My 77 highboy has a locker rear-end. The numbers on the tag are 410 67a gfs on the top row and 66 371 12 L S on the bottom row (as near as I can determine because of rust etc..


Should I be using some sort of special gear oil in it?

Thanks

 

That's not a locker
If there is an L after the 4 in 4.10 it should look like this.
4L10

That's a Limited slip differential.
if it is just 4.10 that is an open diff.

All those numbers identify the plant,date of casting,bearing size & axle ratio.
That's it.

What code is stamped for the axle is on the rating plate on the door ?

 

That's right, if the L isn't like 4L10 then it's a open differintial. If you had a limited slip, then you can use a senthetic gear oil and it should be good, but they also sell some friction lube to put in with a 75W90 gear oil. Lockers don't need any special type of gear lube.

 

Well shoot. I thought the L on the bottom set of numbers meant it was a locker or limited slip.

Funny thing is when I jack it up and turn a wheel both wheels turn the same direction. If one wheel is left touching the ground the other wheel cannot be turned. That sounds like some sort of limited slip to me. I'll try to read the tag a little better there were some spots that were unreadable. If I recall the door axle code was B4.

 

B4 is 4.10 Limited slip

http://www.clubfte.com/users/mil1ion...ruckCodes.html
scroll to almost the bottom of the page

 

Thanks for clearing that up for me Mil1ion. I took a closer look at the tag and the area between 4 and 10 is definately blank but B4 is the axel code on the door so ????. But like I said it acts like a locker/limited slip. Are they actually the same thing?

 

No they aren't the same thing. A limited slip has clutches to allow the tires to slip, and doesn't have a complete lock or even power to rear wheels more like %60 %40 or something around that. A L S also doesn't make the other tire spin until one of the tires spin, not good for rocks, but good anough in mud. A locker locks the rear completally with %100 power to both wheels. They do have some lockers that allow the outside tire unlock in turns, I think these are mechanical lockers.

 

So.....back to my original question. Since I do have a limited slip, then I do need to use some friction modifiers .......Right?

 

That is correct. If you are doing a gear change, then add those friction modifiers. Your local parts store should have right off the shelf.

 

The friction modifiers cost a couple bucks for a small tube which is all you need. You can also run full senthetic gear oil that has the modifier already in it, that's what I did an I don't have any problems.

 

I just found out my new (to me) 1976 highboy has a limited slip diff. The tag on it has two lines. The first line says: 4.10 D6TW GBH The second line starts with the bill of materials number, which is 603717-10 and the second line follows that with a vertical iine that looks like a lower case "L" and the an S. Several sites show you how to identify your differential. For example Axle ID Charts AAM GM Ford Dana, Gear Ratio Chart, U-bolt Identification Chart From that, it appears that the last two markings on the second line of my tag are "L S" and that they are "included on limited slip assemblies only"

But from my research, it seems like the Bill of Materials number is key. On some (maybe all?), it's also stamped on the axle tube.

I found a site which lists all these bill of materials numbers (in a 252 page PDF!!) http://www.therangerstation.com/tech...om=80,-376,792

This list shows that my BOM is for a "trac lock" differential. The key for mine is the "-10" at the end of my BOM number. The same number ending in "-1" is standard type.

So I don't think it's true that a limited slip will always have an "L" in the middle of the ratio numbers (like for example, 4L10). My tag just says 4.10 for the ratio, but the Bill of Materials number tells the full story. I suggest finding your Bill of Materials number and then hunt it down on the PDF at the link above.

Makes me wonder if a lot of folks have limited slip ("trac lock" in Ford speak) and don't know it because their tag just shows the ratio without an "L" in it (like the "4.10" on mine)

And I guess the old test of jacking up one wheel and seeing if you can turn it while the other is on the ground - or lifting both and seeing if they both turn forward when you turn one forward is another way.....