I did it on a 140 2 yrs ago, and now on a 150 FHC.
Took just over an hour. Tested without the instrument outer cover plate, to ensure no leaks etc.
I dont understand why the capillary tube i far, far too long. And you dont want to turn it too tightly. I made a loop and put it under the intake manifold.
And to get the big lock nut through the hole in the firewall, the stiff oil pressure tube had to be pulled out to the engine bay.
Why not 2 holes through the fire wall, each with its own rubber grommet???
The black Heat knob (hot to cold), was always wiggling even though tightened withnthe miniscule nut. So I cut and folded thick paper which I put on the metal bar, then pushed the black knob onto the knpb, and it is now nice anf tight, without the screw.
(The reason for the change was a dead temp gauge. Bought it from Steve Hunt in England, via Ebay).
Peder
I have two temp gauges, one is from a 150 ots and the other from a fhc, The fhc one is longer by at least 6 inches.
Is the dash panel /firewall further back on the fhc ?
Yes its not an easy job.
high morris , if you have a rebuilt one let us talk please.
There should be loads of slack in the capillary, way beyond 6". Temperature gauge capillaries usually have two loops in them between the engine bay mounting and the engine itself, of roughly 4" diameter, on all the cars I’ve worked on. I would expect the XK to be the same. You’d be surprised how easily the capillary fractures if there isn’t a good amount of slack to allow for vibration.
John ,Text me 250713 8420