For many MGB and MGA owners, one of the nagging issues that keeps them from enjoying their cars to the fullest is that the cooling system seems to be inadequate, especially in hot climates. It is true that these cars were designed in an area where temperatures above 90 degrees F. were seldom encountered, and many folks have attributed their cooling issues to the cars' designers not having taken the higher temps in markets like the USA or Australia into account. This is simply not true. These cars were tested under hot climate conditions before they ever entered these markets and found to have adequate cooling systems.

 

The real problem evolved over the life of the BMC/BLMC B-series engine which both of these cars shared. A key factor that has been lost sight of over the years was that the design of this engine as originally conceived was that it was intended to use a rather different thermostat than we have been used to.

 

This thermostat was a bellows design (see 1st photo below) which had a movable sleeve around its bellows which moved up as the thermostat's main valve opened in response to rising engine heat. This movable sleeve, when the engine was cold, sat at a position that allowed a coolant by-pass passage in the head (see 2nd photo below) to remain open, allowing warming coolant to recirculate back into the head and engine while the main thermostat valve blocked coolant from going into the radiator. This setup was used to help to speed up warming the coolant to the engine's operating temperature. As the coolant temperature rose closer to this ideal operating temperature, the bellows type thermostat door opened, allowing coolant to begin circulating through the radiator, while at the same time moving the sleeve up into place to block the bypass passage, shutting off its recirculation of the coolant before it entered the radiator.

 

These bellows type thermostats from Smith's ceased production before the end of production of the MGA and the more typical wax pellet type thermostat we have been familiar with for the last 40 years, was substituted. They were replaced as modern engineering increased the cooling system's pressurization. The higher system pressure played havoc with the functioning of the bellows type design as it relied on expanding gases to make it work, and pressurization counteracted that to the system's detriment.

 

For basic temperature control, the wax pellet type thermostat functioned OK, but they were missing that key sleeve, and from that point forward, coolant would be able to re-circulate into the head and block via the bypass at will and to some extent always defeating the flow of coolant through the radiator. Although not necessarily overly significant with a perfectly operating clean/brand new cooling system, its effect would become a more meaningful contribution to overheating in systems that progressively became more contaminated with scale, rust, and sludge which reduced heat transfer from the head and block and into the coolant. In fact, any factor which reduced the cooling system's efficiency would make this shortcoming a bigger factor in defeating the cooling system's original design and reducing the margin of safety built into the system.

 

The original Smiths' bellows type thermostats are almost impossible to find these days, and I am not sure I would trust a 40 year old NOS unit for too long anyway, not only because of age, but because of the pressurization issue mentioned above.

 

In an attempt regain the advantage of shutting off the by-pass circuit when the engine is warmed up, a very perceptive Australian, Neal Cotty, took a chance to use the factory optional blanking sleeve (originally intended for full flow racing cooling only, with NO thermostat ~ see 3rd photo below) together with a pellet type thermostat. In short, his experiment worked very well, but his only reservation was that he was fearful that in colder climes than his native Australia, it might force the engine to warm up too slowly or, possibly, not at all.

 

This is where I stepped into this experiment by being willing to turn my 68 BGT into a test bed to test his concerns. It definitely gets colder here in the Pacific Northwest in winter than Australia, and like Neal, I too was a bit apprehensive. However, if empirical results are valuable, my experience with this T-stat/blanking sleeve combination did not create as much of (if any) problem with engine warm up during cold weather as anticipated. Yes, my engine does take a bit longer to warm up, but the difference is about ½ to 1 mile more of travel to reach normal when the temps are below about 30 degrees. That is significantly less than the difference I experienced before I started using a cover over the oil radiator in winter! And I have been using this combination since about 1999 and have had no reason to ever look back. It works very well for me as well, and probably, closer to the engine designer's intent, than what we have been generally doing for so many years.

 

The success I had with this combination inspired me to look into the possibility that the choices among modern thermostats might also contribute to allowing better, and better controlled, cooling. I had been made aware of what was called the "balanced" thermostat designed by Robertshaw Controls (now marketed by Prestone ~ see 4th and 5th photos below) by my hot rod buddies and so I got one and used it instead of the common pellet type. Although I will not say that its effectiveness is entirely obvious when you drop one in, it does become a bit more plain when driving at sustained highway speeds (70+ mph) as the temp does seem a bit more stable and when under the strain of mountain climbing, it takes longer for the inevitable rise to occur. The balanced design is such that it is not influenced by coolant pressure increases or decreases as engine rpms rise and fall, and that means that it is better able to more accurately control temperature than pellet type T-stats.