With aftermarket wheels and/or thicker brake rotors, the Fiero’s stock wheel studs don’t provide much thread engagement with the lug nuts. I found that on the front of my ’88 Fiero I was only getting 8 turns with the C4 12″ rotors and my Motegi wheels. That’s 12 mm of thread engagement on a 12mm stud. Marginal at best. With the 3mm spacers I wanted to add behind my rotors to get some control arm clearance, I would be down to a dangerous 6 turns or 9mm.
I scoured the ARP and Dorman catalogs to find a good candidate. The 1988 front hub holes are about 0.490″ with the stock studs removed. Dorman recommends a hole size between 0.017″ and 0.027″ under the knurl OD, while ARP recommends 0.005″ for iron and steel hugs and 0.007″ for aluminum. It’s not clear why there’s such a big range or difference between the recommended interference.
The closest longer studs that would fit are ARP 100-7708 and Dorman 610-323. The ARP 100-7708 studs have an 0.509″ knurl, making a 1/2″ reamer the closest common size to the correct hole size (0.504″ by ARP’s recommendation). When installed, these studs increase the thread length by 0.84″ (21.4mm) over the stock studs.
The Dorman 610-323 studs have a 12.80 mm (0.504″) knurl, which fit in the original hub holes based on Dorman’s specifications. They are 54 mm long, thus increasing the thread length by 12mm over the stock studs.
I pressed the lug studs out of a brand new Rodney Dickman 88 front hub and measured the hole size as about 0.490″ at the smallest and around 0.50″ at the largest. I went ahead and pressed the ARP studs in without reaming the hole to 0.503″, and they pressed in without too much trouble. The hub didn’t split, but a few metal shavings were produced by the knurl pushing through the hole. This fit exceeds the ARP recommendation for the interference fit, but is within spec if Dorman’s recommendation is followed.
Since these studs are so much longer than stock, open-ended lug nuts must be used. For GM wheels the stock ones should work. In the aftermarket, I found some inexpensive Gorilla lug nuts on Amazon.com. The part number is 20033SD for a set of 20 lug nuts and the spline drive socket. These nuts are narrow enough to fit in aftermarket wheels. I tried some White Knight lug nuts but they were too large in diameter to fit in the lug but counterbores in my wheels.
The Gorilla lug nuts fit perfectly in my wheels. The only disadvantage of these nuts is that the require the use of Gorilla’s spline drive key. Some hex drive lug nuts are available from Vorshlag that look like they might fit, since they are small enough to use a 17mm hex instead of the 19mm hex on the White Knight lug nuts that were too large for my wheels.