(this is a post about personal experience, not an update from Bob)
A few months ago I made a set of fuel sight gauges out of the rigid PVC tubing as described here:
https://bearhawk.tips/3789
If there's a way to do something wrong, I'm definitely capable of finding it. In this case I found a few ways, so I thought I'd share. The first symptom was that I was getting a little whiff of fuel smell in the cabin every now and then, and I hadn't been able to find any leaks. Yesterday it was time to refuel, and I filled the left tank to around 3/4. On the next flight when briefly flying uncoordinated, I got a really strong fuel smell. Turns out the sight gauge was leaking at the top due to a crack. When the plane was uncoordinated, it allowed the fuel to come over to the wing root and fill the gauge.
The root of the problem is that pipe threads are tapered, which makes male fittings function as a wedge. The tap is also a wedge. After talking with Bob, here are some things that I did wrong when making them. Making a set of gauges requires cutting four sets of threads. When I did the first one or two, I operated the tap by hand. Then I was kind of lazy so for the last two I chucked up the tap in a handheld drill. In the latter case especially, I quite likely was not removing material cleanly, and thus was actually stressing the plastic outward, while also not cutting very deep threads. If the plastic spreads, then the threads are shallow, making the wedge function of the fitting worse.
When it came time to install the fittings, the threads have to be sealed with something, as all pipe threads do, so I used fuel lube. The down-side of this is that it further reduced friction and made it even easier to overtighten the fittings.
Bob says he has used plastic fittings in this location instead of aluminum. In my next version I think I'll still use the aluminum fittings, but I'll do a better job of getting good threads cut, and also less tightening. I might also try some type of clamping during the tapping operation, to help ensure that I'm cutting threads and not just bell-mouthing the plastic end.
In my case the crack was at the top of the gauge which made it obvious only when the fuel level at the wing root was high. This would have been much less convenient if I had been away from home and filled the tanks all the way up. There were some warning signs such as similar-looking marks at the tube ends. Only one has developed into a leaking crack so far, but I suspect it's a matter of time and I'll be replacing both tubes.
A few months ago I made a set of fuel sight gauges out of the rigid PVC tubing as described here:
https://bearhawk.tips/3789
If there's a way to do something wrong, I'm definitely capable of finding it. In this case I found a few ways, so I thought I'd share. The first symptom was that I was getting a little whiff of fuel smell in the cabin every now and then, and I hadn't been able to find any leaks. Yesterday it was time to refuel, and I filled the left tank to around 3/4. On the next flight when briefly flying uncoordinated, I got a really strong fuel smell. Turns out the sight gauge was leaking at the top due to a crack. When the plane was uncoordinated, it allowed the fuel to come over to the wing root and fill the gauge.
The root of the problem is that pipe threads are tapered, which makes male fittings function as a wedge. The tap is also a wedge. After talking with Bob, here are some things that I did wrong when making them. Making a set of gauges requires cutting four sets of threads. When I did the first one or two, I operated the tap by hand. Then I was kind of lazy so for the last two I chucked up the tap in a handheld drill. In the latter case especially, I quite likely was not removing material cleanly, and thus was actually stressing the plastic outward, while also not cutting very deep threads. If the plastic spreads, then the threads are shallow, making the wedge function of the fitting worse.
When it came time to install the fittings, the threads have to be sealed with something, as all pipe threads do, so I used fuel lube. The down-side of this is that it further reduced friction and made it even easier to overtighten the fittings.
Bob says he has used plastic fittings in this location instead of aluminum. In my next version I think I'll still use the aluminum fittings, but I'll do a better job of getting good threads cut, and also less tightening. I might also try some type of clamping during the tapping operation, to help ensure that I'm cutting threads and not just bell-mouthing the plastic end.
In my case the crack was at the top of the gauge which made it obvious only when the fuel level at the wing root was high. This would have been much less convenient if I had been away from home and filled the tanks all the way up. There were some warning signs such as similar-looking marks at the tube ends. Only one has developed into a leaking crack so far, but I suspect it's a matter of time and I'll be replacing both tubes.
