Join Date: Oct 2014
Location: Southeastern PA
Thanked 11 Times in 9 Posts
I checked my bike last night and here is what I found. MacBandit, you are absolutely correct on how the stock cap, vent, and drain works. Both drain and vents go out of the bike through the rubber hoses underneath the tank and out to the ground. In some model bikes (probably all by now), the vent goes through the charcoal canister for EVAP like you mentioned.
For specifically the TWM cap we have, the vent/drain tube on the base can technically go into either hole as long as it fits. For my bike, the tube will not fit down the vent tube either as it is too small of a diameter like BMila01's bike. Therefore, I used the drain hole as TWM recommends. Based on how the cap base is designed, it looks like it is meant to be a vent and a drain for overflow/water. The underside of the base of the TWM cap (the part that gets screwed onto the top of the tank) has a large rubber washer that is meant to "seal" the filler neck of the tank from the lower drain area. (I say "seal" because it's definitely not a 100% watertight seal, but it would prevent overfills from leaking into that part of the tank area). The fuel would then rise up to the drain/vent hole in the new cap base and drain out.
This leads to question of where liquids from an outside source (rain, hose, super-soakers!?) will drain out if the tube from the new cap goes down the drain tube hole in the tank. I water-tested my fuel cap last night by pouring water all over the tank and fuel cap and letting it sit for about 10-15 minutes. I opened the cap and then unscrewed the base to see if any water got inside. It was absolutely dry and the drain did its part. My bike has also sat out uncovered during 2 rainy (pouring) track days in between sessions, and I have not had an issue then either. Of course, this doesn't mean that is 100% waterproof and everyone should trust me. I am just describing my experiences with the TWM cap.
ON the other hand twist lock caps could work if the outer piece had a machined well around the cap itself so water could drain around instead of being trapped above the seal.
The cap for the TWM is pretty close that what you are describing. The cap fits tight but there is a very, tiny, tiny, gap around the cap that allows standing water to drain out of the vent/drain hole of the cap base. The O-ring of the cap itself also helps keep water from entering the filler hole.
My experience with my TWM fuel cap may not be the same for others, but I can say that it works for me as expected.