Being one who modifies things; twice in the last several years I have had the need to change a Chevy bolt pattern to a Ford bolt pattern. The first time was on a Cadillac rear disc brake conversion I designed, built, and was instilling on a friends 65 Mustang fastback. Rather than find a Ford rotor with the correct Ford pattern and make it work I decided to use the rotor that Cadillac paired with the caliper I was using, which required me to change the bolt pattern. The second time I had to do this was on my Mustang independent rear suspension project
. I am retrofitting a Jaguar IRS unit to fit under my 64.5 Mustang and as part of the retrofit I had to convert the hubs to the Ford pattern.
In both cases, when I suggested to some other car enthusiasts that I was planning to drill the pattern myself, I was told not to do it because it would be almost impossible to have all 5 holes be in the correct position. I pondered the issue and, at first, thought about using a rotor with the correct Ford pattern as a jig, but the problem is keeping the rotor locked in to place while drilling the pilot holes. I then thought about making a 10-hole jig (5 holes each pattern) that I could bolt on and use to drill pilot holes for the new pattern. Problem is, I would still have the issue of getting all 10 holes in the correct position on the jig. One day while contemplating my dilemma, I had an epiphany, instead of making a 10-hole jig, make a 3-hole jig and rotate it from position to position to drill each pilot hole.
I made the jig by drawing a 4.75 circle with a 4.5 circle inside it. I then measured the distance between two studs on the 5 on 4.75 pattern and plotted those 2 points on the outer circle. From there I measured between the two points I had just plotted and plotted a center point on the inner circle. (Note: picture not drawn to scale)
The drawing was then transferred to a piece of steel and the three holes were drilled out. Many measurement were then taken, and a test drilling was done on a scrap rotor to insure an accurate Jig had been created.
3-hole jig, two holes in the Chevy pattern and one in the Ford pattern.
With the jig finished I was able to bolt it to the Cadillac rotor, drill a pilot hole, position the jig on the next two studs making sure the same side of the jig always stayed up, and drill the next hole; and then repeating the process untill all five pilot holes where in the hub. I then removed the jig and enlarged the pilot holes one bit size at a time up to .5. This gave me a rotor with the correct Ford bolt pattern. The beauty of this jig is that even if the new pattern pilot hole is not perfectly centered between the original pattern holes, as long it is on the 4.5 circle, the new holes will be in the same relative location to the two 5 on 4.75 pattern holes/studs on either side of it in each of 5 locations, giving a perfect star pattern every time.
The jig bolted to a Cadillac rotor to drill the new pattern. On the Jaguar hub, I removed the original Jaguar thread-in wheel studs, to make room for the jig. The threaded holes left by the wheel studs were ideal to bolt the jig into place, and from there I used the same technique as above to drill the holes for the new pattern. Once the holes were drilled I pressed new wheel studs into place. This gave me a Jaguar wheel flanges with the correct Ford bolt pattern.
New studs installed in the Ford bolt pattern, with the original pattern holes left open. The Jaguar hub is a perfect fit in this 5 on 4.5 bolt pattern rim.