I have a 1981 F-100 short box stepside truck with a 300-6; and a 4 speed overdrive manual transmission.
I bought this used and am happy with it except for when using it to tow my large 21' fiberglass boat with a 350 v-8. This is a heavy boat and the truck struggles when pulling up hill. Upon investigating I have found that I do not have a stock rear axle. By tach, and by tag, and by jacking up and counting drive shaft to wheel revs, I have found this to be a 2.47:1 . Not very good for towing at all.
It is however a 9" ford with the pumpkin (3rd member) removable from the front. I was wondering about pulling the drive axles, then changing the pumpkin with one from the wreckers with a more desirable ratio. I know that the 9" was used in trucks and cars over a wide number of years, and I don't even know for sure what my rear end originally came out of.
I am unable to read all of the tag I.D. from my axle but here is what I can. WDM DC 1_ _ 0
2 4 7 9 S _ 228
How safe am I to pick up any ford 9" pumkin at a wreckers that looks identical in housing with the same number of splines as mine, and have it work with mine?
Can anyone tell me what my rear end was out of?
The wreckers all ask what vehicle the rear end came from and I don't know , but I wonder if it matters if the spline number is the same.

Thankyou to anyone who can help.

 

you should be just fine picking out a 9" rearend from a junkyard as long as it has your same spline count. One thing to look out for is the difference in front "yokes" to accomodate different u-joint sizes. Also, yoke length can vary from rearend to rearend depending on the application. This is not a big deal though, as you can swap your existing yoke into whatever rear-gear you get.

 

Not positive, but I'm guessing that your 9" had 3.50's. One thing you have to be careful of, if you are 4wd, you need to change BOTH F&R gears. As to changing the rear 3rd member, you are correct. Make sure it is out of approx the same year of truck, splines are the same, and check the ring and pinion for tightness and that the teeth are not worn. It'll save you a lot of money.

 

For towing that boat, I think you would be real happy with a 3.50 gear. That is Ford's usual "Towing Package" ratio, and is a good compromise for highway mileage and low-end grunt for pulling back up the ramp at the end of the day.

Have a look at the mylar Certification decal on the driver's side door jamb--what code does it read? Post that alphanumeric code here;I betcha somebody can decode it for you.

Eddie