Wrong Forum but need answer.....Axle Question....

[rusty ol ranger]

I got a 79 F150 4x4 with a 9 inch rear axle. The poor axle is cooked and i got a buddy with one for sale out of a 81 F150. Its a 9 inch.

My question is...

1- Will the alxe from the 81 Bolt in a 79?

2- Mine is 4wd, and has a 3.50 axle ratio (according to tag) he said his is a 3.55....I know on a 4x4 the front and rear need to be the same...but is this a big enough difference to worry about?

later,
Dustin

 

[heptofite]

yes, it's a big enough difference to worry about, any difference is a big enough difference to worry about.

 

[Mac]

I'd sure worry about the width also. A 3.50 is a 8-9" ratio while the 3.55 is a 8.8 ratio. Spline count? Too many things to be careful of.
Dave

 

[rusty ol ranger]

He told me its a 9 inch, so maybe it is a 3.50 ratio?

later,
Dustin

 

[adsm08]

Well check it. Make a mark on the hub and spin the pinion. Count how many times the pinion turns to 1 rotation of the hub.

 

[dangerranger83]

You are allowed to have .05 difference from the front to the rear gear ratios. Ive seen a larger diffence then that on a truck but it was used in mud, snow, etc....

 

[trail B2]

A nine inch has a drop out third member if his doesn't have a drop out it's a 8.8.If his is a 9" you don't have to replace the whole thing you can pull the axles and change the third member that way you don't have to bleed the brakes plus when I change axles the other one always has worn out brakes.

 

[rusty ol ranger]

I need the third member rebuilt. Im just gonna swap the whole rear end (cheaper)

So if you aloowed .05 difference, a 3.55 should be ok?

later,
Dustin

 

[dangerranger83]

I'm taking those exact gear diffrence words straight from 4x4junkie from another thread.

Its better that the front axle pulls the rear axle then the other way around and the set-up Im seeing that you are going to be doing all will be good.

3.55 (rear) - 3.50 (Front) = .05

 

[1badexplorer]

If the truck he has is an 81 and the rear hasn't been changed ever then it will be a 9" and it will be a direct bolt in fro your 79.

 

[NMB2]

I'm taking those exact gear diffrence words straight from 4x4junkie from another thread.

You sure you arent taking those words from me?

I've had to explain this several times... here it is again.

yes there is a maximum tolerance of .05 difference. You can find these differences from the factory. Its because of the way a 9" gear is cut vs. Dana axes. The 9" does not match ratios in most occurances. Here are a few examples
Dana Ford 9
3.73 - 3.75
4.09 - 4.11
4.30 - 4.33
4.56 - 4.56 (just happens to fall in place)
4.88 - 4.86
5.13 - 5.14

there are ALOT more gear ratios available for the ford 9" but those are just a few of the most common.

In your gear ratio range, the 9" came with 3.45, 3.5, 3.55. .05 rule.

Its better that the front axle pulls the rear axle then the other way around and the set-up Im seeing that you are going to be doing all will be good.

That shit is bogus also, front pulling the rear. There is literally no difference at those gear ratios, you can make up .05 by airing down your rear tires a little bit more than the front. Dont read into all that crap people write about that stuff.

 

[feellnfroggy]

That shit is bogus also, front pulling the rear. There is literally no difference at those gear ratios, you can make up .05 by airing down your rear tires a little bit more than the front. Dont read into all that crap people write about that stuff.

Offroad, there is no difference, but by chance if he locked in on the asphault for any reason, towing etc. Then the difference would be noticable. Fixable but noticable. Even tire wear was enough to ock up our humvee in 4 wheel on pavement. I could reverse slighlty and remove the tension onthe tranfer case, and it would unlock enough to get back to two wheel. But still.

The point I think he was gettin at is We'd rather have the front wheels moving faster than the rear when wheeling. If the front move slower then the 4 wheel drive is gonna bog into the mud rather than pull itself above it. Might as well be 2wheel.

 

[dangerranger83]

Offroad, there is no difference, but by chance if he locked in on the asphault for any reason, towing etc. Then the difference would be noticable. Fixable but noticable. Even tire wear was enough to ock up our humvee in 4 wheel on pavement. I could reverse slighlty and remove the tension onthe tranfer case, and it would unlock enough to get back to two wheel. But still.

The point I think he was gettin at is We'd rather have the front wheels moving faster than the rear when wheeling. If the front move slower then the 4 wheel drive is gonna bog into the mud rather than pull itself above it. Might as well be 2wheel.

Couldn't have said it better for me.

 

[NMB2]

Offroad, there is no difference, but by chance if he locked in on the asphault for any reason, towing etc. Then the difference would be noticable. Fixable but noticable. Even tire wear was enough to ock up our humvee in 4 wheel on pavement. I could reverse slighlty and remove the tension onthe tranfer case, and it would unlock enough to get back to two wheel. But still.

The point I think he was gettin at is We'd rather have the front wheels moving faster than the rear when wheeling. If the front move slower then the 4 wheel drive is gonna bog into the mud rather than pull itself above it. Might as well be 2wheel.

.05 is not noticable on a vehicle no matter what surface you are on. I would hate to doubt you but I have a hard time beleiving that tire wear, unless you had some brand new tires on one end and bald ass tires on the other, which is not typical in the army, would cause issues.

Here is a little bit of hard numbers and facts

3.71 first gear, 3.50 axle gears, 33" tires, 10mph = 1322rpms
3.71 first gear, 3.55 axle gears, 33" tires, 10mph = 1341rpms

1:1 tranny, 3.50 axle gears, 33" tires, 60mph = 2138rpms
1:1 tranny, 3.55 axle gears, 33" tires, 60mph = 2169rpms

Your tires sidewalls flexing as you hit bumps or drive down a hilly/wavy highway literally makes that much of a change.

To further my point, if I need to, I will video my buggy driving down the street in front of my house, with obvious less air in my front tires, probably 1.5" shorter diameter, and it doesnt "lock" my drivetrain up a single bit.

 

[dangerranger83]

I would just stick with the "better to be safe then sorry" saying.

In my eyes almost 40rpm is quit a few to just ignore. I would still rather have my front pull my rear.