Rear End Questions

[ejnonamaker]

Okay. It's cold, snowy, and windy here in Weirton, WV today. As I continue to piece together what I have and what I need I decided to lift the rear end of the car up today to see if I still had the 2.75 limited slip rear end. This is on a 73 Mach1, 351C 2V, FMX trans.

Here's what I found. For one full rotation of the tire I get about 1.4 of a rotation on the yoke for the driveshaft. So I dismissed this as original tires for this car where about a 205 and I have 225/70/14. I couldn't find a tag on the rear end. Not sure where to look honestly. Am I correct that this is a 2.75 rear end? I seems to me that it shouldn't be that far off or am I suppose to count a half rotation of the yoke and not a full rotation? I at least know it's limited slip.

So I'm also use to perhaps a newer style 9" rear end from the F-150's that have the drain plug on the bottom, a fill plug at the top, and a removable plate around the differential case that face the bumper. The only bolts on this rear end are on the forward side of it facing the driveshaft. I think that is correct. I'm guessing if I slide up underneath further I'd find that these are the bolts I need to undo to remove to drain the rear end fluid, change out the seal, and refill. I recall refilling a rear end once with a funnel and a long tube down the axle casing from inside the wheel well when I was a youngster around age 5-7 on a '67 or '70 Mustang.

Needless to say I'm probably all set up for great full economy and highway speeds. This car was originally shipped to Jacksonville, FL. So nothing but flat cruising down there. Especially with the 2V carb.

So the journey continues if I'm correct on all the above. Now it's a 2V to 4V intake, a 4V carb, long headers, mild cam, and upgrading the rear end to probably atleast a 3.5, maybe a little higher. I'm mostly looking to still get some good economy but what a little bit more umps off the line and highway speeds. This is just going to be a daily driver for weekend trips and cruising around town when it's done.

Since I have you all here. I'm use to working on newer cars where when you swap out a rear end you know need to change the shift points in an automatic transmission and recalibrate the speedo. I'm guessing this isn't something that is needed on our 71-73's except changing out the gear on the speedo cable.

Thanks!

 

[barnett468]

one method is to jack both wheels up.

put the right rear valve stem at the bottom or top.

put a mark with white out on the driveshaft.

if you have a posi, turn the wheel clockwise one full revolution and count the driveshaft turns.

if it is not a posi, turn the wheel two revolutions and count.

 

[ejnonamaker]

That's where I'm getting confused I guess. I'm doing just that and one turn of the tire is yielding 1.4 turns of the driveshaft.

 

[73pony]

My guess is you have an open 2.75.

 

[basstrix]

Here's how it works: If you have a posi, both wheels have to be up in the air and both will turn at the same time (if it's not totally worn out) while you're rotating one of the wheels.

If you only have one wheel jacked up, you're reducing the gear ratio by a factor of 2 due to the side gears and spider gears. So when you have one wheel jacked up, you have to rotate one full turn for the tire you're turning and another full turn for the tire that's not turning.

I'm guessing that you're only jacking up one side or only one wheel is turning. If that's the case then either you have a worn out posi or you have an open differential.

 

[ejnonamaker]

Both wheels are turning and off the ground. They turn opposite of each other too. That's why I came here because something doesn't seem right.

 

[rocket366]

Normal for non-posi open differential.

Count how many turns of the driveshaft makes the tire go one turn.

Paul

 

[Mikes73]

By spinning opposite of each other means you have an open differential. I switched from an open to a ford racing traction lok and can't be happier. This thing hooks up goood.

 

[73pony]

Turning different directions it is open not posi. Turn one wheel two full revolutions and see how many revolutions the driveshaft makes. By your earlier post of one revolution of tire equal to 1.4 of the shaft two should be around 2.8 or an open 2.75 rear end. Same thing I have in mine.

 

[Mister 4x4]

Check your door sticker for the Axle Code. If it says "2", then you should have an open-diff 2.75 gear set. If it says "3," then 2.79 gears.

This is all based on what the car came with from the factory, and if your car still has the stock axle and original door, that is.

Check the data sheet on the site for more information about your car here: http://www.7173mustangs.com/misc.php?page=1973

 

[71XR7VERT]

Nice feature to have a drain plug. Mine, and most I think cars came without. I added mine when I rebuilt mine because I don't want to pull the shafts and diff just to change the lube.

Have to do that 500 miles after rebuild.

 

[ejnonamaker]

Thanks all. I think in reading your posts I figured some knowledge I was lacking. Just like GM guys call it posi and Ford guys call it limited slip, I thought open was the same thing as posi/limited.

So I have another rear end. With both axles off the ground the opposite side starts to turn and then stops about halfway through the rotation of the tire. It rotates the opposite direction when it does turn. If you stop turning the wheel and start again the opposite side does the same thing. Starts going the opposite direction for aboit half a rotation and then stops. What type of rear end is this? Is this another open rear diff but it's worn out? It says it's a 3.5 rear end. If we go with rotation count it works out well that it's an open 3.5. It turns aboit 1.75 times per tire rotation.

 

[73pony]

That sounds like a 3.5 open. The other tire may be stopping due to a brake hanging up slightly. It is not difficult to stop the other tire from moving backwards. You can have some just hold onto it to keep it from turning at all. The fact that it spins part way then stops each time tells me a drum isn't true or hanging in a certain spot.

 

[basstrix]

With an open differential, it's easier to just prevent the opposite wheel from turning and go through 2 full rotations. Either both wheels need to rotate 1 full turn in the same direction or one wheel only rotate 2 full turns.

I think GM calls their traction devise a posi-track while Ford called theirs either Equa Lock or Traction Loc.

 

[ejnonamaker]

[emoji2]

 

[ejnonamaker]

Replacing all the gaskets & seals and servicing the rear end was on the itinerary this summer/spring. Just curious what would cause this? My guess is the pinon seal.

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