From Paul Geithner's Triumph Spitfire

Links, References & Technical Information

Upgrading Triumph Spitfire Wheel Studs

Something to seriously consider, especially when using alloy rims, is to upgrade from the stock 3/8 inch, 24 threads per inch wheel studs to ones that are thicker and longer. Being as thin as the stock studs are, they are susceptible to over-torquing, and being as old as many are, they likely have been overtorqued by someone in the past. Fitting wider wheels with wider, stickier tires and making other suspension improvements allows the car to generate more cornering force, which puts more stress on the studs. Furthermore, alloy rims have thicker center sections than steel ones, and so the lug nuts may engage fewer threads, making a somewhat marginal situation even more so. Upgrading to 7/16 inch or 12mm studs that are longer than the stock Spitfire/GT6 ones is good insurance, and a really good idea if you are considering driving your Spitfire sportingly. One very good solution is to upgrade to Land Rover Freelander studs (part number CLP9037L). These are 12mm thick, 1.5mm per thread (i.e., M12x1.5) and are 2 inches long, 1 1/4 inches of which is threaded, versus the stock studs that are 1.5 inches long, 3/4 inches of which is threaded. The comparison photo below clearly illustrates the dramatic difference.  

One particularly good thing about the Freelander studs is that they have the same head shape and shank diameter as the Spitfire and GT6 studs and so will fit without any extra machining and will not cause interference problems. Other studs, such as Dorman studs for Ford vehicles, have the right shank size but not the same beveled and chamfered head shape as the stock studs and so will require some machining to fit absolutely correctly and also avoid possible interference with the Spitfire rear slave brake cylinders. 

Make sure these studs aren't too long for your application (e.g., having your closed-end lug nuts bottom-out on the ends of the studs before they seat on the wheel). If they are too long (unlikely with alloy wheels and typical lug nuts), then you can drill-out the blind hole in each of the nuts a bit more, use open-end lug nuts or just shorten the threaded ends of the studs a little bit. Also, there will be about 1 or 2mm of the unthreaded shank portion of the studs exposed through the hubs and drums (see photo below).  If using steel wheels or alloy wheels with mag lug nuts, check to see that the nuts seat all the way. There shouldn't be a problem, but if there is, then drill-out the first 2mm of thread in the nuts. With alloy wheels that use cone seat nuts, these studs are fine as-is and they work great. 

Changing studs is simply a matter of removing the hubs (at the rear, pulling the hubs with a hub puller, and at the front, separating the hubs and the brake disks), pressing out the old studs, and pressing in the new ones. Do not pull a stud into position by torquing down hard on a nut and a stack of washers as this may very well overstress the stud and weaken it or even plastically stretch it, thereby rendering it defective. Pressing the old ones out and the new ones in can be done simply using a vise and a 1/2 inch or 13mm deep socket.  

With the new size studs, you'll need new matching thread lug nuts too, and these are commonly available from wheel suppliers, auto parts retailers or specialty vendors like Lug Nut King. A good torque range for nuts on the M12x1.5 studs is 66-86 ft-lbs (i.e., 89-117 N-m).

Paul Geithner