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Fuel Cap

E. D. Eliot

Well Known Member
I have a fuel cap that came with my finish kit in 2017. Do I need to drill it out or not in order to properly vent the fuel tank. Please advise. Thanks, Ed
 
Do not drill holes in the fuel cap.
The blue print, {Plans} of the wings should show the fuel vent line routing from the wing tanks back to the fuselage.
 
It depends

Obviously, or maybe not,...it depends on which model you're building.
I'd re-read the instructions, vs. relying on this forum.
The answer HAS TO BE in your instructions.
 
RV12 fuel tank vent

Originally with the RV12 classic, the fuel tank was vented with a hole drilled in the gas cap. Unfortunately this some times spit gas on the rear window and damaged it, especially the original flat cold bent lexan rear window. (The rear window was changed to molded acrylic.)

So, Van’s changed the vent system with a separate vent line. With this new vent system the fuel cap is not used to vent the tank and is therefore not drilled.

See the latest version of Section 37 - fuel tank. Can be downloaded from Vans web site.

https://www.vansaircraft.com/service-information-and-revisions/?aircraft=rv-12&doctype=all

Not sure what the RV12is does.

-Dave
 
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It?s pretty simple. No vent line means you either put one in as per the latest design or drill the cap per the original design. I?ve been flying 8 years with a hole in the cap and no vent line with no issues.
 
Originally with the RV12 classic, the fuel tank was vented with a hole drilled in the gas cap. Unfortunately this some times spit gas on the rear window and damaged it, especially the original flat cold bent lexan rear window. (The rear window was changed to molded acrylic.)

So, Van?s changed the vent system with a separate vent line. With this new vent system the fuel cap is not used to vent the tank and is therefore not drilled.

See the latest version of Section 37 - fuel tank. Can be downloaded from Vans web site.

https://www.vansaircraft.com/service-information-and-revisions/?aircraft=rv-12&doctype=all

Not sure what the RV12is does.

-Dave


The RV-12iS has a vent system and does not have a cap vent hole.
 
Why not drill the cap too? A number of RV-12 drivers have had fuel pressure issues with a totally full tank on climb out. Theory is that fuel upon climb out blocks the new style vent. I drilled the fuel cap as a backup vent and never had that problem again.
 
Why not drill the cap too? A number of RV-12 drivers have had fuel pressure issues with a totally full tank on climb out. Theory is that fuel upon climb out blocks the new style vent. I drilled the fuel cap as a backup vent and never had that problem again.

Sounds like a good idea to vent cap in addition to filler neck vent. I have never seen fuel splash from the cap vent hole on my early S/N 12. A post above said splash from the vent hole was cause for crazing of the Lexan window - never saw that. When filling tank with large diameter nozzle it's important to allow air to vent past the nozzle so you don't get a burb of fuel coming up at you - that I have seen...
 
The only downside I see to a drilled cap is water intrusion. I have been surprised to see how easily it gets in when my 12 has been on the ramp overnight in the rain during cross countries. The good news is that it seems to migrate to the gascolator rapidly and is identified during preflight the next morning. I did not expect that.
 
The only downside I see to a drilled cap is water intrusion. I have been surprised to see how easily it gets in when my 12 has been on the ramp overnight in the rain during cross countries. The good news is that it seems to migrate to the gascolator rapidly and is identified during preflight the next morning. I did not expect that.
All RV canopies will leak. That is why you always put a canopy cover on if there is any chance of rain. The standard cover extends far enough to amply cover the cap.
 
The only downside I see to a drilled cap is water intrusion. I have been surprised to see how easily it gets in when my 12 has been on the ramp overnight in the rain during cross countries. The good news is that it seems to migrate to the gascolator rapidly and is identified during preflight the next morning. I did not expect that.

If the cap is oriented in such a way that the latch cavity is angled downward so that water can easily drain out, in nearly 100 % prevents this.
 
If the cap is oriented in such a way that the latch cavity is angled downward so that water can easily drain out, in nearly 100 % prevents this.

Scott is your comment an endorsement for drilling a tiny hole in fuel caps to make them vented? I'm flying RV-7A and the fuel vent lines are working fine, but it does seem like a good idea to have a backup vent if a bug smacked on the vent line and plugged it.
 
Scott is your comment an endorsement for drilling a tiny hole in fuel caps to make them vented? I'm flying RV-7A and the fuel vent lines are working fine, but it does seem like a good idea to have a backup vent if a bug smacked on the vent line and plugged it.

This is the RV-12 forum
The fuel cap on the RV-12 is almost horizontal on the side of the fuselage, so that was the context of my post..... that on an RV-12, if the cap is oriented so that water can drain out, the likelihood of water entering the tank that way is very remote.

I would not drill a hole in the cap on a wing tank. That would make the possibility of water entry quite likely.
 
Lexan window crazing due to fuel splash?

I have a fuel cap with no vent hole and a tank vent line internal to the fuselage. The internal vent line was a design change that became available while I was building, probably 2015, so I put one in. Despite that I still developed crazing of the Lexan rear window near the filler neck starting after about 3 years in service. I am sure the window was still structurally sound since the pressure loads on that part of the fuselage surface are pretty small, but it got ugly enough that I recently replaced the Lexan window with the current plexiglas part ($500). A pretty tedious task. I couldn't really tell why the crazing showed up. Exposure to fuel vapor is the only culprit that I am aware of but there was not an obvious source.
 
I had crazing develop on the old flat lexan sheet within a couple of years. I finally bit the bullet 3 or 4 years ago and bought the molded plex upgrade. No problems since. I think the residual stress from bending the lexan to the window frame is a major factor. The molded window eliminates that issue.
 
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