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Discussion Starter · #1 ·
Yes this is farmall1206 - and i was wondering if someone could help me!!!!
I have a Ford Rear end that has no tags or markings of any kind on it!!! Can you tell how tell for sure if it;s a Posi rear end and how to tell the gear ratio in it???

I got it in trade and the fellow told me it was a Posi but never told me gear ratio or anything
I'm looking to sell it so it would be good to know the above details!!!

Thank you
 

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Automotive Extraordinaire
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To the best of my knowledge ford never made a factory true "posi" rear end. It shoudl either be a limited slip or an open differential. All you have to do is get both sides off the ground and turn one side. If the other side goes in the same direction it is either a factory limited slip or possibly has a spool installed or the spider gears welded. if the other side turns the opposite direction it is an open differential or a limited slip with worn out clutches. As for the gear ratio, turn one wheel, one full round and count the number of times the pinion shaft (where the drive shaft bolt on) turns. That will get you close enough you can probably figure out what gear it is. Do you know what rear end it is? 8", 9", 8.7, 7.5, 8.8? post a picture that will prob help too. Also if the tag is missing someone has most likely been inside of the differential at some point and left it off. Hope this helps some. I'm sure someone more knowledgeable can give you something that makes more sense. Best of luck to you.

Kelly

By the way welcome to the forum
 

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WFO
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Yes this is farmall1206 - and i was wondering if someone could help me!!!!
I have a Ford Rear end that has no tags or markings of any kind on it!!! Can you tell how tell for sure if it;s a Posi rear end and how to tell the gear ratio in it???

I got it in trade and the fellow told me it was a Posi but never told me gear ratio or anything
I'm looking to sell it so it would be good to know the above details!!!

Thank you
Open or posi
Gear ratio check
 

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High Performance Rear Ends
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3,782 Posts
There is only one way to know for sure what you have, and that is to remove the cover or remove the chunk, depending on which Ford rear end you have, and look inside. ALL OTHER METHODS ARE UNRELIABLE. You can spin the alxes all you want, but you can be easily fooled by this method. Worn clutches, galded shaft, bent axle tubes and other things can cause a false reading. The only way to know for sure is to look inside.
 

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The term Posi refers to GM Positraction, so technically it was never installed on a a Ford. Gov Lock is also a GM-specific limited slip rear end.

Ford used Traction-Lok for most applications, and for a few cars you could order a Detroit Locker. I've owned cars with Traction Lok, but I don't know how it differs technically from Positraction. I'm sure you can find some articles out there on Traction Lok and then pull the cover and find out if that is what you have.

The Detroit Locker was very uncommon in Fords, and usually only available with 4.30 gear ratio for Mustangs and Torinos.

Bruce
 

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High Performance Rear Ends
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Actually Posi is what Eaton calls their clutch type limited slip differential that was used in the Chevy 8.2 10 bolt and 12 bolt in the '60s and early '70s. Eaton has a trade mark on the name Posi. The name was so popular that people started using it for all limited slip differentials, the way Asprin was used for pain killers and Roller Blade is used for inline skates. Governor Lock is also a Eaton differential that is used in GM trucks.

The 9 inch Ford Trac Lock is also a clutch type limited slip differential. Most of them have 4 pinion gears, but the early ones had 2 pinion gears. They use a clutch stack in the left side of the differential and a block with 4 small coil springs between the axle gears for preload. The clutch plates are lined with an organic friction material and are the same as the C4 transmission clutch plates, with the exception of the plate that goes against the clutch hub. The axle splines on the left side are split between the axle gear and the clutch hub. The differential cover on the left side is weak and is usually the first thing to fail. I've seen many of these that were broke by stock engines with automatic transmissions.
 

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Carolina's method has worked for me for years at least in ballpark ,you can tell a 4.56 from a 3.00 mabey not a 308 /300 also early Fords mostly wagons were marked between the reinforcing ribs a large P or an N the n meant nodular iron,which is much stronger ,most but noy all marked with a p were limited slip,ddont know how yo id Detroit locker fron outside used in late sixties early 70s probably wont find one of these anyway also a 9 in you cannot get a socket an bottom bolt the others you can
 

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High Performance Rear Ends
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Most, but not all 9 inch Ford nodular cases were marked with a N directly above the pinion. The P cases are not 9 inch Ford cases, but '55-'64 Chevy cases. The P cases were made with one of the braces inside deleted so that the limited slip differential would fit in them. These were used mostly in Corvettes, but also in the high performance full size Chevys from '55 to '64.

Ford put the gear ratio, differential type and ring gear size on the tag. If you had a 9 inch Ford with a 3.25 gear and the Trac Lock differentail the tag would say 3L.25 9 xxx on the bottom line. If there was no L in the gear ratio then it was a standard differential. If it was a 8 inch rear end instead of the 9 inch then it would have 8 instead of 9.
 
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