Mustang 10-hole wheels are 15x7" and the correct offset for an early RX-7. Good.
They are the wrong bolt pattern, albeit a tantalizingly close one. Bad.
It has a 4.25" (108mm) PCD and the hub faces are solid, not hollowed-out as in most wheels. Additionally, the front face has eight depressions for lug drillings, even though only four are used. (I have learned that some do not, those in theory would be even better) There is not enough material to redrill the existing lug tapers out 1mm (and thus 1.4mm deeper) but thanks to the design, there is plenty of material to make four *new* holes.
This is where I come in.
Unfortunately, nobody local would be willing to touch the project on liability reasons. So, then, I set out to make a jig to do this myself. (This also means, do not try this at home, or even at a friend's house, anything you do with your car is your responsibility, and if you do it anyway and take out a bus full of preschoolers, nuns, or preschool nuns, or in any other way cause bodily or property damage to anybody or anything... you were the one who picked up the tools)
I was wheedling around for a few days thinking of how to mark out the 110mm pattern so that they are 55mm from center and 90 degrees apart. I kept coming back to somehow chucking it on a lathe and scoring a line. Not thinking creatively! Then basic trig hit me.
The basic premise is simple: Make a plate with two holes spaced so that a 4.25" wheel can be bolted to it with two adjacent lug holes. Then calculate where the new hole in between goes, using nothing more than the Pythagorean Theorem and deduction.
Here is the diagram and the numbers involved for calculating *where* the holes have to go.
The math works as follows: The two 2.125" radii from adjacent lugs form the legs of a right triangle, which I calculate the hypotenuse to be 3.0052", as marked on the left.
A 45 degree angle is formed when a line is drawn halfway between two of those legs. It thus splits the hypoteneuse into two 1.5026" halves, and since the new triangles made by this line are 45-45-90, we know that this line meets the hypoteneuse 1.5026" from the center of the wheel.
We can then subtract that from the 2.1653" radius of our new bolt circle, to find that the new hole must be .66275 (less rounding here) outboard of a line drawn between two adjacent lugs on the old pattern.
As a further check, I calculated the hypotenuse of the new right triangle defined in part by the new lug location, which is 1.6422".
When I make the drill jig, in reality I will mark the two mounting holes on a scribed line, then scribe two arcs 1.642" in radius from those points, verify that where they meet is .663" away from the line, and call it good enough. The drill bit will probably wander a thou on each hole, despite using a drill press.
The lug tapers will be reamed on a jig consisting of a genuine Mazda brake rotor anyway, so this is an awful amount of preciseness for something that only has to be in the general ballpark.
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modified March 05, 2007
Pete Remner aka peejay or sometimes ???