I bought some used Spitfire-size 13 x 5.5 American mag wheels several years ago and I put off getting them installed, because it's difficult to find the proper 3/8" mag lug nuts. Since this car "might" autocross occasionally, and it will see heavy street use, I felt the wheel studs should be upgraded a tad. TRF and Moss each want about $4.00 for a TR6 bolt, which is "supposed" to fit; but I decided I would find a better way (read: less expensive!). Therefore, I scoured the local PepBoys display rack looking for something of the 7/16" variety with the appropriate length and knurl diameter. There may be other sizes very close, but I found the size listed to suit my needs. I have no idea what kind of car it's cross-referenced to, and they may be available over-the-counter from other chain auto parts stores.
The brand name is WHEEL-TITE, part #28020. The specifications are:
7/16" - 20R wheel bolt
0.525" knurl dia.
9/16" shoulder length
1 3/8" overall length
PepBoys @ $1.39 ea.
The shoulder of the wheel bolt protrudes slightly from the face of the hub after installation. This is okay for mag wheel installation, however, it may interfere with the original steel wheels or other custom wheels. Check this carefully before driving. You may also have to re-drill larger holes in your wheels to accommodate the larger studs.
This is not an advertisement for PepBoys Auto Stores, but a fact-based reference as to where I found the parts available locally. Your results may vary. As always, be careful using bench grinders and large hammers - use proper eye protection and lots of common sense.
Turn head diameter down from 0.75" on each of the bolts to about 0.62" (similar size to Spitfire 3/8" wheel bolts) and bevel lower head edge to about 45 degrees - again similar to taper or bevel on lower side of head of Spitfire 3/8" bolt. I used a bench grinder, using a short piece of 1/2" pipe to help hold on to each of the wheel bolts. Just tap the bolt into the end of the pipe so that the knurled bolt "grabs" inside the pipe - just enough to hold it tight while you rotate it against the bench grinder.
You will have to remove the front hubs and take the rotors off to get these in from the back side. Hub and rotor removal will also save wear and tear on your wheel bearings. A large hammer and a couple of 2x4s to block up the hub will allow easy removal of the old 3/8" wheel bolts. I "beat" the 7/16" bolts in with a large hammer from the back side of the hub. You might want to use a couple of pieces of metal square tubing instead of 2x4s - it's much sturdier to support the hub. Seat the bolts all the way, and you are ready to re-install the rotor to the hub. Assemble the hub & rotor to the spindle, replacing seals and/or bearings if necessary, and of course with the proper quantity of appropriate wheel bearing grease.
The new rear wheel bolts will have to have an additional slight bevel cut on the top part of the head. When these are installed, there will not be much clearance between the head of the new wheel bolts and your brake cylinder. After removing the brake drum, "tap" out the existing 3/8" wheel bolts. There should be plenty of room to remove and install without having to remove the brake pads. Get a hold of some 7/16" grade 8 nuts and some 1/2" washers to use as a puller and spacers. Insert the new 7/16" wheel bolt from the back of the hub, put on the spacer washers, and tighten the grade 8 nut until it pulls the wheel bolt all the way in to the back of the hub. Remove the nut and washers, re-install brake drum and wheel, and you're done. Happy motoring!
FYI, Ford Taurus studs will also work, are longer than stock for thick wheels, and have the advantage of being found everywhere!