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-   -   Rear Axle Question ... 9 inch v 8.8 inch (

johnson.jeff.a 07-08-2015 08:58 PM

Rear Axle Question ... 9 inch v 8.8 inch
Rear Axle Questions:
Putting a 302/AOD in my 55 f100.
Project Goals: Daily driver type "work truck,"
fun to drive but no dragging, etc,
60-70 comfortably on the occasional freeway trip
Occasional small load in the bed, or maybe pull a small trailer with a couple of dirt bikes, wave runners, etc (no big boats)

Considering: 8.8" out of a pre 94 explorer/monteiro (drums in back) OR a 9" out of a 57-72 F100. Here are my questions:

About the 9"
Is it true that a 9" from a 67 to 72 f100 will bolt up (no mods at all)?
Is it tough to find posi? Given goals above, will I care?
I've had a recommendation to go with 4.10 gears (because they'll meet my goals above, and are pretty common). Thoughts?
Is $250 a fair price?

About the Pre 94 Explorer 8.8"
How much of the stock re-suspension do I need to modify to us it? Same u-bolts etc?
How about spring plates and shock mounts.... compete re-do? (I don't weld and will need to outsource any of this work)

ben73058 07-08-2015 10:03 PM

Hey Jeff,
Here is an article that goes through the rear axle solutions with some thoughts on each.

1948 through 1960 Ford F-1 and F-100 Rear Axle (differential) Swaps -

Ben in Austin
1950 F1

drptop70ss 07-08-2015 10:26 PM

9" posi rear will not normally be found for $250, usually much more. 8.8 4:10 is easy to find, posi will take longer but can be found for around $250, depends on who is seling. Myself I would go with a 3:73 gear.
There are plenty of differences between the two rears but myself I would get an explorer 8.8 3:73 posi, especially if you want to run passenger car wheels since the explorer rear has the 4.5" bolt circle.
I just picked up a non posi explorer rear for $80, a F150 8.8 3:55 posi rear for $100, and I will be putting the truck posi and gears in the explorer case. Open 9" rears are out there but posi center sections are much more expensive around here.
I would update the rear springs and hangers as I did with my trucks, other than welding the rear axle pads it was a bolt on deal after drilling new holes for the hangers. It isnt necessary but I think it will make a big difference in ride quality.

ben73058 07-08-2015 10:35 PM

Hey Jeff,
If I were going through all the work to swap out rear ends - I'd get exactly what you want. We went with an
8.8 in. from a 2001 Ford Explorer - disc brakes, limited diff., 3.73 gearing. There are 2 issues to know on these 8.8's -
they have a car bolt pattern 5x4.5in not the 5x5.5 original. You'll need to take that into account with wheel selection.
The second issue is these are a little narrow at least for our '50 - we had to buy some 1.5 in. wheel spacers.

There isn't a lot of welding involved. You have to grind off the original spring perches & relocate them - I would say minor welding to re-install
the perches.

Good luck over there.

Ben in Austin

ben73058 07-08-2015 10:38 PM

Like Dave said - I was typing at the same time. We paid $125 for our
Explorer rear end. We went with the 5x4.5 in bolt pattern because it matched up with our Mustang II style front suspension.

Ben in Austin

johnson.jeff.a 07-09-2015 11:11 AM

Originally Posted by ben73058 (Post 15487909)
Hey Jeff,
If I were going through all the work to swap out rear ends - I'd get exactly what you want. ...

Ben in Austin

Thanks, Ben.
Mostly I want to limit complexity as much as possible (within reason), but I also want to keep using my old steelies. Honestly, if the old transmission weren't making noises, and if I had to spread less cardboard to catch all the oil dumping out of the old y-block, I'd just keep the old drive train...Leaning toward 9" for my new one.

I found this site: Engine RPM / Engine Speed Calculator

It's telling me that with an AOD, my 29 inch tall tires, and 4.10 gears I'll be winding at about 2,387 RPM to go 75 MPH. That sound about right to you?

