73 rear end
Drifting
Joined: Jan 2002
Posts: 1,645
Likes: 11
From: Schaumburg IL
There are several ways.
1. There may be a tag on the read by the filler bolt on the right side above the 1/2 shaft that listed either a ratio or p/n to cross in the manual of chevy by the numbers. Or go to Mortec.com and they usually have charts of things like that.
2. The build sheet for the car if you have it.
3. Hard way - jack up the read end, block the front !!! wheels, turn over a tire and count 1 revolution to how many the drive shaft turns over and divide it out.
4. Pull rear end cover (maybe a good idea to change the lube after all these years) and count the teeth on the large ring gear to the small drive gear that is fed by the driveshaft and divide out the ratio.
Or most manuals had 3:73:1 maybe a 3:55:1 and I believe all 73's came with limited slip read ends.
1. There may be a tag on the read by the filler bolt on the right side above the 1/2 shaft that listed either a ratio or p/n to cross in the manual of chevy by the numbers. Or go to Mortec.com and they usually have charts of things like that.
2. The build sheet for the car if you have it.
3. Hard way - jack up the read end, block the front !!! wheels, turn over a tire and count 1 revolution to how many the drive shaft turns over and divide it out.
4. Pull rear end cover (maybe a good idea to change the lube after all these years) and count the teeth on the large ring gear to the small drive gear that is fed by the driveshaft and divide out the ratio.
Or most manuals had 3:73:1 maybe a 3:55:1 and I believe all 73's came with limited slip read ends.
Le Mans Master
Joined: Sep 2003
Posts: 8,812
Likes: 2,029
From: eastern NC
First, I would recommend you jack the rear of the car up with both rear tires hanging free. Place the car in gear or park if it's an automatic.
To determine if you have a posi, rotate one wheel while watching the opposite wheel. If the opposite wheel rotates in the opposite direction, you have an open rear. If it does not want to rotate or resists rotating you have a posi.
To determine the ratio, place a chalk mark on the tire, preferably at the bottom. Mark the drive shaft and then with the car in neutral, rotate the drive shaft by hand and count the number of rotations it takes to make the tire rotate one full revolution. 3.5 drive shaft rotations to one tire rotation equals a 3:50 rear end ratio, 3.75 rotations equals a 3:73 rear gear... etc. This will work if the rear is a posi. If it is an open rear the tires may try to rotate in opposite directions, so you will need someone to help keep the tires rotating in the same direction.
With the rear wheels dangling, you may get some binding in the halfshaft u-joints. They will rotate with some minor binding, but not enough to hurt anything for the purpose of the test.
Good luck... GUSTO
To determine if you have a posi, rotate one wheel while watching the opposite wheel. If the opposite wheel rotates in the opposite direction, you have an open rear. If it does not want to rotate or resists rotating you have a posi.
To determine the ratio, place a chalk mark on the tire, preferably at the bottom. Mark the drive shaft and then with the car in neutral, rotate the drive shaft by hand and count the number of rotations it takes to make the tire rotate one full revolution. 3.5 drive shaft rotations to one tire rotation equals a 3:50 rear end ratio, 3.75 rotations equals a 3:73 rear gear... etc. This will work if the rear is a posi. If it is an open rear the tires may try to rotate in opposite directions, so you will need someone to help keep the tires rotating in the same direction.
With the rear wheels dangling, you may get some binding in the halfshaft u-joints. They will rotate with some minor binding, but not enough to hurt anything for the purpose of the test.
Good luck... GUSTO
Le Mans Master
Joined: Oct 2000
Posts: 6,362
Likes: 30
From: Oak Harbor WA
The easy way is to drive the car at exactly 60 mph in the final drive ratio which will be 1:1. Mark down the resulting RPM and then look up the numbers by typing in the required info at sites that have this....Randy's Ring and Pinion I think has this info, but there are many others. You will also need the tire height but that can be gotten from Tire Rack or the website that manufactures the tire.
The build sheet will have this info it came from from the factory but that doesn't guarantee those are the gears you have installed.
The build sheet will have this info it came from from the factory but that doesn't guarantee those are the gears you have installed.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
All '73s had a posi rear end. Posi and limited slip are the same thing, just two different names. The date of manufacture and ratio are stamped on the bottom of the diff unit at the mating flange with the rear cover. Many units are very corroded, so it might take some scraping to see the code.
