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)

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 - Ford-Trucks.com


Ben in Austin
1950 F1

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.

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

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

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.

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)

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!

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.

Ross,
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

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.

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

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.

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.

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...

Yes. Assuming your AOD has a .667 OD gear.

The big advantage of the 9" is the ability to change rear ratios with ease in an hour or two. With an 8.8, it's a much bigger deal.

Just my $0.02

I have a Ford Racing 302 340/HP 350/TQ bolted up to a 302 with a AOD. Disc brake conversion up front, stock suspension (posies springs). My original Dana 44 with a 3.92 holds up well to ocasional beatings and burnouts. It I am on it hard it has a 1 -2 gear chrip. I did convert it to a posi. cruises 70 at just about 2100 rpm with 15x275x60 tires, 28 inches tall. I have been know to run it 80 for a couple hours on end at about 2400 RPM. Dana 44 is rebuilt all new bearings, seals etc. (no leaks) Posi unit was purchased from Randy's Ring and Pinion.

I'm pretty sure all factory AOD's have the same .667 OD gear.

Where is it leaking from? It would be a lot less expense and work to change 3 seals and a rear cover gasket, since any one of those 4 places is all that can leak unless something is broken, than to do a complete axle swap for no other good reason. The original rear is pretty tough and is probably the last thing to break on one of these trucks. Who are these people 'advising' you to replace it?

Wayne. I believe it is leaking from both the front seal and the cover. No leaks out at the end of the axles.

Understand your point about not fixing what's not broken, and seals are easier than a swap.

How can I tell what gear ratio is in it now? I don't have a tach. I know the transmission is a T-98, but i don't know anything about its internals (other than that I have bad synchros). I've looked for an external indication, but not sure where to look under the 60 years of crusted oil and grime.

If I could get some comfort that the gear ratio would meet my goals (in original post), or that swapping them out wouldn't be a bigger deal than the 9" swap, I'd love to consider the Dana.

As far as who's told me to replace it.... It just seems like 90% of the folks that I talk to (on-line and in local "Classic Car" shops) say something like "I assume you'll want to get that Dana out of there" To my discredit, I have not done alot of digging into the specific reasons... maybe they all thought I was going for 400+ HP... which I am not.

Thanks!

There originally would be a small metal tag under one of the rear cover's bolts, with the gear counts on it.

To find out your rear ratio, leave one tire on the ground and jack up the other side. Mark your driveshaft with a chalk. Put it in neutral and rotate the wheel two full revolutions. Count the number of driveshaft revolutions. I got almost 4 revolutions out of the driveshaft. It's a 3.92 . Dennis

If you've been driving it and it works, makes no strange whines or clunks, you're probably good to go for a long time. The number one reason most guys swap the rear for a 9" is because it's the easiest way to get a higher, more highway friendly rear ratio when running a 1:1 top gear in their trans. There are (or were) thousands of 3.25 9" truck rears laying around for the taking. Since you're going OD, that's not an issue. The most common ratio for your truck is 3.92:1. A 4.27 was optional. Either way, it will be well suited to use with OD. If you pull the cover to fix the leak, you can count the teeth on your ring and pinion gears and calculate the ratio that way. 2 birds killed with the same rock.

Ross, Bill, Wayne, Bryan, Dennis (and probably missed someone), based on your comments, I spent time examining the old Dana today. This is what I found out...

The gear tag is still there and says 47/11 ... I think that means 4.27, right?
I can turn the drive shaft slightly (maybe 1/4 inch +/-) with the tires firmly on the ground. Is this level of "slack" ok?
Based on the calculator I've been using, 4.27 gears, AOD (.677 Transmission Ratio), and 28.5 tires should get me to 70 MPH at under 2,400 RPM. I'm fine with an ave highway speed of 65-70 (for this truck).
The leak seems to be coming from the front (where the drive shaft enters housing).

So bottom line... If (a) I don't need posi, (b) don't care about easily being able to swap gear ratio in/out, and (c) will not exceed 400 HP (probably won't exceed 300) for this build....

