How to identify what type rear end

[1968sublime]

Hello,
How do I tell what type of rear end is in my 1968 Roadrunner?

 

[HYRDGOON]

Passing on what I was told from another thread

look at the center section on the driver side right near the u joint. The casting number is pretty big and easy to read and ends in 741, 742 or 489. (there are a couple other lesser known ones but those 3 are 99% sure to be it.)

 

[1968sublime]

in decoded terms what do those # mean?

 

[HYRDGOON]

I'll let someone with more knowledge than me chime in with the right answer but from what I understand those are just the casting numbers for non-interchangeable internals that were essentially provided for the same cars (I.E. you can't take ring & pinion from a 489 and install in a 742) people feel the 741 in the weakest of the bunch but unless you race it with some big slicks, it don't matter at all.

The info I'm passing on is pretty much copy and paste from thread's where people helped me out so I'm sure someone with a more definitive answer will be along with more info.

Mike

 

[WP29440SE]

I'll let someone with more knowledge than me chime in with the right answer but from what I understand those are just the casting numbers for non-interchangeable internals that were essentially provided for the same cars (I.E. you can't take ring & pinion from a 489 and install in a 742) people feel the 741 in the weakest of the bunch but unless you race it with some big slicks, it don't matter at all.

The info I'm passing on is pretty much copy and paste from thread's where people helped me out so I'm sure someone with a more definitive answer will be along with more info.

Mike

that's assuming its an 8 3/4

otherwise ....

 

[HYRDGOON]

that's assuming its an 8 3/4

otherwise ....

True!! It didn't occur to me he may have a Dana! See that's why I said I'll leave it to the wise one's!

 

[1968sublime]

Awesome guys ! great information. I have the 8 3/4 rear end.
Now I have to figure out why it locks up when I role the car. It has set for 23 years and just fired it last weekend. Have not driven it yet but in pushing it around the rear axle locks up and wont turn. If I then role backwards it unlocks and will roll a little more. Any ideas whats going on?

 

[1969CoronetR/T]

I have the 8-3/4 in my R/T. Does anyone know how to tell what gears are in it without opening it? I think I have 4.10's in it as it screams like a banshee doing 60 MPH and 4000 RPM.
Mark

 

[HYRDGOON]

Awesome guys ! great information. I have the 8 3/4 rear end.
Now I have to figure out why it locks up when I role the car. It has set for 23 years and just fired it last weekend. Have not driven it yet but in pushing it around the rear axle locks up and wont turn. If I then role backwards it unlocks and will roll a little more. Any ideas whats going on?

Again I'm not a lot of help in diagnosing the problem but check out pages 7 & 8 of this restoration thread... another member was having problems with his diff so there is plenty of advice offered to what an issue might be...

http://www.forbbodiesonly.com/mopar...d-runner-restoration-don-t-laugh-its-paid-for

- - - Updated - - -

I have the 8-3/4 in my R/T. Does anyone know how to tell what gears are in it without opening it? I think I have 4.10's in it as it screams like a banshee doing 60 MPH and 4000 RPM.
Mark

I think to be certain you will need to open it up although not what they were intended for though you may be able to plug your numbers in to a calculator like this one and come up with a good idea

http://www.ringpinion.com/calculators/calc_rpm.aspx

 

[WP29440SE]

I have the 8-3/4 in my R/T. Does anyone know how to tell what gears are in it without opening it? I think I have 4.10's in it as it screams like a banshee doing 60 MPH and 4000 RPM.
Mark

my understanding is that you can jack the rear up off the ground, put it neutral , mark one of the u joint caps in relation to the carrier ,spin the wheel one full rotation , count the number of times the drive shaft turns.

approx. :

2 3/4 = 2.76
approx. 3 = 291

3 1/4 = 3.23
3 1/2 = 3.55
3 3/4 = 3.73
approx. 4 and some change = 4.10

you get the idea ..

 

[HT413]

I have the 8-3/4 in my R/T. Does anyone know how to tell what gears are in it without opening it? I think I have 4.10's in it as it screams like a banshee doing 60 MPH and 4000 RPM.
Mark

60mph at 4000 RPM's is really screaming. That gotta be at least 4.56's. How long have you had your car, do you think your converter is slipping? The way to test is to cruise at like 3000rpm and nail it. If the RPM's come way up without much acceleration, your converter is really loose. Or you can hold onto the brakes at a dead stop and gradually bring the RPM's up until the engine overwhelms the brakes.

