Suitable mechanical seals are very commonly available from almost everywhere. Google “Mechanical Seal”. All you need to know is the shaft and casing diameters and the height. There are very sophisticated types available, Jaguars use the simplest, most basic.
Some, like Tadek’s pics, are just a stationary part that presses into the pump casing, with a spring loaded carbon surface that seals against the rotating impeller. Issue with Jaguar versions is that the relatively soft cast iron impeller wears very quickly. One solution is as per Post #4, ceramic runner accommodated by shortened impeller. Sometimes (XJ6/12) there is a rubber seat between the ceramic runner and the impeller, this is to ensure that the runner spins with the impeller.
Setting this type of seal is tricky, pressing on the impeller simultaneously sets both the seal spring compression and the impeller/casing clearance, i.e. these cannot be set independently. You want the seal spring to be lightly compressed; compression is not so much for sealing as it is to accommodate wear. At the same time you want the gap between the impeller and the casing to be as small as possible without touching, for best pump performance.
Another type of mechanical seal combines both stationary and rotating parts and press fits to both the casing and the shaft. I.e. this type DOES have an interference fit onto the shaft.
As per Bills posts. Aftermarket for all classic Jaguar; imo newer and vastly superior because the seal and impeller can be set independently. With the appropriate setting tool (e.g. Bill’s mandrels), you can just blindly tap the seal into perfect position. Because the rotating part gets pressed onto the shaft it does not rely on the impeller, the impeller can be separately set to minimise the clearance with the casing, I usually press it on till it starts to scrape then pull it back a hair. Generally requires shortened impeller so that there is plenty of separation (Bill’s 3 mm) between the seal and the impeller.
Photo: On the left are from SNGB (stationary part, runner and rubber seat) on the right is aftermarket combined seal type together with a setting tool.
Imo for my car (S3) biggest issue with seals is the matching (aftermarket) impeller that is usually supplied these days (Bill’s pic). These impellers (disc and vanes) are very much thicker than Jaguar which reduces the flow passage increasing potential for the impeller to “choke” at high speeds, restricting flowrate thereby increasing overheating. Also the vanes are too long, they protrude out of the shroud into the pump inlet where they create “washing machine” turbulence to further disturb/restrict flowrate and thereby potentially increase overheating at all speeds.
Photos: the first is with the aftermarket impeller, the second is with a genuine Jaguar impeller:
