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Ford Mustang fastback GT390
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Discussion Starter #1
When I bought the car, some paperwork i got with it said that it was a 9” 370 posi.
can anyone tell me what the casting numbers relate to.
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cheers Mark.
 

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Casting numbers can't I.D. what is inside it.
It would have to be original, and have a metal Tag with stamped I.D. numbers to identify it. Since this is a 1972 casting # D2SW on the pinion. This is obviously not the original pinion to this car. So the entire rear is in question.
So you can either believe the prior owner. Or. Crack it open and see what you have.
 

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When I bought the car, some paperwork i got with it said that it was a 9” 370 posi.
can anyone tell me what the casting numbers relate to. View attachment 761088 View attachment 761089 View attachment 761090 View attachment 761091
cheers Mark.
It is a 9", It's easy to tell because the bottom bolts on the carrier are almost impossible to get off and even more impossible to torque to spec without a specialized adapter.. As for the internals, jack up the rear so that both tires are off the ground. Insure the gear box is in neutral. Rotate one of the tires by hand, if the other tire rotates the same direction, you have a limited slip differential. If the tires rotate in opposite directions, you have an open differential.
To determine the rear drive ratio, Place a mark on the tire side wall and another on the prop shaft. Count the revolutions of the prop shaft to cause one revolution of the tire. If the prop shaft rotates approxmately 3 3/4 revolutions to one for the tire, you have a 3.7 ratio
 
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2B28 tells us that the differential housing was cast on Feb 28 1962? 1972? 1982?. The D2SW narrows it down to 1972 or 1982. I think the 9" was still available in F150s in 1982 but it may have already been replaced.
 

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You can always "cheat" and do the following....

Rotate the pinion yoke. If both axles turn in the same direction then mark the pinion yoke and either axle flange at 12 O'Clock then rotate the pinion yoke, counting the number of revolutions it takes to get the axle flange to go around once. If the pinion goes around 3.7 times then you have a 3.70 ratio.

If only one axle turns or they go in opposite directions, use something to stop one side from turning, mark the other axle and pinion yoke as described above, but when you are finished counting the number of pinion revolutions multiply it times 2.
 

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Ford Mustang fastback GT390
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Discussion Starter #8
I’ve already checked the rotation of the wheels, they turn in opposite directions, so now I need to check the rotations of the wheel & shaft to determine what the gearing is.
So now ive Determined this is an open diff, just need to check the gears, is this a good or bad rear end & would this explain why I can’t get the rear wheels to light up, god I’ve tried, full throttle from a start the car just takes off without any spinning. ?
 

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1966 Mustang GT 4sp Nightmist Blue
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I’ve already checked the rotation of the wheels, they turn in opposite directions, so now I need to check the rotations of the wheel & shaft to determine what the gearing is.
So now ive Determined this is an open diff, just need to check the gears, is this a good or bad rear end & would this explain why I can’t get the rear wheels to light up, god I’ve tried, full throttle from a start the car just takes off without any spinning. ?
That’s interesting. I used to own a S code 67 with a C-6 completely stock and 3.00 gear/peg leg unit and could roast the tires at will(usually tire) from a stop up to about 20 mph. It was a torque monster. You should be able to spin at least one tire at will?

Chris
 

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I’ve already checked the rotation of the wheels, they turn in opposite directions, so now I need to check the rotations of the wheel & shaft to determine what the gearing is.
So now I've determined this is an open diff, just need to check the gears, is this a good or bad rear end & would this explain why I can’t get the rear wheels to light up, god I’ve tried, full throttle from a start the car just takes off without any spinning. ?
It's bad news. Even the garden-variety 289 2V auto trans puddle jumper could smoke the right tire. Heck, on a good day a 200 cid six-cylinder could do it.

The stone-stock 390GT I built for my Dad could smoke 'em both.
 

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Discussion Starter #11
Ok so what’s wrong, engine is a 351c c6 3s peed Auto, 2800 stall converter ?. Just in the process of putting in a Holley sniper & full Hyperspark ignition, so this weekend I’ll give it some wellie.
might need to wear lead shoes or learn how to drive like an American.
 
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