How do I install my mechanical temperature gauges and capillary tubing?

When installing one of our mech. temp gauges, you should first be aware of the fact that you will need to drill or cut a 7/8″ hole in your firewall to route the capillary tubing. This is must be done because the capillary tubing can not be removed from the back of the gauge or the sealing nut without damaging the line itself. We include a grommet to fill the hole you drill, so there is not a gaping hole in your firewall.

Once you have determined where the gauge will be mounted, you need to carefully route the capillary tubing. The capillary tubing needs to be routed away from any excessive heat sources, such as headers or the exhaust manifold. This is done for a couple of reasons:

  1. To prevent or cause damage to the line
  2. So the line does not absorb the extra heat and reflect a higher temp than what you are actually trying to monitor.

Also, be sure to keep the tubing routed away from any moving components, such as any suspension parts or fan blades.

Obviously the tubing will have to be bent somewhere down the line, or you may have some excess line you will need to coil up. This is where some people start to cringe. No worry, though, as the capillary is quite flexible. With this in mind you can, if need be, coil the tubing to a diameter as small as 1-1/2″ without concern for damageing the gauge or the capillary.

Coiling is also a good idea to help absorb any vibration that the line may be subjected to, as the smaller coiling can actually help to “deaden” any excess vibration that may be transmitted to the gauge. Coiling also helps protect the tubing. HOWEVER, these bends cannot take place at either the steel crimp end for the sender bulb or right off the back of the gauge. To be safe, leave at least 8″ unbent; or, if a bend must be made, be sure it is as large a diameter as possible, as the ends of the capillary are very sensitive to severe bends. Another concern is that you do not want to double-up the line. Doubling-up the line can create a kink and can cause the gauge to become inoperative.

Now, we can briefly discuss also how the gauge itself works. What we do is run a hollow copper tube from the end of the line (commonly referred to as the sender “bulb”) all the way into the gauge, then cover it with a protective plastic/rubber-like covering. This line is filled with ether gas, so if you have ever broken or cut one and get a funny smell or some white fluid on you, don’t worry, it is not harmful. When the sender is in the intake, head, trans or oil pan (or whatever you are monitoring) the temperature heats the ether gas inside the tubing. This makes the gas expand and thus “push” the meter that is inside the gauge. This moves the pointer and shows you the temperature. However, if this hollow copper tube is kinked or doubled-over itself, the line will be shut and the gauge will not work. This moves the pointer and shows you the temperature. However, if this hollow copper tube is kinked or doubled-over itself, the line will be shut and the pressure would not reach the gauge, so you would never see the gauge reflect any temp readings at all. This is also true if the line is ever cut or the sender bulb is removed; once the ether gas escapes the line, there is nothing to “push” against the meter to register any temperature readings at all.