Fuel Tank Fittings
Posted: Sat Jul 18, 2020 10:41 amIt seems that fuel tank fitting leaks are one of the more common issues that I've read about since starting my project. The Sonex Fuel Tank Installation and Warranty guide states, "Do not over-tighten the fittings as they can break an insert loose in the tank". "It only takes a snug, not tight, fit to achieve a leakproof installation". I have a problem with subjective terms like "snug" and "tight", they're not measurable. What's snug to me might be tight to someone else.
I decided to make the torquing of the fittings measurable and ran a series of tests with different Loctite products. I chose Loctite products because I was introduced to a Loctite pipe sealing product called PST back in the 1980's and was extremely impressed. For my test I used Loctite 545, 565, 567 and 5452; Loctite 565 is similar to the Loctite PST of the 80's and Loctite 5452 is designed for hydraulic fittings and is pretty expensive. To better replicate actual conditions I used several 1/8" NPTF raw aluminum couplings with brass 1/8" NPTM fittings.
After applying the four different Loctite products to the brass fittings I screwed them into the aluminum female fittings. I torqued the first set of fittings to 10 inch pounds using a torque wrench (10 inch pounds is basically hand tight). I then torqued the next set of fittings to 15 inch pounds the the last set to 20 inch pounds. Once finished I let all of the fittings set-up for 5 days. After 5-days I connected the fittings to a fuel line and to a lawn mower gasoline tank and filled the tank with gas. I let the fittings sit for a week while making several checks a day looking for leaks.
THE RESULTS: After one week had passed I was surprised to see that NONE of the fittings had leaked, not even the hand-tight 10-inch pound fittings! I decided to use the Loctite 545 for my Sonex fuel tank since that's what this product was basically designed for. I then proceeded to install the brass fittings in the Sonex tank and decided on 15-inch pounds as my target torque setting. This was easy to do with the straight fittings (such as the finger strainer and petcock) but was a little more difficult to achieve with the 90 degree fittings. It seems that when the 90 degree fittings were torqued to 10-15 inch pounds the fittings were facing the wrong direction. When they were facing the right direction the torque wrench was reading 20-30 inch pounds.
Bottom line. I filled my tank with gas and had zero leaks, not one. My take away from this test is that keeping the fittings torqued to very low torque settings, such as 10-inch pounds, does a great job of sealing. A little more torque is still OK but I found no need to torque past 15-inch pounds to insure a no-leak situation. I hope this helps someone.
Steve
I decided to make the torquing of the fittings measurable and ran a series of tests with different Loctite products. I chose Loctite products because I was introduced to a Loctite pipe sealing product called PST back in the 1980's and was extremely impressed. For my test I used Loctite 545, 565, 567 and 5452; Loctite 565 is similar to the Loctite PST of the 80's and Loctite 5452 is designed for hydraulic fittings and is pretty expensive. To better replicate actual conditions I used several 1/8" NPTF raw aluminum couplings with brass 1/8" NPTM fittings.
After applying the four different Loctite products to the brass fittings I screwed them into the aluminum female fittings. I torqued the first set of fittings to 10 inch pounds using a torque wrench (10 inch pounds is basically hand tight). I then torqued the next set of fittings to 15 inch pounds the the last set to 20 inch pounds. Once finished I let all of the fittings set-up for 5 days. After 5-days I connected the fittings to a fuel line and to a lawn mower gasoline tank and filled the tank with gas. I let the fittings sit for a week while making several checks a day looking for leaks.
THE RESULTS: After one week had passed I was surprised to see that NONE of the fittings had leaked, not even the hand-tight 10-inch pound fittings! I decided to use the Loctite 545 for my Sonex fuel tank since that's what this product was basically designed for. I then proceeded to install the brass fittings in the Sonex tank and decided on 15-inch pounds as my target torque setting. This was easy to do with the straight fittings (such as the finger strainer and petcock) but was a little more difficult to achieve with the 90 degree fittings. It seems that when the 90 degree fittings were torqued to 10-15 inch pounds the fittings were facing the wrong direction. When they were facing the right direction the torque wrench was reading 20-30 inch pounds.
Bottom line. I filled my tank with gas and had zero leaks, not one. My take away from this test is that keeping the fittings torqued to very low torque settings, such as 10-inch pounds, does a great job of sealing. A little more torque is still OK but I found no need to torque past 15-inch pounds to insure a no-leak situation. I hope this helps someone.
Steve