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PostPosted: June 26, 2016, 3:22 pm 

Joined: November 19, 2014, 11:42 am
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Note that I do not recommend this - the reasons are described in the post.

I put a traction lock in my 66's 9" rear axle and decided that I was really over drum brakes, so I set out to put disc brakes on it. I am too cheap to spend hundreds on the kits that are out there, so I started researching junk yard alternatives. I found some information that suggested that the Explorer rear disc brakes could be used on the 9" (and found a pretty decent writeup about using them on a car 9" rear) so I decided to acquire some parts and do that with my truck.

Note also that the later Crown Vic, Grand Marquis, and Town Car have the same concept of rear brakes as the Explorer but the parts are not identical - the Explorer has a left and a right backing plate so both sides end up with the pads behind the axle. The TC application uses the identical backing plate on both sides so one side has pads behind the axle and the other ahead. Also, the TC uses a metric banjo bolt in the caliper and the Explorer uses a SAE banjo bolt. I actually used the TC setup, but the same issues apply to both.

The Ford 8.8 axle flange is smaller than the big bearing 9" axle flange, so the bolt pattern and center hole need work to attach:


You can see in the photo above that I rotated the pattern and drilled new holes, you could also egg out and enlarge the existing ones.

The first issue with this approach is that the thickness of the drum backing plate where it attaches to the axle flange is about .150" and the thickness of the Explorer backing plate is about .400". If this is ignored, then the axle will have the opportunity to slide in and out of the housing by about .250" as the side forces on the wheels change, because the axle bearing (which is what captures the axle in the housing) will not be secured in place. My solution to this was to make a spacer out of 1/4" key stock. I chose aluminum because it is plenty strong enough to handle the loading that will be experienced, and it is much easier to form. The plan was to put this spacer between the outer side of the bearing and the bearing cap (basically in the hole in the backing plate) so I made it in a "C" shape so it would slip over the axle outside of the bearing:


When I attempted to assemble this, I discovered the second issue with this approach. The Explorer brakes are set up for a 2.50" offset between the axle flange and the wheel mounting surface. However, the 9" truck axles are (at least mostly) set up with about a 2.0" offset. This makes the wheel mounting hub too close to the parking brake assembly and the disc too close to the backing plate. The disc can be spaced out by putting a spacer between the hub and the disc, but we are still left with the hub too close to the parking brake shoes, so that the lug studs were interfering with the return springs.

So, I put the 1/4" keystock shim into the axle housing to space out the wheel mounting hub. There are too issues with this:
First, the axle needs to be long enough that when it is held out 1/4" that the splines still engage the differential to drive the vehicle. In my case, the factory axles were plenty long enough.
Second, this results in the outer axle bearing not being fully supported in the axle housing. The outer big bearing race is about .875" wide, about .870" of that being encased in the housing the way the factory set it up. By spacing it out .250", this means that the outer race of the bearing is only supported .620" in the axle housing. This will not be a problem for the axle housing, but it reduces the capacity of the axle bearing because of how the outer race is only partially supported. In my application, I judged this to be an acceptable compromise, but one needs to consider this carefully before deciding to do it. Note that if you are replacing the axle bearing, you can put the spacer outside of the bearing before assembling the bearing - this would resolve the bearing capacity issue but would reduce the axle capacity somewhat because of the longer overhang outside of the bearing.


Then, I was still left with a 1/4" difference between the offset that the brake assembly was expecting and that of my modified axle. So, I got a pair of 1/4" wheel mounting spacers and turned down the outside diameter to 6.75" so they fit inside the Explorer rotors (which I had redrilled for the 5 on 5.5" pattern of the truck) and assembled the rotors and calipers. The spacing is now just like the brake assembly expects.


Note that there is no picture of it assembled because it is a little challenging to get the nuts on the axle flange bolts behind the wheel mounting hub because it all gets pretty cozy in there, but it does go together and all fit.

It is also important to make sure that the bolts that hold the axle in are long enough so the locking nuts are fully engaging the threads. In my case they were long enough, but in another case it might be necessary to put in longer bolts. And also the wheel mounting studs must be long enough to engage enough threads on the lug nut to be safe, since we have spaced the wheel out by 1/4" that must also be confirmed.

I used the front brake hose from the Explorer to go from the hard line to the caliper. Note that since the Explorer uses SAE banjo bolt and the TC uses Metric banjo bolt, and the Metric bolt is slightly larger diameter, I had to drill out the hole in the end of the hose to allow the use of the Metric banjo bolt. This is a reason to use the Explorer brake parts rather than the CV/GM/TC parts.

So, here are the issues:
Explorer brake assembly expects a 2.5" offset between axle flange and wheel mounting surface, most truck 9" axles are 2.0"
Spacing out the axle is required if you have the 2.0" offset and want to use the parking brake, this requires the axles are long enough to still engage the differential, and it reduces the capacity of the axle bearing or the axle itself.
A 1/4" spacer is required to capture the axle properly unless the axle bearing is relocated on the axle.
A 1/4" spacer is required between the wheel mounting hub and the disc to properly locate the disc.
Longer axle flange bolts and longer wheel studs may be required.
The Explorer disc needs the hole pattern redrilled.

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PostPosted: September 17, 2016, 3:30 pm 
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Sounds like a major effort, I guess I will stick with the drum brakes. Thanks for the how to article, it is very informative.


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