- - - Updated - - -

my understanding is that you can jack the rear up off the ground, put it neutral , mark one of the u joint caps in relation to the carrier ,spin the wheel one full rotation , count the number of times the drive shaft turns.

approx. :

2 3/4 = 2.76
approx. 3 = 291

3 1/4 = 3.23
3 1/2 = 3.55
3 3/4 = 3.73
approx. 4 and some change = 4.10

you get the idea ..

Yup, though that only works with a limited slip. Open gears you need to do it the other way - put it in N then turn the drive shaft and see how much the wheels turn.

- - - Updated - - -

Awesome guys ! great information. I have the 8 3/4 rear end.
Now I have to figure out why it locks up when I role the car. It has set for 23 years and just fired it last weekend. Have not driven it yet but in pushing it around the rear axle locks up and wont turn. If I then role backwards it unlocks and will roll a little more. Any ideas whats going on?

Probably the brakes hanging up. Pull the drums and try again. Just put a drum on backwards so you can turn the axles easier.

 

[Mopars & Missiles]

I have the 8-3/4 in my R/T. Does anyone know how to tell what gears are in it without opening it? I think I have 4.10's in it as it screams like a banshee doing 60 MPH and 4000 RPM.
Mark

Mark, HT413 is right. You must have at least 4.56 gears to do 4k rpm at 60 mph.

I calculate that 4.10's would put you at 75 mph at 4000 rpm. (26" tall tires)
Also calculate that 4.56's would put you at 67-68 mph at 4000 rpm. (26" tall tires again) If your tires are significantly taller/smaller than 26", let me know and I'll recalculate with that information.

All it takes is to do the math throughout the driveline to tell what gears, or rpm, or mph, or tire size, etc. I have an excel spreadsheet set up for this purpose. Just plug in the knowns, and the unknowns are calculated.

I should add that my spreadsheet is set up assuming an automatic transmission. To calculate for a manual, I would need to make a minor adjustment to the equation.


Wow, just tried a couple more combinations for you. 4.86's put you right at 63.5 mph, BUT, 5.12's would be almost exactly 60.429 mph at 4000 rpm with 26" tall tires.

 

[1969CoronetR/T]

Sorry, I should have started a new thread. I try to stick around 3500 RPM when I take it to work on the highway since I owned the R/T (2009). They speedometer is going to be fixed this winter so I have been using an app on my iPhone and I will be going about 55 and to stay around 3500 RPM. If I take it to 60 MPH it will do 4000 RPM. It is a monster from red light to red light acceleration so I try to stick to back roads to save the excessive wear due to the high RPM's. Now, maybe my tachometer is wrong, but it screams at highway speeds as the little Prius blows by me doing 70 MPH. I had the automatic transmission rebuilt two years ago so I assume the converter is not loose and the engine is a 440 and the wheels are 26" tall and it has the sure-grip.

 

[HT413]

Sorry, I should have started a new thread. I try to stick around 3500 RPM when I take it to work on the highway since I owned the R/T (2009). They speedometer is going to be fixed this winter so I have been using an app on my iPhone and I will be going about 55 and to stay around 3500 RPM. If I take it to 60 MPH it will do 4000 RPM. It is a monster from red light to red light acceleration so I try to stick to back roads to save the excessive wear due to the high RPM's. Now, maybe my tachometer is wrong, but it screams at highway speeds as the little Prius blows by me doing 70 MPH. I had the automatic transmission rebuilt two years ago so I assume the converter is not loose and the engine is a 440 and the wheels are 26" tall and it has the sure-grip.

Well, if the tach is right and the app is right, and those really are 26" tall tires, looks like someone stuffed 5.17's in that diff. Lol, makes my 4.10's look like granny gears.

 

[1969CoronetR/T]

I will be swapping in a new alternator this weekend and I will ask my uncle's friend, who is a mechanic, to check the gears. I like the acceleration, but top speed sucks. Thanks for all hte help and will let you guys know this Sunday. Have a great Thanksgiving.
Mark