I need to order a water temp sensor for late flathead. Is it 1/8 thread? I've searched, a lot. I'd take the cylinder head to the hardware store and find out for myself but I'm no good standing up, still healing up from surgery. Thanks [Some info: I Scored a set of Classic Instruments gauges on eBay, that don't include the senders (oil pressure, water temp). The seller has CI temp senders but in different sizes and can include them for a few more bucks.]
3/8" NPT thread. Two types: single pole and 2-pole. Single pole passeger head makes the gauge move. 2-pole drivers side is a switch that opens @ 26* and allows dash gauge to go full HOT.
Assuming that it's a V8 flathead and unless you have two water temp gages you are only going to be reading temps off of half the engine. (with after market Classic Instrument gages.)
Although you can wire up two senders via an on-off-on toggle switch to a single gauge. Then you can flip the switch left or right to read the temp on that side if you're worried about it.
Henry's original King seely system actually protected (?) both side of the motor. If the left sensor (switch) opened @ around 206* the dash gauge goes to full hot. If the right side sensor sees hot the dash gauge also goes to hot . Here's a print for the original system and don't know if it will marry with your new dash gauge. You should be able to test wire it with an original single pole sensor, a battery and a pan of hot water.