Air Venting

When the cooling system is initially filled, coolant displaces air. With the old thermosiphon systems, air was simply vented out the filler neck. But when a thermostat is introduced, air can be trapped in the block and head.
This condition only occurs when the system is first filled Once the trapped air is "burped", the engine will remain air free unless there is a leak.

With air trapped behind the closed thermostat, a dry engine will overheat before the thermostat becomes hot enough to open. And even if the thermostat opens, the coolant level will be insufficient. Something needs to be done to allow coolant to rise at least to the level of the thermostat before that first start. The very simplest solution is a bleed port which can be opened while filling the system. The port may be controlled with a valve or simply with a screw plug. 

GM Diesel

For many applications, this function is handled by a air bleed port drilled directly through the thermostat itself. Sometimes it's just a small notch cut in the edge of the poppet. The port allows air to bleed past the thermostat and be released through the radiator filler neck. A bleed hole is a permanent leak that can bleed a quart of coolant a minute in normal operation, so it really has to be kept small. A vent can be added to any non-vented thermostat, just keep the size of the hole to around 1/16".

Gates 33188 For E-Type Series 2 or 3

The most common venting provision for British applications is a jiggle valve in the thermostat cap or poppet. This a simple brass or plastic stopper which is loosely anchored in a hole. While the engine is filled with air, the jiggle valve won't seal. But when the system is full of coolant and the engine is running, the stopper in pressed into the hole by the force of coolant flow. This is an automatic solution that's relatively leak free. 

Smiths 85025 for most early British Applications
Western Thomson for S1 E-Type

When retrofitting a generic thermostat to an old application, care must be taken that the thermostat has a jiggle valve that is set up correctly: if the thermostat is designed to operate on the cool side of the engine (more about this in the bypass discussion), then the valve will be in the reversed position. Installing such a thermostat in a warm side application will result in a large, permanent leak.

Backwards Jiggle Valve? The "business end" is on the top side of the flange. Gates 33747 for cold side or reverse flow applications.

A variation of this theme is the ball valve. This is a stainless ball caged in a seat. As with the jiggle valve, it will be pressed closed by the force of flowing coolant. Because the ball and seat are precisely matched, the ball valve may be somewhat less likely to leak than the jiggle valve. 

Behr 2.136 80 for Mercedes 60X cold side application

My last example is a very silly design. This is the thermostat housing from a Series II E-Type. This housing has a separate jiggle valve, which vents air into a port, which connects to a hose, which connects to the radiator. Thus very expensively doing what a vented thermostat could do just as easily. When a vented Waxstat is installed, then the bleed is redundant. I don't think anyone can be proud of this one. 

A few final comments. When a thermostat with a vent provision is installed, the vent port must be oriented to the highest point. Otherwise, air won't rise above the wax capsule or bellows and the thermostat may open late. It's important to fill the radiator to the top, then give the air plenty of time  to bleed out through that tiny hole. (If the system has other bleed points, and XJS folks know all about this, make sure they are open until all the air is out.) The thermostat bleed port is very small, so it takes time for all the air to rise out. Top it off, let it sit, repeat until it won't take any more. Patience is needed to prevent air pockets. 
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