One other question. I'm assuming that the AOD/9" combination will require a different length drive shaft... will that mess with my pinion angle enough to have to mess with the spring perches on the 9" anyway? If so, that negates some of the 9" advantage.

ben73058 07-09-2015 11:56 AM

Hey Jeff,
I like the 9 inch because it's width matches up a little better &
you should be able to avoid using spacers. You also get to keep your existing wheels if you like them. What are you plans for the front end?

Moving to a Mustang II style front end dictated our decision in the back to go with the 5x4.5in. bolt pattern 8.8 in.

As with all changes - you get benefits plus other adjustments you have to make. I think if you change your rear axle you'll have to look at your drive shaft, pinion angles, shock set up etc. I'd definitely plan on changing your ubolts -I wouldn't reuse the old ones.

Ben in Austin
1950 F1 (351W/AOD)

johnson.jeff.a 07-09-2015 02:09 PM

For the foreseeable future, I am staying with the stock front axle (plus the disk brake conversion I already did). "Done" for this iteration of the truck involves this 302/AOD drive train change and the addition of the toyo power steering box. I will then drive it for years before considering any other drastic mechanical changes. I really just love to have it as my weekend transportation, and honestly do not mind the bouncy/hard truck ride. Just don't want it to strand me, and want it to leak less... much less.

Yeah, the way I phrased that question made it sound like I cared about the expense of u-bolts. I don't, and will plan on replacing.

Thanks again, Ben. Can always count on you for some good answers!

ALBUQ F-1 07-09-2015 02:14 PM

Do you really need posi? What axle ratio do you have now? Most stock (high) ratios work fine with an AOD. Mustang GT's with 302/5.0, a T5, and factory axle were spinning about 2200 2300 at 75. With increased wind resistance of a truck I wouldn't want to be much below that.

johnson.jeff.a 07-09-2015 02:26 PM

Not sure I need posi at all. It was one of the original purposes of my post. I'm becoming convinced that, for my goals, I don't need it.

If the calculator I found is right, my tire size with an AOD would get me to 75 mph at about 2,250 RPM with 3.73 gears and 2,450 with 4.10. I'm leaning toward 9" with 4.10 gears. Won't get hung up on posi

fladoodle 07-09-2015 02:37 PM

4.10 is a highway ready gear with an OD. Unless you're planning on mainly being a cruiser, I'd go with the 4.10 for the added fun factor. Id also agree that an open 9 inch makes the most sense for you from what I've read here. Just my two cents! But you'll probably be happy with whichever option you choose.

ALBUQ F-1 07-09-2015 02:44 PM

What ratio do you have now? I would expect it's about a 3.92? Why change anything?

johnson.jeff.a 07-09-2015 02:56 PM

Ross, good question. I only have two answers.....
the stock Dana is leaking badly
Most have advised that I replace it now, while I have the box off the back, and am replacing rest of drive train.

Also, I can't find any indication of what gears I have, I just know it's wound out at 55 .... I know the aod will change that.

49willard 07-09-2015 05:35 PM

As Ross suggested you could stay with the stock rear. Unless you push the 302 to significantly higher performance the stock rear should do fine. I ran a 1956 322 Buick nailhead thru the stock rear for years, no issue. I changed my rear to a 9 since I am running a sbc 383HT clone that dyno'ed at 440 ft-lbs so I wanted the 9. If you do change to a 9 with the stock leaf springs you do not have to remove and re weld the spring perches however you will need to adjust pinion angle with wedges. That is what I did. I also had to modify a drive shaft-not expensive. I run 3.70 gears with an AOD and 28"+ tires. I am very happy with the setup.

johnson.jeff.a 07-09-2015 05:45 PM

Ben, Dave, Ross, Bill, "Fladoodle" thanks for the valuable input. As usual, you have helped me make a decision that I was a little stuck on.

Will post pictures, update as I get into it...

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