Jaguar Repair: 1984 XJ6 fluke, plumbing section, vacuum test

Question
Howard,
You have helped me quite a bit with my XJ6 and it is now a weekend cruiser. I check all fluids and poke around under the hood at least once a week, but usually before and after each drive. With that, I think I have what I would normally write off as a fluke, but thought I should ask anyway. About ten miles from home tonight, the generally ghostly quiet running turned to a loud exhaust sound and I thought perhaps a weld in the exhaust had cracked, no biggie to have repaired on Saturday. Then, I heard and felt a piece leave the vehicle and I quickly pulled over to find the second bolt which holds the heat shield on top of the exhaust manifold, nearest the firewall; was gone. In the time even I have been working on cars new and old, I know bolts don't get up and walk off without good reason. I was able to find a bolt at a hardware store to temporarily plug the hole while I completed my drive around town, and once back home tonight, I located a fitting in my vacuum test kit which fits with precision, and plan to use that to locate a suitable plug tomorrow. So, question one is, is that foretelling of something more serious, or really just a fluke, in your opinion? Secondly, I imagine I will do best to match the thread pitch in the plumbing section at the hardware store with a brass plug of some sort. Do you see any problem there? Otherwise, I imagine that is not a readily available piece as far as an O.E. exact replacement goes.

Answer
Hi Michael,
The second plug is larger than the front one and is used to hold down the heat shield. They are not noted for falling out. It must have been loose or had striped threads. It is an odd size as I remember. I don't remember the size but do remember doing a lot of digging to find one to fit. You can't use a pipe thread as it holds the heat shield down.

Howard