For clarity, ACD's problem does not seem to have had anything to do with the fact that the Jaguar lug nuts have stainless steel covers. The entire top halves of the lug nuts broke off, not just the stainless covers. The most likely explanation is that the lug nuts had been grossly overtorqued by a previous mechanic (probably with an impact gun), and broke off when ACD's mechanic attempted to remove them (probably also with an impact gun). Another contributor to this failure is when a tech does not fully seat his socket onto a nut before applying force. Many of the damaged stainless steel covers I have seen on client's cars show evidence of a socket having been carelessly inserted about halfway onto the nut when the impact gun was then run at high torque.

For the record, many of us prefer the appearance of the original stainless-covered lug nuts and the fact that they have greater torque/tension characteristics when used with alloy wheels, are better protected against corrosion on their threads, and won't pit or flake like chrome. In 30 years of Jaguar ownership I have only had to replace a couple of nuts due to abuse by tire shop techs.

Jaguar is not the only automaker to use stainless-covered nuts. Mercedes-Benz, Volvo, Aston Martin, Lincoln, Cadillac, Lexus, Porsche and others also use them. Here's a set of M-B stainless-capped lug bolts:



MacLean-Fogg, the global Original Equipment Manufacturer that patented the stainless steel-covered lug nut, claims to sell 220 million lug nuts per year around the world, so the major automakers must be convinced of their advantages. Here is some information from the company:

"Chrome plated wheel nuts are typically constructed from two carbon steel components, a threaded nut portion and a small end cap. The cap is welded to the hex end of the nut and this assembly is chrome plated. Due to the nature of racking in the chrome plating process, the threads are not fully covered with plating. The result of this incomplete plating coverage is the introduction of rust in the threads during the plating process. Stainless steel capped wheel nuts are constructed from two different components, a carbon steel nut and a stainless steel cap. The nut is plated with the customer’s preferred plating and friction modifying topcoat prior to assembly with the cap. The cap serves to cover the entire hex portion of the nut, terminating at the bulge, where the crimp is achieved. Since plating is accomplished prior to assembly with the cap, full plating coverage exists on all surfaces, thus preventing the occurrence of rust in the threads."

"The torque/tension tests show that the stainless steel capped wheel nuts clearly exhibit a distinct advantage over chrome plated wheel nuts in all but one application, that of steel wheels. Chrome plated wheel nuts installed on steel wheels showed marginally better performance. SAE J2316 requirements were met in all of the stainless steel capped wheel nut tests, whereas chrome plated wheel nuts failed in one of the three categories."

These days, when I have to have tire work done, I take my own quality 6-point socket to the tire shop and insist the nuts be loosened and torqued manually. I don't blame anyone for replacing the originals with solid nuts, especially those who can't do their own work on their Jaguars, or those whose originals have been damaged by careless shops. But the original Jaguar nuts are superior and can be maintained indefinitely when cared for properly. They are not "junk" until they are abused, just like premium leather seats.

Cheers,

Don

 

The real problem is the dumbash behind the impact gun who used a 3/4" socket wrench because it fit and he didn't know that the correct size is 19mm. 19mm is smaller than 3/4".

I still have all the original 2-piece lug nuts on my 2009 XKR Portfolio Convertible and they're still like new because I keep a 19mm deep socket in the trunk.

 

I spoke to the rep at Discount Tire. I recently replaced the tires on my wife's CX9. He told me all of their pneumatic wrenches are set to 55 psi and then hand tightened to prevent a problem.