TR2/3/3A tr3 temp guage capillary tube thread size

does anybody out there know the thread size for a tr3 water temp capillary tube thread where it goes into the t stat housing. I am trying to block off the hole so I can run my engine without the gauge for now? I read it was 5/8 but that seems to large.
Thanks to all,
Dave

It's not a pipe thread. It's a 5/8"-18 NF thread. The bulb on the capillary tube seats inside the housing. That is what prevents water from getting past the threads. You might be able to get away with a 3/8"-18 NPT plug. I measured one, and the bottom is a little larger than 5/8", so I don't know how far it would thread in. I would be very careful trying to use one of those. If your old gauge is shot and you want to send it out to get repaired, why not just cut the capillary tube a couple of inches from the fitting and just screw the bulb and fitting in.

I haven't tried it but would think a sender from a TR4 would make an okay plug (if you can lay hands on one).

But depending on what you want to do with the engine with the gauge out you might get away with a rubber plug and leaving the radiator cap off.

I once stuffed an ear plug into the metal heater pipe (threads to the valve failed) and duct-taped it in place. Drove home like that with no problemo.

Yes, the TR4 sender fits fine (I have one in my TR3).

Another alternative might be to just buy a cheap mechanical temp gauge. I don't know if the electrical ones come with the same thread; but every mechanical gauge I've seen uses the same fitting as TR3; even when the package says otherwise!

Sorry, I have absolutely no idea of what the application was. But years ago, I was trying to get a TR3 home by driving it; when the Tstat housing fell apart. I carried the remains into a nearby FLAPS and scouted through the water outlets until I found one with the same bolt spacing. Of course not the same size & shape, and no room for a thermostat (or bypass fitting); but by adding a long flexible hose, it worked for 1500 miles or so. ISTR I used a sparkplug & hose clamp to close off the bypass hose.