Wouldn't it be nice if some of the currently manufactured aftermarket wheels fit FIATs? If FIAT used a 4x100mm bolt pattern instead of the 4x98mm pattern, nearly any currently available style wheel could bolt right on. Why not change the bolt pattern? This is exactly what I did, first on a 131/Brava, then on a 124 Spider. The conversions took a lot more time than money, but both proved to be successful. On the Brava, the hubs were drilled out and permanently changed to a 4x100mm bolt pattern. With the 124, adapters were made that bolted to the hubs as the wheels normally do and accept a 4x100mm wheel. Each method is outlined below. If you attempt a similar conversion on your car, pay attention to all details so that safety and reliability is not jeopardized. Unless you are skilled on the proper equipment, machine work should be trusted to a reputable shop that welcomes custom projects. They may also be able to provide the necessary materials for this conversion. Good Luck!
CHANGING THE BOLT PATTERN
Permanently changing the bolt pattern has pros and cons. The obvious pros include a much larger selection of wheels, no increase in unsprung weight (at the hub), and the reliability of a simpler design. Cons include among others the impossibility of returning to a 4x98mm pattern without sourcing new hubs or axles. Don't forget to obtain a suitable 4x100mm spare tire/rim for the trunk. If you chose this method, count on moderate downtime for your car as you disassemble the hubs and axles, complete the necessary machine work, and source wheel studs and nuts. This is not a weekend project! Below is what must be done to each involved component, and remember that some changes may be necessary for a 128/Strada or X1/9.
Front Hub: Remove from car and dismantle completely so that no bearings, retainers, or grease remain. Inner races can stay in place. The existing threaded wheel bolt holes must be drilled out to accept a knurled wheel stud. This is actually quite simple, but remember that placement of the new hole is crucial. Let's focus on just one of the four holes. The existing threaded hole is 12mm in diameter (major thread diameter) and 49mm from the center of the hub. The new hole must be 50mm from the center of the hub and 14mm in diameter in order to completely remove the original threaded hole. This diameter may not be exactly 14mm depending on the knurl diameter of the chosen wheel stud. There should be an interference fit of about .3-.4mm for proper fit of stud as it swedges into the hub. Now the rear surface of the hub must be spot faced to provide a flat surface for the wheel stud head to seat against. Don't forget this step or the studs will not seat properly or locate correctly. The spot face diameter should be just larger than the diameter of the wheel stud head (no contact around the outer edge here) and just deep enough for the knurled portion of the shank to flush-out with the mounting flange of hub. The remaining three holes are done in the exact same manner. This work is best done on a rotary table to insure that every hole is exactly 90 degrees from the next. A full scale technical drawing of this work should accompany the hubs to the machine shop for illustration and clarity. Remember to stress to your machinist that in addition to adding the studs, the location is changing. After completion of machine work, press wheel studs into place and verify that is bolt pattern is 4x100mm as desired. The prototype that I had made during the design stage of this project turned out to be 4x99.2mm. An inattentive machinist was the cause; the result was me having to obtain another hub for completion of my conversion.
Rear Axle: The existing threaded holes need to be drilled out just as they are for the front hub. While the front hubs can be machined on a bench, a lathe with attached mill head may be necessary for spot facing the rear surface of the axle shafts. Alternatively a back-spotface bit can be used. These are expensive, so check out all possibilities. All wheel stud dimensioning should be the same as for the front hubs. Keep in mind that rear axle flanges are not the same thickness as front hub flanges, so spotface depths will be different. The distance between the spotface and mounting surfaces is what is important.
Wheel Studs: This could be the part that causes the most concern. Ideally, the wheel studs should have a 14mm knurl diameter, a knurl height that is just under the hub/axle flange thickness for minimum spotfacing, a low profile (flat) head, and sufficient length for mounting the desired wheels. The nuts must contact at least 7-8 treads with the studs. Wagner and Bendix both offer a variety of studs that should fit your application. Check all dimensions carefully. Many studs will fit three out of four requirements, but few will satisfy all four. Be sure they will work.
Wheel Nuts: These obviously must have the same threads as the wheel studs. Most wheels have tapered seats, which require tapered wheel nuts. Some wheels have ball shaped seats for use with rounded wheel nuts. In any case always use the proper type wheel nuts. Avoid the use of cylindrical wheel nuts as centering problems can occur as the bolt holes in the wheels wear. Both tapered and ball type wheel nuts simply seat deeper when wear occurs eliminating any chance of false centering.
Brake Rotors/Drums: The wheel bolt holes may have to be enlarged slightly for the increased bolt pattern size. The nominal size of the original hole is 1/2 inch. I drilled them out to 9/16 inch, providing sufficient clearance.
Stock Wheel Spacers: These must also be drilled out slightly to accommodate the larger bolt pattern. Because wheel offsets vary so much, multiple spacers (or none at all) may be used to achieve proper placement. Be sure to check that enough threads are in contact.
WHEEL ADAPTERS
Using wheel adapters also has pros and cons. Pros include easy changing between 4x98mm and 4x100mm bolt pattern wheels, no permanent changes to your car, and little downtime as only the rims/tires needs to be removed to complete the conversion. Cons include increased unsprung weight and the use wheels with a large amount of offset (30-40mm). Because this conversion is "bolt on", it could be moved from one car to another with little effort, increasing it's utility for multi-FIAT owners. The adapters should be fabricated from an appropriate grade of plate steel. Thickness will depend on your application, but use at least 1/2 inch for strength and reliability. Your machinist can help you with this selection. Both surfaces should be machined to assure flatness and parallelism. The outer diameter needs only to be large enough to accommodate the wheel studs. Make sure, however, that it is not less than the diameter of the stock wheel spacer. The inner diameter needs to be large enough to fit over the hub/axle of your desired model(s). Check the hub/axle diameter close to the brake rotor/drum as it is largest there. Next, eight holes must be machined: four for the knurled wheel studs at 4x100mm and four for the original tapered wheel bolts at 4x98mm. The holes for the knurled wheel studs should be machined as described above, but make certain that the head is completely recessed below the surface of the adapter (by counterboring) so that a flat contact can be made with the hub/axle. The tapered bolt holes should be 1/2 inch drilled and 60 degree countersunk 1/4 inch deep. This will insure exact centering of the adapters, and therefore the new wheels. The countersink side MUST BE OPPOSITE the counterbore side, so that the wheel studs protrude away from the hub/axle. Obviously, the holes will alternate every 45 degrees, creating the pattern of eight holes. After the wheel studs are pressed into place, the adapters can be bolted onto the hubs/axles just as the original wheels are. The small bolts securing the brake rotors/drums will have to be removed to allow flat contact with the hub/axle. Because the original wheel bolt heads remain exposed, the new wheels must have clearance cavities between bolt holes. Many wheels have these voids to save material during the manufacturing process. Selection of wheel studs and nuts is the same as above. Don't forget that large offset wheels are required for use with this conversion method.