pawoSD. The 420 is very different from mine. On the 500 the thermostat is in the housing on top of the waterpump. On the 420 it is on the side of the water pump. That change was made in the 86 my. The thermostat that came out and the one that went in were both Behr made in Germany. In my testing I installed an aftermarket gauge in the front of the intake. It clearly showed that the engine temp was going higher than the original gauge was reading. The original gauge sensor is in the same housing as the t-stat. The original gauge would start going up and at the same time the add on gauge would start going down. They would meet and go down together to a normal reading. This told me the t-stat and sensor was not seeing the real engine temp when the t-stat was closed. Drilling the 2 holes made the 2 gauges read the same (well close) from cold to normal temp of around 185 with no spikes. If there is something wrong with the system that I just covered up, I would love to know what it is and fix it. However, for now every thing is working as it should and I feel safe driving it. The only drawback I can think of with the holes in the t-stat is that it may not warm up as quick. So far, I can't tell any difference in when I get heat. The normal highway temp is the same as before.

Paul

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84 500 SEL (307,xxx miles)