OEM Thermostat Pre-drilled?

I've been experiencing low heat output from my heater and have been troubleshooting the problem using the guidance on these forums. I'm down to flushing the heater core and replacing the T-stat. I've read where many people have optioned to drill a hole in the flange of the replacement T-stat for closed cooling systems at the 12 and 6 positions to aid in burping the system. I just picked up an OEM T-stat from the dealer for my XJ (1988, 4.0L, 200K) and noticed that the flange has a hole drilled at the 12:00 position with a "floating ball" in a tri-crimped housing attached the the back (engine) side. I'm assuming that this is a one way ball valve, but am noy sue of the purpose. Is this a mod my Mopar to aid in bleeding the system of air? Would this work for a closed system, or do I still need to burp from the back of the block?

That is a "burp" valve, used to free air trapped behind the thermostat when the RENIX system is filled. I do much the same thing - I just use Stant instead, and drill a pair of 1/16" holes at 1200 and 0600 on the dial. Install with one hole "dead up" and one "dead down," and the system can purge itself of air (air passes out of the engine through the top hole, and water passes in through the bottom.)

The purpose of that little "poppet" - or whatever they're using this week - is to break up air bubbles so you don't get a logjam at the one hole. That's why I just drill two, instead.

If you haven't installed it yet, take a moment to drill another 1/16" hole at 0600, and that should make it far simpler to burp the engine.

And, don't neglect vacuum as a problem - the plenum "switch" doors are vacuum controlled - and I think the blend door may be as well (I'd have to check to be sure.)

5-90