I picked up a 489 and a 742 rear center section today, both are open, I need to build one, which one should I build? What are the pros-vs-cons of each?
The gear ratio selection is not a factor, I am set on a 3.73 gear. I am thinking of the Mancini aggressive style diff. Which is easier to set up myself, i want to do it myself this time?
I went through this a while back. All the research and questions I asked ended up pretty much the same way. Either one will stand up to reasonable abuse if built properly with good parts. The whole "you gotta have a 489" thing is a little overrated. Hell, I know a guy with a pretty hot car that runs a 741... go figure. I suppose if you look at it from an engineering standpoint, the 489 with the tapered pinion is a bit stronger and if you have a 489 takeout to build, with all other parts going into either case being about the same price, by all means choose the 489. You might offer yourself a little piece of mind and you'll be more "fashionable" with the 489 also. Just my 2c.
The 742 is a little easier to rebuild because you use shims to set pinion bearing preload. In the 489, unless you buy the replacement shim adjustable crush sleeve, it can be a little tricky.
I have a 742 and 489 also I'm in the process of building. I believe some people said the 742 is better for manual trans. I'm building the 489 because my 742 is complete and my 489 is just an empty case. Oh yea and bigger is better.
With the 742 you don't have to spend an extra $30 on the solid pinion spacer/crush sleeve eliminator.
I'd do whichever cost less to have as a complete unit.
As far as strength... ALL 8 3/4's reduce to the same diameter at the pinion yoke. Short of busted teeth, that's the other place that EVERY one I've had let go broke. Nothing like banging the limeter while the driveshaft is exiting the transmission. )(/*
The cases you currently have is worth the value of the case, IMHO... $50 or so. If it has a preferred style yoke on it, maybe more. I see some real $$$ crazy fools asking $175-200+ for 489/742 open 3.23 cases and they've had them for YEARS!
The gear ratio selection is not a factor, I am set on a 3.73 gear. I am thinking of the Mancini aggressive style diff. Which is easier to set up myself, i want to do it myself this time?
Just noticed you said it was open. doh I mentioned the 2 piece side gear on the some/all??? 742 sure grips that can cause problems with aftermarket axles. I'll leave the reminder in case someone reading this may benefit.
The 742/741 axles spline got us once. '70 slant auto valiant green bearing 489 sg 3.23 We went to the track,my brotherinlaw put in my 741 3.91's SG. At the line something looked like it broke. The car would spin the driveshaft and the wheels didn't turn. We called for the 3.23 set and swapped everthing back to get home. The axles bumpers kept the axles out of the inner spline just alittle. He stripped the axles splines on the end just enough to not engage. doh **(*) remove the bumpers when installing green bearings. The 489 didn't care.
If I had both I would use the one in the best shape as they are both about the same in every way other then the 489 uses a spacer for pinion preload but that can be changed. Ron
Both 489 and 742 pinion are the same size at the smallest point. The taper bearing of the 489 was made for the rear end bango flexing. As it bends or there is alot of torque put on it the pinion will want to push its self off the ring gear out the front of the pot. This is why Ma mopar made this pot. IT today is still the best gear set because of this. You can still make the 742 take as much power but the amount of power to the ground will be different.
The 9" has a rear bearing support stopping this flex too.
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