for all of is, a back brace really helps.I don't know everything, I just know what I have experineced myself.
Thanks for all the negative reply's.All I am trying to do is share the experience's I have had, and pass them on to help others.
Please read the notice at the beginning of the threads.If your having a bad day, then don't respond please.
My mom always told me," If you don't have anything good to say, then don't say anything"
Happy Racing

:hello1:
 
I've only broke one 8 3/4 in my life and it was a 489 in my old pickup. And you guessed it, the caps broke.
 
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