I successfully repaired my Mark V temperature gauge which had the tube cut off at the capillary bulb.
I read Roger’s writeup and a couple of others I found on the web. Just recording here a few other details and observations.
This gauge X.70915/1 is unique to Mark V among Jaguars, may have been used in other marques, but I did not find any reference on the web.
My '38 SS and '51 XK120 have gauge X.42638/6 which was used in all '38-'48 saloons and XK120/140, but not Mark VII nor Mark 1. There were some detail changes in this one over the years.
The Mark V tube is 1/16" diameter and about 56-58" long with a steel spring wound around it for protection. There is a steel protector sleeve at the gauge end but not at the capillary bulb end.
The tube on the SS is 1/16" OD and about 80" long and does not have the steel spring protection, nor does it have the steel sleeves at either end.
The tube on the XK120 is 1/8" OD and about 80" long with no spring but has the end sleeves.
I have a broken spare from an April '53 120 with 1/16" x 80" tube with spring and end sleeves. The capillary end is longer to accommodate the sleeve.
Removing the short cutoff tube from the capillary, I discovered it extended nearly to the bottom of the bulb. I did not duplicate this aspect when I inserted the remaining end of the tube, left it protruding 1/2", as I was having to remove some of the wound spring and didn’t want to shorten the finished length any more than I needed to. This may have been a mistake, as I will explain later.
The first step using a propane torch is to solder the tube into the capillary body with the end bulb off so you can check the tube is not blocked afterwards. I used a .017" guitar string.
Then solder on the bulb.
Check it for leaks.
At the gauge end remove the oil pressure unit.
On the temp unit I found a short tube which was used at Smiths in the filling operation and then plugged with solder. The gauge from the '53 120 does not have this tube, so the Smiths people must have found they could fill without it.
Remove the solder from the end with a small soldering iron.
I found the end was crimped before soldering, which is a common practice with refrigerator piping, so I had to cut off about 1/8".
Check it is clear with the .017" guitar string.
Then put a blob of solder near the end but not blocking it, ready for the final step.
I did not disturb the gauge end of the long tube, but if you have to replace that, be careful not to overheat the body and loosen the Bourdon tube, that curved copper thing that moves the gears.
To fill, I used starting fluid which is mostly ether with some heptane and propane, sprayed into a jar with a lid. You need a container with an opening big enough to fit in the Bourdon mechanism with the small fill tube submerged at the bottom.
Heat the capillary bulb in boiling water and watch the bubbles in the ether jar.
Put the bulb in the ice water and the ether will suck in.
I couldn’t tell how much was going in, so I went through the heat/cold cycle 4 times until there were very few bubbles the 4th time.
Then leaving the bulb in the ice water, pull the mechanism out of the ether, close the jar lid, and solder the end of the fill tube closed with the small soldering iron, and not too much heat. You have to hold it downwards so the blob of solder will flow down and cover the end. The ether tries to burp out a little bit with a little pssst, but you don’t lose much before you get ahead of it and get the tube closed.
You can test it now in the hot water, and if the pinion gear moves a little bit you know you are successful. It only moves half a turn, about 4 or 5 teeth.
Assemble and test.
I mentioned that I was not sure about the tube insertion in the bulb, and the filling amount. In water boiling at near 100C, my needle ran around into the oil pressure side, so maybe I should have inserted the tube farther or I may have put in too much ether.
When I get some running time on this engine, if it is too inaccurate I may try again.