Fell down a rabbit hole and might have climbed out finally.
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Years ago I built a brake swap kit for myself using ’98 Mustang Cobra rotors, C5 Vette calipers and a simple flat plate to bolt it all up to a stock disk brake spindle. Been on the car for several years now and seems to work well even with the manual brakes and large MC. But…there are some things I don’t like about it and I’ve since owned and sold a 2015 Challenger and have lusted after a Scat Pack for some time now.
About a year ago I played with the idea of swapping Scat Pack calipers onto the car, but with either the same 13” rotors I am running now or maybe 14” GT500 rotors. In both cases, the size of the caliper and the spacing of the mounting bolts made it look near impossible to make work.
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But for some reason around the end of last year I tinkered with it again and discovered that with a 14.2” Scat Pack rotor, it just might work. At that time, I was working with a traced shape from an image of a caliper off RockAuto so I order a single caliper and a set of pads so I could take actual measurements. The theory being that if my sketched layout was way off, it would be easier to return one caliper rather than 2 calipers and 2 rotors. Order enough to validate the theory but not enough to waste too much money if it was wrong.
After taking some measurements off the actual caliper, it looked like it just might work. After some full scale cardstock templates I moved to a wood mockup and crashed hard. The layout and the theory of the swap wasn’t the issue, it was trying to accurately drill the holes in some wood blocks and cutting them to shape. It might have been fine if the glue I used to stick my paper template to the wood hadn’t swelled the paper and moved hole locations by about 1/8”. At that point, I decided to jump into an idea I had toyed with for awhile and bought a 3D printer. So much better!
One of my criteria was I wanted something that didn’t require a machine shop to build it and I wanted to avoid the below results.
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I think the above failure was partly due to the amount of cantilever on the plate. And when my new layout with the actual caliper showed that the distance from the mounting plane to the outside of the spindle was just under 1/4" (later revised to just under 5/16”) I knew I was going to have to use the rear surface of the spindle much like the bracket above did.
This resulted in a multi-plate setup that uses a 1/4" plate on the front of the spindle and a 3/8” plate on the back of the spindle and either plates or tubes to fill in between. This (in my mind) gives plenty of meat to the overall bracket but also some triangulation, so the caliper isn’t cantilevered off the main plate. Plus the plates are all just flat plates and can be laser cut from
SendCutSend.com.
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Truth be told, I wanted a 1/2" plate on the back, but since I am using the threaded holes for the stock caliper mount and a nut on the outside plate, I need a bolt both long enough to allow for the nut and enough thread length that I could tighten it down on the back plate. The standard thread length for a partially threaded 1/2x20 bolt is 1.5” and I needed a thread length of 1.625” so it had to a be a full thread bolt. Best I could find was some 2” long full thread bolts from McMaster-Carr, resulting in the decision to use a 3/8” rear plate as that makes the bolt long enough to get the nut on with full thread engagement. The 1/2" plate would have left the bolt short and I couldn’t find a grade 8 bolt over 2” long that was full thread. Overall, there is 5/8” of plate supporting the caliper so I think it ok. Worse case, I could drill the treads out of the spindle and use a partial thread 2.25” long bolt with a 1/2" plate, but at that point there wouldn’t be any going back.
The plate spacers add some complexity too. I need 7/8” total for the spacer thicknesses, but SendCutSend only cuts up to 1/2 plates. Add that the tolerance on a 1/2" and 3/8” plate could add up to potentially being too tall or too short by over 1/16”. So, my current idea is to get two 1/4" spacers and two 3/16” spacers as the tolerance on those thicknesses adds up to much less. But still not perfect.
The other option is some 1” x .188 wall DOM tube. The drawbacks are that now you need a machine shop to accurately cut them to length and keep them true, and it doesn’t give as much surface area to support the two plates. And that tube size is as economical as I could find ($22 at
MetalDepot.com for 8”) and still fit. The upper tube gets pretty close to the spindle and could still require some clearancing on the spindle depending on the shape of the spindle forging so a bigger tube would be hard to make fit. But it would be easier to accurately match the thickness of the spindle, plus you could weld the tubes to one of the plates making assembly easier.
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The last hiccup was figuring out how to get the rotor centered on the hub. The stock hub has a pilot that is something like 2.82” OD and the Scat Pack hub has a pilot of 2.85”. Not a lot of difference between the two, but enough that the rotor moved around some, but also not enough to just get a centering ring made up. There is a fairly large chamfer on the back of the SP rotor and I thought maybe that could be used, but the one quote I got for machining one was over $300 each, or I could get it 3D printed in metal for about $200 each. Nope not an option for me. Add that I doubt the chamfer has a tight tolerance on it making it less than useful unless I made them to a specific set of rotors.
In the end, I think I am going to probably redneck it and slip a feeler gauge of the correct thickness between the rotor and the hub and just leave it there. Wrap it around the hub and slip it in. The hub is close to 9” in diameter so it might take two of them on each hub, but a couple of bucks for 4 sets of feeler gauges has to be cheaper than $400 in printed metal parts that might or might not work depending on how much attention the rotor manufacturer pays to the process of adding the chamfer.
Another (and maybe better) option would be ARP 100-7707 wheel studs. These have a shoulder that stands above the face of the hub flange and would probably do a great job of filling in the holes in the rotor and centering things up. One potential issue is the published specs that say the rotor has a 115mm bolt circle while the hub is a 4.5” bolt circle, but I know of one guy that did exactly this and it worked great for him. The other issue is that I already have 3” wheel studs in the hubs I am running right now and would hate to spend another $200 on different studs if I can make them work.
On a side note, I printed up some bushing to take up the difference between the hole in the rotor and the 1/2" stud in the hub to get the rotor centered for mocking this up. Even though the bushings had a consistent wall thickness all five dropped into the holes fine and the rotor slides off and on happily. This suggests that used OEM rotor I am using for mockup has a 4.5”/114.3mm bolt pattern, despite the published data that it should be 115mm.
I bought the one caliper for $128.79 (which includes the core charge) plus like $17 in shipping. The new rotors I am looking at are just over $50 each with shipping being close to $30 (for both). The plates for both sides, with a black zinc finish, were $120 or so last I checked. So around $550 plus whatever the pads cost? Oh, and brake hoses haven’t been worked out.
So, there you have it. 14.2” rotors and Brembo 4 piston calipers on stock spindles.
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And yes, the adapter plate clears a lower ball joint.
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