William Brady, what makes you think Jaguar did the long stud to reduce weight? The 4.2 short stud block was prone to cracking on the intake side area above the distributor just below the deck area. Probably a thin casting from the larger, repositioned cylinders. Shorter block and head used from 64 to 1968. When the XJ6 came out in 1968 they needed to recast the new long stud block for new motor mount position, also made a longer head with extra cooling holes at the back. Jaguar realized the engine would require catalytic converters to meet emissions and this would add to overheating. Jaguar had no control over the cars after they left the dealership. If the owners chose not to flush the cooling system or keep up with the coolant, it allowed the cooling system to corrode. Every block I have seen with broken studs were half full of rust, also the cause for blown head gasket. A clear indication of a lack of cooling maintenance. If you own one of these cars, do yourself a favor, and knock out the freeze plugs and flush the system. Take a look at the studs when the freeze plugs are out. Easy to replace the studs BEFORE they break.
Cheers Tom
