I got a lead on 2 different 8/34's One is a 742 case and the other is a 489. Which or is both re buildable and which would be a better diff ? Cost wise, rebuilding if need be which would be the better choice ? Not familiar with the 8/34. Thanks.
Recommended 8/34
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mischiefdart
Well-Known Member
I would go with the 742. The rear pinion bearing is more robust that the 489. Both are equally rebuildable with good selection of gear ratios. The 741 has limited gears available.
Thanks for the reply. I have 3 center sections but are 741 case, ratios are 2.94, 355, and 391 all open. I have considered swapping the gears, say the 355's over into another case that I was looking at and using the sure grip unit. Both of the other cases have really low gears, and I'm not real keen on twisting the motor on street cruises. I figure the 355' may be kind of ideal for both purposes behind a stock motor. I did find some info on those 2 diffs. but I wasn't sure which would be the better of the two and easiest to rebuild if needed. I'm a novice so I'm not familiar with those cases.
grimreaper
Cross Member
The 741, 742, & 489 all require different gear sets due to the pinions so swapping the gear set from one of your 741s is not an option if that is what you are considering. There is an ongoing argument as to whether either the 742 or 489 housings is stronger. The 741 housing is the weakest of the three but is still no slouch. They hold up to a lot of hammering and for a street car that will not be launched with slicks will probably hold up very well.
Start with this info. http://www.bigblockdart.com/techpages/875axle.shtml
Start with this info. http://www.bigblockdart.com/techpages/875axle.shtml
Thanks, that was my intension. So if I go with either the 742 or the 489 case, can the sure grip work with the 741 case as you and my research has shown that the 741 is no slouch? I have considered using a DOT type tire or a set of slicks on different wheels and just switch wheels when I want to do some track time. I have an 8/14 in it now, but also have an A body 8/34 and the shorter B body rear as well, that I could swap in, and only change the perches, or swap in the A body, but then purchase aftermarket axles. I have thought of another option to. Installing the A body rear and using wheel adapters that came on the rear for the big bolt pattern wheels I have then when wanting to go to the track, just remove the b bolt wheels and adapters and install small bolt pattern wheels with whatever type of tires I decide on. slicks or DOT. I guess I have a few options to go with. I figured it up and to beef up an open 8/14 vs an 8/34 it doesn't seem to cost effective since I have the 2 housings and gear selections. Thanks, I'm glad you cleared that up on the gear swap.
grimreaper
Cross Member
Sure grip units will interchange across the 8-3/4s so you can swap one of the others into your 741 housing. If you are planning to do much on slicks you probably better figure on either the 742 or 489. If only an occasional blast on slicks you may get by on the 741. (Some people make them last for a lot of 1/4 mile passes with no problem but you know the old saying, better safe than sorry...) 4spd or auto will also make a difference... As previously mentioned, gear sets are readily available for the 742 or 489 and the bearing sets are all "approximately" the same price. So the net difference you would have in 742 or 489 vs 741 would be a couple hundred bucks for a higher ratio gear set. It would depend on how much abuse you actually plan to subject on the assembly .
The 489 has the largest pinion at 1-7/8" vs. 1-3/4" for the 742. The 489 replaced the 742 starting in the 1969 model year. The 741 is the weakest of the three and I think it goes back to at least the early 60's. I think the typical 741 used the smallest u-joint also.
If it was me, I'd use the 742 simply because the sure grip unit is more easily re-buildable as it uses replaceable clutch discs. The 489 uses a cone-type clutch that eventually wears out thereby losing its locking ability so you're back to a one legger. I know someone on here rebuilds the 489 by machining and using shims, etc. But if you have both on hand, I'd go 742 just because the clutch plates are still available (try Mancini's for instance).
The B-body 8-3/4 is definitely wider than an A-body axle. And there were different width B-bod rears depending on the vintage. You can make up the difference by re-welding the perches and carefully choosing the rear wheel backspace. I know some guys just spread the springs apart where they meet the perches but I personally wouldn't do it. If you already have brakes with the B-bod then you are good to go (or stop as it were). IIRC they originally used a B-body rear in the 68 SS Hemi A-bodies with the offset spring kit. You can always sell your a-body rear to help pay for some of this.
Or use the A-bod and replace the axles. I had C-body axles shortened and splined by Mosers for approx $100 plus the 35 I originally paid for them. But then you have to re-do the rear brakes. I used backing plates, drums, hardware from a B-body (10" x 2-1/2")
The various center sections swap into the housings no problem. 741-742-489. I think you mentioned using SBP to BBP wheel adaptors when switching to racing tires which are going to exert a greater force on everything? Uh....no.
The wheel selection in BBP is much greater than what is available in SBP.
If it was me I would convert to the 4-1/2" pattern now so you don't have to screw around with adaptors, stacks of SBP, BBP wheels, etc. Just do it once and don't look back.
Oh yeah, at one time the factory recommended the 3.55 ratio as being the best match for a 340 on a new car but that's your call.
If it was me, I'd use the 742 simply because the sure grip unit is more easily re-buildable as it uses replaceable clutch discs. The 489 uses a cone-type clutch that eventually wears out thereby losing its locking ability so you're back to a one legger. I know someone on here rebuilds the 489 by machining and using shims, etc. But if you have both on hand, I'd go 742 just because the clutch plates are still available (try Mancini's for instance).
