I didn't start drilling until I was a mechanic, and it was to allow faster filling. The hole pictured is too big, all it needs is a small one to let air pass. Many FWD and Asian car thermostats have a 'jiggle valve' built in.


With the amount of coolant in a typical V8 cooling system, I figure the amount bypassed is tolerable, and as the previous poster said, it allows the flow of heated coolant to open the 'stat a bit quicker. If you are taking your time and can let the system burp all the air out before running, you can install it without this mod. My is that if you are in a hurry and drill a small hole, you aren't going to cause any harm, provided it's properly located, you make sure all drill shavings are removed, and you don't damage the stat during the process.