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Discussion Starter #1
I've got a weird issue with the wife's 66... the rear axle pinion seal was leaking badly so I replaced it. It has a 7.25" rear axle (I6), which according to the parts listing uses the same seal as an 8" rear end. That's all good and well, but I took it for a test drive today and it flung gear oil EVERYWHERE! I just finished pulling the pinion yoke back out and the seal looks perfect, as does the sealing surface on the yoke. I took out the trusty calipers and measured the yoke OD as ~1.65"... the seal is 1.81" ID (the rubber bit)! Well crap, no wonder it leaked so bad!!!

I can't seem to find any seal dimensions that give me something other than the 1.81" ID. I even checked the axle tag and it is indeed a 7.25" axle (3.20:1 ratio). Where do I find a pinion seal for this thing? Anyone got a part number?
 

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apparently it depends on which U joint you have as indicated on pg 201 of the npd online catalog;

2017 65-73 Mustang Catalog

it shows two different part number for the pinion oil seal

4676-3 with the 110 series u joint
4676-1 without the 1100 series u joint. this one is the same as the eight inch rear end.

my uncle had this same issue a number of years ago when he had the rear end resealed in my 64 falcon.
 

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I just replaced mine on my early '67 7.25" rear. It was NAPA part SKF 18100.
https://www.napaonline.com/en/p/NOS18100

Not sure why the web site lists it as $22. My receipt says $11.

If you want to hold one in your hand, it may be as close as your local NAPA.

I will say though that the new one is substantially lighter weight than the original and easy to bend out of shape. Though that could just be me and my hammer heavy style of seal installation. To get it installed evenly I ended up trimming down a 2" PVC end cap to use as a seal installer, though I'm sure there's a proper tool for that out there somewhere.
 

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Discussion Starter #7 (Edited)
Ok, finally got it sorted. The new seal is the correct diameter and no more leaks. Below is a pic of the two seals side by side (7457N on the left and 8181N on the right) and the wrong seal slipped over the yoke to show the gap.

A word of advice; measure your yoke before you buy a pinion seal!
 

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Since the determining factor is which u-joint (does that then change the yoke?) the car was equipped with, is there a distinct visual or easily measurable difference between the two? It would be nice to have the parts on hand before you take it apart. I've seen the parts listed in other catalogs as well, again with the u-joint, not the date of manufacture, as the determining factor.
 

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PS - Was the outside diameter of the seals the same? If not, is the whole axle housing different, not just the yoke?
 

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Thanks Woodchuck. In a parts catalog I see the front u-joint has two varieties as well. Are there two different yokes up there too then?
 

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I wasn't planning on it now, (but you never know with a car this old) but if I was to pick up a spare driveshaft assembly from the junkyard before its gone, I better mind my business.
 

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"The housings are no different. The OD of the seals are the same. The ID is different to match the yoke."

Woodchuck is right... The U-joints have absolutely nothing to do with it...

It all has to do with the size of the yoke which will then determine the seal in the end..

:eek:)

Tony K.
 

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But will the u-joints be the visual "tell" as to which pinion seal you need then? I see on another parts list "inside" and "outside" clips for u-joint varieties, must be the snap rings. Are they different enough you can tell from under the car, despite the decades of dirt?
 

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But will the u-joints be the visual "tell" as to which pinion seal you need then? I see on another parts list "inside" and "outside" clips for u-joint varieties, must be the snap rings. Are they different enough you can tell from under the car, despite the decades of dirt?
Best thing would be to bring your cheap vernier caliper with you when you go...
 

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The difference is whether the transmission is a C4 automatic or 2.77 3-speed. The 3-speed uses tiny U joints while the C4 (except for early models) uses the larger V8 style U joints.
 
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