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25 Posts
Discussion Starter #1


Thought I'd share this painstaking axle conversion in my 1973 Caprice Classic. Let me preface this by saying the only reason I did this is because the GM factory installed some oddball 12 bolt axle that parts are extremely hard to impossible to find. I wanted to install some kind of positraction and a change of gear ratios, both which turned up to be harder than I thought.

GM installed a type "A" axle. This was not a Chevrolet axle. The build sheet even called it out. I finally found out 20 years ago what axle it was. I went to a Chevrolet dealership and went to the parts department and brought the build sheet. It stumped the young lads working there, then they called an older chap to the counter. He looked at the build sheet, raises his head and said that's an Oldsmobile axle. He disappears for several minutes and brings a big Oldsmobile book full of dust to the counter and cracks it open. He points to the exploded view and asks is this it.

I said emphatically it certainly was. They then did a nation wide dealer search on gear sets and differentials. None, nada, nine, nyet. There is also no aftermarket support for this 9-3/8" massive ring gear Oldsmobile passenger car axle.

An axle change was inevitable. This is a long process and I'll break this up into parts as there are loads of pictures.

This is the Olds axle.

The rear brakes are 11" x 2" drums.

The axle solution I had chosen was a Moser Engineering Ford 9 inch custom made to fit the 71-76 B body. It came replete with 31 spline axles.

Here's the axle housing. Now for brakes I wanted OEM brakes and had asked if they could make the bearing flange work for my old drum brakes. Moser said no, they do not make that flange for those big brakes. So I asked if they could make '94-'96 Impala SS (WX3)/Caprice police interceptor (9C1) disc brakes fit. They said yes.

Cool beans.

However, there was a possibility of needing a shim, they supplied the shim, but it was up to me to figure it out if I needed it or not. Well since the shim goes between the axle flange and the bearing, installing a set of bearings is a 50% chance of being right. Not wanting to waste a set of bearings. I modeled up the Ford big axle bearing in 3D CAD and 3D printed one out with reduced outside dia. and increase inside dia for a slip fit. Otherwise the plastic faux bearing was withing a few thousandths of the real one.

Here's the shim. So I did a test fit with the shim and the faux bearing. Turns out I needed the shim.

Since GM discontinued the backing plates for the 94-96 full size rear disc brakes I had to locate a used set. This turned out to be loads harder than I thought. Turns out most of these have been crushed or pillaged for other 5 on 5 rear disc brake conversions. But I got lucky and found a set. Next was cleaning and powder coating them.

Then rebuilding them.

Next problem was the supplied studs. The '73 B body uses 1/2"-20 studs and the '94-'96 are metric. The English/SAE studs wouldn't fit through the rotors on the shoulders.

I downloaded the Dorman catalog but couldn't find anything that really would work well.

I ended up turning down each shoulder on all 10 studs.

Press the studs back in and the bearings on.

To be continued.

Next up will be installing a differential and gears then setting them up in the brand new 3rd member housing.


p.s. If you are having trouble viewing the embedded pictures you may want to try a different browser.

1,315 Posts
As an FYI to anyone trying to view the photos i found while at work behind a firewall chrome, firefox, and edge failed to view the photos (firefox got close by at least showing a picture should be there but it never loaded it). the tapatalk app was able to view it, but i had to follow the path listed at the top of this page.

Just before i posted this comment I went back and tried Internet Explorer and that loaded the pictures properly...which saddens me deeply that i had to use

this was an interesting read, thanks for sharing!

25 Posts
Discussion Starter #4 (Edited)
As an FYI to anyone trying to view the photos i found while at work behind a firewall chrome, firefox, and edge failed to view the photos (firefox got close by at least showing a picture should be there but it never loaded it). the tapatalk app was able to view it, but i had to follow the path listed at the top of this page.

Just before i posted this comment I went back and tried Internet Explorer and that loaded the pictures properly...which saddens me deeply that i had to use

this was an interesting read, thanks for sharing!
Hello Joes74challenger,

I think I may know what's going on with the pictures and not showing in the more mainstream browsers. The problem is most likely this forum is accessed under HTTPS (has the security link layer) certificate and my pictures are from my domain which is only HTTP. This forum when you view it forces your browser to switch to HTTPS then instead of downloading the pictures on its server simply points to my domain that does not have the SSL layer and the browser will not display them.

Two avenues for possible correction are for me to see if it's possible to get an SSL layer certificate from my domain host and or see if this website will upload the pictures and resend them under HTTPS.

It will be a few days before I can see if it's possible to get the SSL. If I can, then the pictures should display for the common browsers. Or if the website changes the way it handles pictures.

We shall see.


25 Posts
Discussion Starter #6
Part II

Picking up where I left off. Setting up the third member.

I ended up buying a brand new Eaton True Trac differential and Richmond 3.25:1 gears.

First thing is install the new bearings.

I will say this, not all Ford 9" axles are created equal, neither are the third members. There's a host of different size bearings depending on application. You have to do the research to see what yours takes.

Just for comparison, the used looking bearings were out of a old Ford 9" and those are smaller than the ones I need.

