New Finish Kit Fuel Cap

I just received my Finish kit. The fuel cap still was shipped with instructions to drill out a vent hole in the cap. I thought with the new Vent kit, also shipped with my kit, drilling out the cap was not needed.

I think you're right. I just picket up my SLSA and the fuel cap is not vented.

Does anyone know if the fuel cap requires a hole drilled for venting if the new tank vent system is not utilized? I have installed the fitting for the new vent but may not hook it up at this time.

Do Not Drill Fuel Cap

With the fuel tank vent line installed there is no need to drill fuel cap...

D&M Dan said:

Does anyone know if the fuel cap requires a hole drilled for venting if the new tank vent system is not utilized? I have installed the fitting for the new vent but may not hook it up at this time.

Click to expand...

If you do NOT utilize the new vent line, the cap MUST be vented.
Is there a good reason I am unaware of why one would not use the vent line?

Early RV-12 kits did not have a separate fuel tank vent line. The early design utilized a couple of holes drilled in the cap for fuel tank venting. Apparently, Van's Aircraft considers either design to be acceptable although current kits have the separate tank venting system (i.e., with a sealed fuel cap).

Larry, l have one of the early kits and I was unsure if I wanted to add the optional venting. I think I will however so as not to drill the cap. Thank you for the replies.

Cap vent

I have a fairly recent kit,797 and have the tank vent. I didn't drill the vent in the cap originally. I have had three minor "events" during takeoff ,a strong fuel smell in the cockpit. The engine did not skip or stumble, just smelled fuel. I decided to drill the vent holes in the cap. I did, however drill with a #43 drill and tapped 4-40, so I can plug the hole with a 4-40 screw if I find I don't need the vent hole. I have 4 flights (6.2 hours) with the cap vented and no fuel smell yet. I will update as I accumulate more hours.

Kept both

I added the vent last year but kept the vented fuel cap instead of replacing it with a new sealed one. Interesting idea to tap it and put a screw in the hole. Advantage of vent is when filling the tank, less chance of fuel blow back (and getting on the back window) if it is being filled too fast. Doesn't entirely eliminate that risk, but improves it. If the new vent gets plugged by insects etc... I feel better having the original cap as an alternative means to vent the tank.

Saw a guy taping over an RV-12 cap at Oshkosh a while ago to prevent water from leaking into the tank when parking outside for several days. Probably only downside to keeping the vented cap.

While we're on the subject of vented fuel caps, I make it a point to physically blow through the fuel cap vent holes as part of my preflight inspection and/or during refueling operations. Just making sure that no critter has taken up residence in my cap.

One drawback to installing the vent system is that I now have a much stronger fuel smell in my hangar. Never had it before I installed the vent so it looks like the vent really vents the fuel vapor.

If I could do it over again, I would just stay with the vented cap.

I intend to keep the vented cap and not install the vent tube. Reason for this is to minimize air / vapor exchange when the airplane isn't flying. I run 93 Octane E10 auto fuel and don't want to have alcohol in the fuel absorb moisture from the air. Alcohol is very hydrophilic (“water-loving”). Also best to have fuel tank nearly sealed to prevent degradation of octane rating over time.

I certainly agree that a different fuel tank venting system is needed. I installed the new venting system recently. I have an automatic shut off on my fuel nozzle and found that now the shut off worked as before except that now when the shut off activated a small amount of gasoline would spew out the new venting system on to the floor each time I filled the tank. To stop this from happening, I plugged the new fitting on top of the tank and installed a new vent fitting in the elbow just above the level of the end of my fuel nozzle.

I stuck with the original tank and cap design. I saw no reason to mess with success. No vent lines, Moeller gages or frangible bolts on my tank. Sorry ,Mom, no tank mods and I still run with scissors!

If you need to know what your true tank level is use a dip tube. Just stick a flexible nylon tube down the filler neck, put your thumb over the end and pull it out

Now I am confused. Is this mod worth the time and effort or should I just drill the cap and close off the vent fitting I installed while the tank was apart?

2johns said:

I certainly agree that a different fuel tank venting system is needed. I installed the new venting system recently. I have an automatic shut off on my fuel nozzle and found that now the shut off worked as before except that now when the shut off activated a small amount of gasoline would spew out the new venting system on to the floor each time I filled the tank. To stop this from happening, I plugged the new fitting on top of the tank and installed a new vent fitting in the elbow just above the level of the end of my fuel nozzle.

Click to expand...

John, are you saying that your vent is now in the filler neck, near the top? This sounds like a simple and viable solution. That vent line could be routed out the bottom of the airplane, and fuel from over filling, or expansion would exit this path. In fact, why couldn't you use a sealed cap with this venting method?? Sure sounds like a good fix to me.................Tom

D&M Dan said:

Now I am confused. Is this mod worth the time and effort or should I just drill the cap and close off the vent fitting I installed while the tank was apart?

Click to expand...

You will need to decide whether the benefits of the new vent outweigh the perceived detriments. It is not mandatory to install the new vent design. The biggest benefit I think is that it allows more air to escape the tank during refueling and lessens the chance of significant blowback of fuel when the tank is nearly full. If the fuel gets on the cut edge of the rear window, it can cause crazing of the window. Many of us have replaced the rear window already.

The detriments, besides having more parts and complexity, is that the fuel smell is much more noticeable with the new vent. Since my -12 is always hangared at home and my hangar is attached to my home, the fuel smell is problematic, but not dangerous. My wife has a nose like a bloodhound and she can smell the fuel in the house a little.

Just my two cents. In my opinion, the frangible bolt change to the tank is a significant safety issue and should be made. I chose to keep the sight gauge and not install the Moeller gauge either. With my fuel computer adjusted properly, I always know what fuel is remaining in the tank within .25 GAL and I visually verify this via the sight gauge prior to each flight. These are just my opinions. Each owner must make their own decisions.

Cap vent

Update on the cap vent. I now have 50.9 hours total, with approximately 25 hours since venting the fuel cap( also have the tank vent as per latest version of KAI). I have not had any fuel smell issues in the cockpit since adding the cap vent. All previous occasions when I smelled fuel occurred after a shutdown/ passenger swap when the engine was at temperature. No reoccurrences since,. Just a Data point for those that might be interested.