I have one of those Rev9 aluminum expansion tanks and I've heard some people say you need to drill a hole in the cap for venting. I have not done this yet, am I doing any damage by not venting it? or is it ok the way it is? thanks!

 

I had the same concerns as you did till I found this article... it covers expansion tanks vs overflow tanks pretty well, in addition to in depth info in how cooling systems work, it's gets into the weeds pretty good.
https://motorverso.com/radiator-overflow-tank/

 

The stock coolant reservoir is vented so that the reservoir is at atmospheric pressure. The stock radiator cap is designed around that and won't open and close at correct cooling system pressures if you seal the reservoir.

There are actually two spring-loaded valves inside the radiator cap. The big spring clamps the seal shut against the radiator and won't allow fluid to escape until the spring is pushed off the seat at 16 psi radiator pressure or whatever is the rating of the spring in your cap.

The small spring is another valve that opens in the opposite direction when the radiator pulls a vacuum. That allows fluid to dump back into radiator from the reservoir.

If the reservoir is sealed, then it's going to have increased pressure in the reservoir after the first time the radiator dumps to it. That's going to upset the balance of forces on the valves inside the radiator cap, and radiator purge and fill will happen at different pressures than intended. Pressure in the engine cooling system will INCREASE. By how much I'm not sure without being able to measure parts and run the math. I just know it won't operate like intended.

Vent the tank.

 

my #Rev9 Expansion tank for 3rd Gens arrived today...
AND it's made in f-n #CHINA 🤬

Needless to say this CCP Crapola from Rev9 is going back.

I will never buy anything from Rev9 again.
https://rev9power.com/aluminum-coola...2-3rd-gen.html