Pressing the new bearings onto the new carrier.

Before placing the ring gear onto the carrier, you want to make sure there are no burrs.

Same with the ring gear.

Chase the threads and clean with parts cleaner.

Clean the threads on the bolts and use Loctite.

New pinion pilot bearing. Again there different sizes depending on the third member housing you bought. Always verify.

Inset the bearing and install the retainer.

In case you're wondering the third member is a brand new nodular iron piece. It's not worth your time and money to try and find one from Ford. Chances are it has seen hard use and they can have cracks in the nose pinion bearing area. I think this case was around 200 dollars brand new. And never trust a used limited slip, more than likely you'll have to rebuild it anyway, by the time you're done dickering with something used and abused, chances are it would be the same price or cheaper just buying something new.

Now after reading about the True Trac, I opted to spend the additional money on it. No clutches to wear out, no additives in the oil needed. Acts like an open differential under normal driving but stomp on the go pedal and it locks both wheels to together, smoothly and without noise. When I called Eaton to ask what their differential could handle as far as a torque differential they said I'd break a set of gears before their diff broke.

I was both impressed and I kind of took that as a challenge :wink:

Set the carrier roughly in place.

Prepping the pinion carrier for new races. This is the only piece on the axle (aside from backing plates) that was used. I have a couple Ford galaxie parts cars and nicked this off one of them to save a few bucks. I read no evidence that these suffered from cracking from long term use.

Installing new bearings onto the pinion gear.

Installing the bearing into the pinion carrier. I also use a slight amount of RTV around the perimeter of the seal case to help fight against leaks.

Not less forgetting the new crush sleeve

Ready for tightening down.

This is the shim pack you need to set the gears. Order this from Rock Auto. I think it was around 12 bucks. Some places were charging upwards of 50 dollars for this.

Now the fun easy part. Follow directions on the gears.

Let me interject here and say, if you haven't done this, buy a book for your rear axle type. This book was invaluable.

You do not need fancy tools to properly set up gears, just some patience and the ability to utilize common sense.

There's loads of little tidbits in order to set up a gear set properly. Let me touch on a few. New gears come with dimensions written on them. The idea is you set up the gears to match those dimensions and that will get you really close to the ideal gear tooth pattern. It's to shorten the time spent and make your life easier. Again read the directions.

Something in the book that wasn't on the directions I'll note here. There are hunting, non hunting and semi-hunting gear ratios. This means the same teeth will always mate together all the time, some of the time or it's random. The 3.25 gear set if I remember properly was a semi-hunting set. Usually for non hunting and semi hunting there are timing marks on the gear sets as to how they were machined. It's important to align the timing marks, if present, to keep gear noise reduced.

An example of getting the pinion depth to match what's written on the gear.

Timing mark. The gear set directions didn't even mention this. The book did. Glad I found it.

Once you set the pinion depth to what's hand written on the gears, next you set the backlash.

In this case following the hand written depth and setting back lash led to a near perfect tooth contact pattern.

That was easy. As a side note don't waste your time and money trying to find gear mating paste. Gray anti-seize works just as well.

I did experiment with other shims and it turns out first try (matching the numbers) was the best.

Set the bearing preload on the carrier bearings and lock the rings.

New axle housing gasket, copper washers and lock nuts.

New '94-'96 Chevy full size rear disc brakes hoses and misc.

Everything loosely installed for now. Starting to come together.

In part III I'll cover rough installation, prepping the engine and transmission and how I redesigned the rear controls arms to correct the idiotic propshaft to pinion angle GM bestowed on these cars.


25 Posts
Discussion Starter #7
Part III

Picking up where I left off. There is a design fault of the rear axle pinion angle. It's far greater than 3-4 degrees. It's so prominent that GM had to fit a CV joint at the axle. The further foible of this is they didn't put one on the transmission side which also shares an extreme angle.

Now Moser made their axle to the same specifications as the GM. That means copying the idiot pinion angle.

This is a good comparison of axles. That Oldsmobile 'A' axle is very large. This allegedly was used in GM C body cars such as the Olds 98 and Electra 225.

The big bearing Ford 9" is no feather weight but that damn Olds rear axle must weigh 100 pounds more. I can lift one end of the Ford 9" replete with brakes, I couldn't get one end of the Olds off the ground.

The propshaft Cardan joint angles are optimal when both ends are equal and between 0- 4 degrees.

In order to set the axle pinion angle the engine and transmission mounts need to be in excellent condition. Whilst the transmission mount was acceptable, the engine mounts were obliterated.

The thrust side (drivers side) mount was collapsed.

Changing the mounts was quite the undertaking. I had to enlist the help of a good friend and we wrestled that 900 pound engine for 45 minutes till it finally sat in the mounts.

With the engine mounts replaced I could now focus on the repositioning of the rear axle. But how to make this easy....

I was at the Tractor Supply store (similar to Farm and Fleet) and saw these tractor 3 point hitch arms and had a light bulb moment. I could use these as tools to adjust the rear axle into place and then take measurements.

