Thermostats and Blanking Sleeves

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Written by Malcolm Eades

When our engines were first conceived, the designer was clever enough to realise that if the thermostat in a cold engine shut off the water circulation completely, the engine would warm up quickly but the water in the head would be static, leading to hot-spots and overheating around the combustion chambers.  The solution was to allow the water in the engine to circulate without passing through the radiator. If you look into the water outlet on the front of the cylinder head (where the thermostat sits), you will see a slot cut into the side. During warm-up, this passes water direct from the head to the water pump, which keeps it circulating through the engine and overcomes the hotspot problem until the engine is warm.

Of course, as soon as the engine is up to working temperature, there is no need for the by-pass. The thermostat opens and water is then allowed to pass through the radiator for cooling. The last thing you want is hot water that avoids passing through the radiator but instead goes via the by-pass straight back into the engine. The solution is to arrange for the by-pass slot to be closed off simultaneously with warm-up. But how is this achieved? Well, this is the clever bit. If you look at an original type Smiths thermostat, it has a broad metal blanking ring around it. The ring moves with the bellows, so as the thermostat opens, the ring moves up to blank off the by-pass and all the water has to pass through the radiator to be cooled. The picture shows the AC version of the original design.

So far, so good. But what if your thermostat does not have the blanking ring? Later ones don't. The bellows type was superseded soon after its use in the Magnette because it was found to be susceptible to the varying water pressure in the system. Higher pressure would push on the bellows and partially close them. The later wax pellet type was better at resisting this, but had no blanking ring. Modern thermostats tend to be quite short and do not close off the by-pass either. Unfortunately, this means that even with the engine at full working temperature, some of the water is going to be recirculated straight from the head back into the engine. In low temperature weather this is unlikely to be a big problem but in high temperatures, just when you want all the cooling you can get, some of the water does not get cooled and this can lead to overheating.

The same thing happens if you take the thermostat out in hot weather. It is common for racers to take out their thermostat to ensure maximum water circulation, but, of course, engine warm up takes much longer because the whole contents of the system have to be heated before operating temperature is reached. To avoid the recirculation by-pass problem they fit a blanking sleeve in the thermostat housing. This is full depth, so the by-pass is closed permanently and all water passes via the radiator at all times. These blanking sleeves fit all B-series engines, so are readily available from MGA & MGB spares specialists. The picture shows a Moss version.

On the basis of the above, I have reached the following conclusions:

1. that it is unwise to remove the thermostat permanently unless you are operating in extremely high temperatures, in which case, a blanking sleeve is desirable.

2. if you have an original-type thermostat with the blanking ring, don't throw it away just because it is old. Keep it as long as you can. A leaking bellows is the only likely failure and they were built to last.

3. If you see an original-type thermostat for sale, buy it. Even if it is second-hand, there is a very good chance that it is still serviceable. You just need to check that it is designed to operate at the right temperature.The workshop manual states that the thermostat is set to operate between 80 degC and 85 degC, which I think indicates the spread between the temperature at which it begins to open and that at which it is fully open.. On the Smiths thermostat, the operating temperature appears at the end of the part number after a "/" (e.g. A12345/80). On the AC equivalent, the operating temperature is stamped on the top, along with type number TF2. When you get it home, you can check its function in a pan of water. It should open just before the water comes to the boil. (Borrow the co-driver's jam thermostat for total accuracy).

Original type thermostats are difficult to find now and you may be a bit dubious about fitting something that old, even if it is NOS. Fortunately it is possible to achieve a compromise solution. You can fit a blanking sleeve WITH a modern thermostat. The thickness of the cork housing gasket is normally enough to accommodate the two rims at the top of the housing. However, to allow partial circulation during warm-up, you need to allow a bit of water past the thermostat, even when it is closed. This is achieved either by drilling the top of the thermostat or, if it is the type that already has a hole with a little toggle in it, just cut out the toggle. Warm-up will still take longer but the water in the head can move. There is evidence in bulletin boards and articles that this compromise solution is being used by a growing number of MG owners.

The blanking sleeve will have holes in the side. These are only to let air bleed through when you are refilling the system, so avoid positioning them over the by-pass slot. Position them either side of it.