With out circulation at the thermostat, the water in the block is going to heat up faster then the water in the thermostat housing. It is going to take some time to transfer heat to the thermostat (remember, no circulation), so while that is going on, the block is going above the thermostat setting. Once the thermostat has enough heat to open, the rush of hot water will cause the radiator to spill out due to high heat and pressure. After a brief time the system averages out and works as normal.
No blockage, or broken parts, just a incomplete system.
If the heater hoses were attached, or the heater core bypassed with a loop hose, this would not be a issue. Water would circulate around and under the thermostat creating a smooth even heating.
The owner created his own bypass with three small holes. Its working, he's happy, and the truck is happy. The only down side to drilling a lot of holes is you can slow down the heating up of the engine.
What's so hard to understand?
Joe
Joe is exactly right with his description. This yoyo effect is called thermal shock and it can cause some serious problems that have not been discussed here. Most engines suffer from this problem to some degree, but Stovebolts are especially prone to it because of the thermostat location as Joe described.
Drilling holes in the thermostat may not be a very elegant solution but, it is effective. In fact my aluminum radiator came with instructions for doing exactly that to prevent thermal shock from damaging the radiator. Also Ron Davis Radiators states in their installation guide "Aluminum radiators must not be subject to pressure surges that result from air pockets in the engine. When air surrounds a cylinder or area next to a combustion chamber, the metal becomes very hot. When water comes in contact with the hot metal, a volume of steam is produced that is larger in pressure than the cap can release. The resulting pressure bulges the tubes in the radiator and reduces the airflow and cooling capacity."
This thermal shock works the other way too and can wreak havoc with the engine. That is why GM installed a crude but effective 2 stage or bypass thermostat on the 261 engines. Since the cylinders are Siamesed on these blocks there is no water flow between them. This resulted in steam pockets building up which produced localized hot spot that cracked blocks and heads. The cure was to drill steam holes between the block and head and install the afore mentioned bypass thermostat thus insuring constant water flow through the engine. Thermal shock can also cause abnormal and excessive wear on the cylinders and rings. Engines are designed to run in a specific temperature range and running them too cold is nearly as bad as too hot. Anyone who has torn down many engines and measured the wear on them knows that the front cylinders almost always show more wear than the rest. This is because they are operating at a lower temperature than the others. In fact some savvy engine builders hone the front cylinder slightly larger to compensate for this. On my 1958 Apache 235 I have seen the temperature climb to 220 degrees (195 degree thermostat) on initial start up in the morning. Then I have watched it plummet to 160 degrees in a matter of seconds as the thermostat finally opens and a slug of cold water rushes into the engine. I can only imagine what is happening to the front cylinder when this happens. Cold climate or a mechanical fan or an electric fan that runs constantly will amplify this problem.
These are some of the reasons all modern vehicles have some type of bypass thermostat or heater bypass system. Since I am using a Robert Shaw "balanced thermostat" that doesn't easily lend itself to being drilled and the fact that drilling the thermostat slows engine warm up I decided to use a more modern fix. I installed a " AC Delco 15-5675 Heater Bypass Valve" on my truck. It is inexpensive, OEM quality, and easy to install.
I apologize for the long post and the info is worth exactly what it cost you, but it is based on many years of working on classic cars and some of the most famous race cars in the world. As with all the info you read on the internet you should do your own research before you decide what if anything you should do.
Thanks, Diceman