The lowers were good to go but I had to make a little adapter for the uppers to work.

Installed these and started experimenting with the pinion angles.

Now we're getting somewhere. This is where the pinion should normally reside (pointing slightly up instead of down).

I took loads of measurements and made a spreadsheet. Moser gave me 4 lower control arm attachment holes and so I took measurements in each to see how stable the pinion angle was at different axle ride heights.

Armed with this data and the new fixed angle of the centre line of the engine/transmission I determined I needed to lower the transmission mount about 5/8" of an inch to reach an optimal Cardan joint angle.

There's only one recourse to lower the transmission and that's to lower the mount pad on the cross member. There is quite a bit of wiggle room in this area to alter the engine and trans centre line with respect to chassis. Got to love big cars.

This is a spare cross member. As a side note this car is over built so much. I do love that. People balk in disbelief when I tell them the car has a curb weight of 5000 pounds. Rounded to two significant digits.

The wall thickness of the cross member is nearly 1/4" compressed at the ends it's nearly 1/2" thick steel. This could support a Detroit diesel never mind a Turbo 400 behind a Mark IV 454.

One of the flaws of the GM cross member is that there are no drain holes in the low sections and water just sits and rusts. In fact there is really no way for the rust to even come out. The logical choice is to bore some 1 inch holes in the low spots. Even 1 " holes will not weaken the brace for this application it's so overbuilt.

Just look at all that.

This was about 4 pounds of iron oxide. If a newer car rusted this much it would be a death sentence as chances are structural integrity was lost somewhere critical. Even with this removed, the trans cross brace weighs nearly 50 pounds.

I want to make an note here. Now I don't plan on keeping this Turbo 400. I would like to replace it with an overdrive transmission. A 4l80E to be specific. I will make a new lighter weight and just as strong transmission brace out of 4130 Chromoly when the time comes. But it never hurts to clean and paint this brace up in the meantime.

Further more I did refresh the Turbo 400. It started leaking badly several months ago and when it was cold out, reverse wouldn't engage till it warmed up. All that was fixed. I'll start another post on the refresh of the Turbo 400 if anyone is interested.

Once the transmission was set back in place and my pinion angle finalized next up was to make the set of actual adjustable upper control arms. But first in order to add stability, the lower temporary tractor arms were removed and measured and a temporary set of more stable lower controls were cut and welded to that length and installed to rigidly locate the lower portion of the axle. The arms I used were spares off an old 4th generation Ford galaxie. These were similar enough to use for this purpose.

Now with the new pinion angle the upper mounts on the axle housing needed to be moved inboard a tidge to avoid bind. So they were removed along with the spring perches as they were practically off in another quadrant now.

In the next part I'll cover the making of the rear control arms, misc brackets and adapting the existing GM propshaft to the Ford axle via Lincoln parts.

Hope you enjoyed.


p.s. please feel free to ask any questions or voice any concerns.

25 Posts
Discussion Starter #8
Part IV

Right, moving on. One thing I forgot to mention in the previous post is that along with optimizing pinion angle via the temporary tractor hitch adjustable arms, this was also a good time to put the axle in such a position that the wheel looks aesthetically pleasing fore-aft wise in the rear wheel well.

Because I like a luxury ride, I opted to use the standard GM full size rubber bushings for the rear control arms. This needs to be pointed out and I'm sure I'll repeat this numerous times, but there are loads of substandard, sometimes dangerous replacement parts being made. Just because it's the lower price doesn't mean it's the same quality. Sometimes even the more expensive parts are made so poorly they are only fit for the rubbish bin.

I have here a collection of brand new good and very substandard bushings. The good ones are the Harris branded ones (sold in Moog boxes) and then there is some no name imitation crapola ones. Now I bought the cheap ones because I needed a temporary set to install for locating the sway bar. That's coming up.

The substandard ones are made with such an indifference to quality the inner sleeve is placed almost willy-nilly with respect to the outer shell. Unlike the Harris ones.

Another topic I wish to cover is metallurgy. Do not use ordinary construction grade steel for anything structural or safety related on a vehicle and here's why. Ordinary construction steel is rated around 50-55 ksi. Most of the frames and suspension parts that are steel used on cars are higher carbon and other elements added for resistivity to cracking from flexing and then it's heat treated.

The metal I'm fabricating the rear control arms is 4130 Chromoly, 3/4 hard (tempered) aircraft certified steel. Chromoly is extremely resistant to cracking from continued flexing. Ordinary steel you buy a chain hardware store or even a steel supplier is not. It will crack.

I fellow car chum of mine is a big Oldsmobile fan and he mentioned some of the Oldsmobile clan bought tubular control arms as upgrades to their classic Oldsmobile's and are having cracks develop. The last thing you want is a crack leading to suspension or brake structural failure at high speeds. I need not elaborate further.

My bushing housings start off from a seemless pipe of 4130.

I inside bore with my cheap little lathe for a friction fit of the GM bushing. I even step the inside for the shoulder.

You can just make out the slight shoulder the GM bushings have.

These are the beginnings of the cup of the upper control arms.