Should I just replace the seals on the Dana, and use it with my new AOD/302? Would make the project easier/faster, and get me on the road sooner (big deal to me).

Really appreciate any opinions to consider.

In short, yes.

I'd say that would be my 'plan A'. You can always change out the rear later if you find you hate it or it isn't working for you. But for now, you can use it and save a bunch of money, time and hassle.

I bought my 2002 Explorer 8.8 rear end for $200. I got a bit of a deal due to knowing the owner of the junk yard. Here is something to consider....

The 9 inch, great selection of gears and parts to make it bulletproof and to fit your exact taste. You will have drums so that can be good or bad depending on what you want.

I wanted the 8.8 for these reasons:

8.8 is stronger, the explorers come with trackloc (posi) and 31 spline (beefier) axles. I got the 3.73 rear gear and disc brakes.

If I were to but a 9 inch posi rear end I wouldve cost me more to have the same setup, even though It was closer to a direct fit.

No, it is not.

That said, they are very good rear ends. They are more than strong enough for all but the all-out race-style engines out there. They're about the same as a GM 12-bolt...But they're not stronger than a 9".

The downfall of the 8.8 is the axle tubes not being welded.

The downfall of the 9 inch is the flex in the housing.

The upside of the 9 inch is the endless amount of parts available (at a $$ price)

The upside of the 8.8 is the standard Trackloc and disc brakes for a fraction of the cost.

The Ford 8.8 Mustang Rear End - Car Craft Magazine

If you are concerned about 8.8 rear end strength, get the 31 spline axles (Explorer and late model Mustang)

The strength of the 9" comes from it's pinion snout bearing. The 8.8 is an excellent rear, and I do not doubt its strength - they are on par with a GM 12-bolt. The 9" is still king when it comes to all-out strength though and why they're used in every form of racing (even if there isn't a single 'Ford' piece in them). If your housing is flexing, you've got bigger issues to fix! ;)

Another huge advantage for many people using the 9" in this particular niche of the hobby is the ability to retain the stock 5 on 5.5" bolt circle...those don't exist with the 8.8.

The bottom line is, they're both good axles.

I got a chuckle out of that magazine comparison where the pinion shaft diameter of the 8.8 was much larger than the 9", like that was an advantage. In reality it's a testament to the effectiveness of the snout bearing versus the overhung design of most other diff's.

Just like a bolt in double-shear is stronger than a larger bolt in single shear...those gears are trying to get away from each other and there's a lot of force working it...

what is the strongest rear axle... ha ha ha...

what's the best carb will be the next discussion.

I think the OP has received some good advice keeping/ rebuilding the stock axle.

When replacing a Dana 44 out of a 1955 f100 the link here says 72 is a direct exchange...some other info indicates 76 works. Does anyone know what the difference is if any?? Thanks for any help!

An unmentioned possible disadvantage of the 8.8 is it is too narrow for our trucks, requiring strongly offset wheels or wheel spacers. That can be an advantage tho if you want a different bolt pattern, you can use all in one spacers and BP converters. Advantage of the 8.9 is they are very available (hence inexpensive) and come in a number of different gear ratios, with/without LSD, with/without disk brakes. I am running 3.70 ratio with my double OD T56 6 speed (.66 5th, .50 6th) and find it a perfect combo, loafs along at 80MPH in 6th while getting 26+ MPG.

That's the ultimate goal is 5spd and larger engine. Right now it is 272 V8 with a Mustang 4spd but the rear diff. needs rework. didn't want to redo it but switch to 9 inch for its advantages. Just found some 73 model up rear assy. and didn't know what the difference from a 72 if a 72 was pretty much a direct bolt in what would be the issues in using later model.

The '73 - on F-100 axles are quite a bit wider, don't have the numbers handy but I think it's about 4".

For me the down side of a 8.8 is the c clip axles and rear axle bearing design. Mid 80's thru 1996 f150 have 5x5.5 axle pattern