The B-body 8-3/4 is definitely wider than an A-body axle. And there were different width B-bod rears depending on the vintage. You can make up the difference by re-welding the perches and carefully choosing the rear wheel backspace. I know some guys just spread the springs apart where they meet the perches but I personally wouldn't do it. If you already have brakes with the B-bod then you are good to go (or stop as it were). IIRC they originally used a B-body rear in the 68 SS Hemi A-bodies with the offset spring kit. You can always sell your a-body rear to help pay for some of this.
Or use the A-bod and replace the axles. I had C-body axles shortened and splined by Mosers for approx $100 plus the 35 I originally paid for them. But then you have to re-do the rear brakes. I used backing plates, drums, hardware from a B-body (10" x 2-1/2")
The various center sections swap into the housings no problem. 741-742-489. I think you mentioned using SBP to BBP wheel adaptors when switching to racing tires which are going to exert a greater force on everything? Uh....no.
The wheel selection in BBP is much greater than what is available in SBP.
If it was me I would convert to the 4-1/2" pattern now so you don't have to screw around with adaptors, stacks of SBP, BBP wheels, etc. Just do it once and don't look back.
Oh yeah, at one time the factory recommended the 3.55 ratio as being the best match for a 340 on a new car but that's your call.
rbdart
Well-Known Member
The sure grip will interchange in all 3 cases,the gears themselves are by case only,be very careful running those adapters I have shredded a couple sets in the past,put the a-body rear in and redrill the axles to big bolt pattern and you will have an axle with 2 patterns,there is a fella on here that has a set for sale already to go...unless your pushing 500+ hp /torque then run the 741's they will take alot more than you think,good springs and bracing the housing/tying frame rails will make any 3rd member live alot longer life,even the 8 1/4 was used for testing by Ma Mopar in a 400 low comp bblk duster with 4.30's,10''slick ran mid to hi 11's,the R/T Dakotas have 8 1/4 rears and they are heavy trks,had one so I know how tough they are,never broke any of them and trk had a fresh 360mag slightly modified ran 13.60's...you can break anything thats already worn out to begin with...
Car in question is a 75 dart sport 360. All stock, I have thought of pulling the stock cam and putting in a small grind for some lope. A few years ago I had a 340 with the purple shaft in it. I believe it was 430 on the intake side and 450 on the exhaust at .50 lift. Was a good torque cam with some lope. Moved my 73 roadrunner pretty good with 3.23's or the 3.55's. This 360 is all stock and a smogger engine and low compression, cat exempt with stock duals. Not looking to go fast, just consistant. I have read about the re drilling of axles, but will have to do some research on the strength issues. No I would never race with the whell adapters. They came with the A body rear I got, and was kind of skeptical on using them on the street, but never heard of any failures. I believe they are trans dapt or something like that. Not sure as the rear diff is in storage right now. Thanks for the info on the re drilled axles listed, I will check them out. I'm actually not sure if any local machine shops would do the re-drill because of liabilities. Shame things aren't what they used to be. Was chatting with an old mopar guy about the 741 case and he said back in the 60's that's what mostly was run on the strips, even when they did the pushbutton neutral drops. I agreed but I believe that traction may not have been as good back then as now with the compounds of the tires and the track surface.
dodge71demon
Well-Known Member
I'm going to purchase a new Strange Dana 60 once you put the figures together It's just as cheap if not cheaper to go this route rather than to buy and rebuild an 8 3/4 espically if your upgrading to big bolt pattern. Quoted $1820 no shipping charge from local Napa without brakes I'll be using disc brakes
Been doing some figuring. If I get aftermarket big bolt axles from Dr.Diff. Make a deal on the 742 case with sure grip. Swap out the B body backing plates and brakes, drums onto the A body housing I should be well under a grand, but you gave me something to think about. I may just put the 8/14 up for sale to recoup some of my money and should come out even less. Good winter project.
What, maybe a 12 second car? Why spend money on a case you don't need. Run the 741's you have and spend the money that you'd be wasting on a 489/742 case on anything worthwhile.
Find a sure grip to swap into any of the current 741 geared cases you have and run it. The 3.91's are a bunch of fun if you don't plan to freeway drive much with stock height tires.
Find a sure grip to swap into any of the current 741 geared cases you have and run it. The 3.91's are a bunch of fun if you don't plan to freeway drive much with stock height tires.
Dartnut
Don't hate me because i'm beautiful
If you decide to go with the 489, make sure you get the crush sleeve eliminator.
Worth every penny.
Worth every penny.
dodge71demon
Well-Known Member
Just to clarify my statement on the Dana that would be if you started from scratch on an 8 3/4 and replaced everything using the 489 and going big bolt pattern - Housing and third member - Auburn sure grip - ring/pinion - axles - bearings etc (been there done that) do you need the Dana probably not but $ for $ it's the wow factor
I'm not sure it would even make it the 12's. I would think in stock form 15's or so. I'm going off 1/4 times from write ups from back in the day. These center sections from my research seem to be a decent deal. Even if I don't use the gear set I will still have a better case and a sure grip, which I could use in one of my 741 cases that I have, 294,355,391. At this point I feel sure I'm going to go through the A body rear and install it and just transfer the B body backing plates/ brakes and drums onto the A body rear and get aftermarket axles. Appears to be the lowest cost effective solution, with all of my options. I may even swap out the stock convertor and install the dynamic one I have that came out of my mild bracket car, which I 'm sure will pick up the car's et's.
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