Big Block Dart Forums banner
1 - 11 of 11 Posts
G

·
Discussion Starter · #1 ·
This is really a newbie question, but I need help telling if I have a 742 or a 489 casting. My problem is pretty simple, I need better highway gears as I will never see the track and only use my Dart for a cruiser. Using the Richmond Gear rear ratio calculator, it says I have around a 4.56 in the rear. That would explain my 45mph @ 2500 rpm readings, but I need way different gears for my purposes. I know it can be hard to get anything higher than 3.55s, so I may end up running taller rear tires in addition to the gear change. Currently I am running 215/75/14s in the rear. Any hints on how to identify the rear would be helpful. Thanks!!
 

·
Registered
Joined
·
47 Posts
DRoCk,

From the Tech section,

8-3/4" Axle Center Section Types
The 8-3/4" axle was available in three basic types. The types are differentiated by the pinion stem diameter....1-3/8", 1-3/4", 1-7/8". The choice of axle pinion assembly was determined based on the application.

1-3/8" small stem pinion... (aka. '741')...
Carrier casting numbers: 1820657 (1957-1964), 2070741 (1964-1972). This assembly was typically used in low weight/low horsepower applications through low weight/medium horsepower and high weight/low horsepower applications. Pinion depth and bearing preload is set with shims. Differential bearing setting (ie. backlash ) is set with threaded adjusters.

1-3/4" large stem pinion... (aka. '742')...
Carrier casting numbers: 1634985 (1957-1964), 2070742 (~1961-1969). This assembly was replaced by a phase-in of the 1-7/8" pinion starting in the 1969 model year. 1970 RW (Plymouth and Dodge mid-size) were the last models to use the 1-3/4" which appeared in a 2881489 case. This assembly was typically used in high weight/medium horsepower applications through high weight/high horsepower applications. Pinion depth and bearing preload is set with shims. Differential bearing setting (ie. backlash ) is set with threaded adjusters.

1-7/8" tapered stem pinion... (aka. '489')...
Carrier casting numbers: 2881488, 2881489 (1969-1974). This assembly was introduced in 1969 and was phased-in to relace the 1-3/4" unit through 1970. Note: the 1-3/4" pinion also appeared in some '489' carriers during this period. By 1973, the '489' was the only unit available in passenger car applications. This assembly was typically used in high weight/medium horsepower applications through high weight/high horsepower applications. Pinion depth is set with shims, preload is set with a crush sleeve. Differential bearing setting (ie. backlash ) is set with threaded adjusters.

All 8-3/4" carrier assemblies can be identified externally by the casting numbers. Additionally, the '741' commonly has a large X cast on the left side, the '742' may have a large 2 cast on the left side, and the '489' has a large 9 cast on the left side. Through 1965, the factory ratio was stamped on the identification boss, followed by an 'S' if Sure Grip equipped. After 1965, a tag was affixed under one of the carrier mounting nuts to identify the ratio. If Sure Grip equipped, an additional Sure Grip lube tag was sometimes affixed; later years sometimes had the filler plug painted orange.


hope this helps.
68Valiant
 

·
Registered
Joined
·
50 Posts
The common ratio for the 8-3/4 was 3.23 unless special order. You should be able to find them at scrap yards or many racers will have ones that came out of their axles. Go to the barter/trade section and I'll bet you can find some for your 4.56 unit.
 
G

·
Discussion Starter · #5 ·
Thanks for the replies, Guess I should have checked the tech section. doh brrrrrt I'll check it out to make sure of what it is and I will get back to you all. I've been in "house remodelling mode" lately and my GF says that takes priority.
 

·
Registered
Joined
·
129 Posts
As far as the case # goes , look on the side of the case and it will say what # it is. As far as ratio just count the cogs on the ring then the cogs on the yoke/pinion shaft and divide one into the other. Bingo..... the ratio then will be known.
 
G

·
Discussion Starter · #8 ·
Drove it again today to check out my new tach and I think the more I drive it, the more I like my "mystery gears". Still not sure whats in the rear, but I do know its got a spool and it actually hooks up pretty darn good. Think I'm going to leave it that way for a while, its great in town and still does ok at about 70 for short distances.
 

·
Registered
Joined
·
392 Posts
put a floor jack under the rear end and lift it up, put a piece of white tape on the pinion yoke or d-shaft, put another piece of tape on the tire. Turn the tire one revolution while counting how many times the yoke turns, you can get a pretty close idea of the ratio.

use jack stands for safety..of course.




mike.
 

·
Registered
Joined
·
58 Posts
If you are still looking for an economical set of highway gears, I have a set of 2:94 gears.They are great for fuel economy.
 
G

·
Discussion Starter · #11 ·
Thanks for the offer, but I think I am going to stay with my current set up for now. I'm really starting to like the way it drives around town, highway driving isn't going to happen too much for now, probably should worry about my rusting trunk floor and gas tank supports that I noticed today. First things first right?
 
1 - 11 of 11 Posts
Top