A fellow club member who regularly drops in to my place had been having some trouble with a series 3 XJ6 blowing head gaskets. He eventually checked the engine block and found the typical warp which in this case was about 9 thousandths of an inch [.009]
He finally bit the bullet and removed the engine for some serious machining work to get the engine block re-decked i.e. ground back to get rid of the warp. . Along with that he had a typical problem with a broken head stud when he removed the said studs for the machining work. He requested the workshop carrying out the machining to extract the broken stud.
He was put onto an alert by the workshop that he should also check the length of the old head studs as they do have a history of stretching. When he did this he found that they had in fact stretched. This may not sound serious but it is a problem in that Jaguar use a domed [acorn] nut to hold down the head.
If the stud has stretched the nut may feel as if has tightened down onto the head but it may have only tightened down onto the stud and may be exerting little or no pressure onto the head. This is a condition known as becoming “thread bound”. I.e. the nut is tight on the thread but is not holding the parts together with sufficient pressure.
Upon checking the head studs he found that they had all stretched beyond limits and a new set of studs was definitely needed. The XJ workshop manual does give guidance on checking the stud length but how many of us amateur mechanics read the manual before tackling the job?
In fact one wonders if the stretched head studs were not the problem in the first place as the engine had not really done any serious distance since an “overhaul” by a previous owner
In discussion with this member who like myself has lived in remote areas we agreed that if we struck this stretched stud problem well away from any reasonable engineering facilities we would either add an extra washer onto the head and use the Jaguar acorn nut or ditch the acorn nut and use common plain nuts. That would at least allow us some chance of carrying out a repair sufficient to get us home.