Overheat Indicating Melt Tabs: The Whole Story
The melt tabs we put on all the cylinder heads and rebuilt engine long blocks are just the messengers reporting the truth: If they are melted, the engine was overheated. They are just the GoWesty messengers, and should not be shot! Hopefully, this article will shed some light on how these melt tabs function and why they should be trusted.
The first thing to understand is that 99% of the heat generated by an internal combustion engine originates in the combustion chamber, which is in fact very “internal,” hence the name “internal combustion engine.” That is the hottest point of the engine, where combustion temperatures are in the range of 1500 degrees F. Most of the heat exits via the exhaust, the rest through the parts surrounding the combustion chamber: Piston, upper cylinder wall, and cylinder head.
The heat that passes through the piston eventually gets to the coolant indirectly via the oil, but the heat coming through the cylinder and head goes directly to the coolant. If the whole system (thermostat, water pump, hoses, radiator, radiator fan) works as it should, coolant temperature stays around 200F, and never gets over about 220F, tops.
Overheat indicating “melt tabs” are commonly employed to determine if the coolant got too hot (engine was overheated). The entire “melt tab” is actually mostly an aluminum donut that does not melt, and contains solder in the middle “donut hole” that is designed to melt at a set temperature.
When the solder melts, it drips out and leaves a hollow center in the aluminum donut indicating the solder reached its designed melting point.
We install two melt tabs on the bottom surface of each cylinder head we build. These modified heads are sold individually, and are also utilized in each engine long block we build.
One common misconception is that the surface of the cylinder head is or can be hotter than the coolant, and that is why melt tabs cannot be trusted. But this is simply not possible when it is placed over a coolant passageway, which is where we place both. This is because the surface of the cylinder head over a coolant passageway is always cooler than the coolant, not hotter. This surface is located between the coolant (heat source) and the relatively cooler ambient conditions (heatsink). So, if the solder in the tab is melted away, the coolant must have gotten even hotter than the melting point of the solder. In this way, the melt tabs are a conservative indicator of the temperature the coolant actually reached: the coolant actually got hotter than the melting point of the solder.
If that is not enough logic to convince you that the solder melting is not lying and could be melting too soon, we also install two different threshold tabs that are both well over what constitutes engine coolant overheating. On top of that, there are two of each tabs on each engine: one each on both heads. All this is to say that the precise temperature at which they melt is not critical, and nobody is lying—unless all four melt tabs have grown brains and decided to conspire to fool us all…..
One of the melt tabs we install on each head has solder designed to melt at 225F, the other at 250F. If the 225F melt tab is melted, that means the coolant was hotter than 225F, which means the engine was overheated. If the 250F tab is melted, the coolant got over 250F, which indicates severe overheating. Again, these melt tabs do not lie, and they do not exaggerate. In fact, I personally have seen the temperature gauge (electronic ScanGauge) on my many of the Vanagons I have test driven and raced read over 225F, repeatedly for short periods (yes, I am hard on our own engines, on purpose…), and not melt the 225F tab. So I can tell you based on the how they are designed, installed, and from personal experience:
- If the 225F tab is melted, but the 250 melt tab is not: the coolant must have gotten over 225F, and for a sustained period. This sort of overheating often goes unnoticed, and does not necessarily result in the coolant boiling over or immediate engine damage. We refer to this as “good luck.” Even still, it could lead to an eventual, premature failure of the engine in the way of a failed water jacket seal or burned valve, for example.
- If the 225F and the 250F tabs are both melted, there must have been a very severe and noticeable overheating episode involving coolant boiling-over. This type of event typically does not go unnoticed. Depending on how quickly the engine is shut down, severe engine damage may or may not occur. When it does not, we call that “really lucky.” But once the coolant boils (turns into a gas), it is like a nuclear melt down: there is nothing to absorb the energy the 1500 degree combustion gasses are generating, and things go south fast. We call this a “Chernobyl event,” and rarely does it not lead to severe, if not catastrophic engine damage.
Clearly, if you put yourself in our shoes, you can see why we clearly state in our written, GoWesty engine warranty that melted tabs voids the warranty, even if only the 225F tabs are melted. Nobody can know for sure how close it got to melting the 250F tab, or for how long it was run in this condition.
The main takeaway here is this, the overheating being reported by a melted tab was not caused the engine itself. The engine’s job is to make power, which necessarily means making heat. It is the job of the cooling system to keep it the heat under control so the engine can continue to make power without falling apart. You can rest assured, if one or more of your melt tabs is melted, something in the cooling system must have failed to do its job at some point in the past, pure and simple.
Related articles:
Cooling System Overhaul: A $3 Hose Takes Out a Fresh Engine!
A GoWesty Engine Warranty: Is it Legit?
