So no one has mentioned the downside of metal tanks.
They are vented. They had to be, but not vented like today's high quality plastic tanks. I'll get to the vent in a minute.
metal does an excellent job of allowing condensation to form inside the tank. This is especially true if the tank is allowed to sit out in the elements and where temps can change drastically. Like here today, this morning it was 30°, this afternoon's high was 69°. Almost a 40 degree change. As the tank get hot, then cools off, water vapor in the air condenses.
With the vent working both ways (allows air in and air out), condensation occurs faster and more severe. Also, today's fuel pretty much sucks. 40 years ago when metal was the only option, gasoline would not evaporate like it does today. Take a fully vented full tank of fuel and let it sit outside for a few days. I bet you money that some of it will be gone. During the evaporation process, the fuel cools off, which also invites condensation to form in the fuel. You can see this quite well by using a quart mason jar about 3/4 full of fuel, lid removed, and let it sit out for a day or so. Even in a few hours you will see the effects, I tested it a bunch of times at work and on hot humid days, you could watch condensation form on the outside and inside of the jar. In about 2 hours, about 30% of the fuel had evaporated and there was an ounce or two of water (condensation) in the bottom of the jar.
This is why the "new style" fuel tanks were invented. Well that's one reason, the other was EPA regulations requiring low permeation materials so that fuel would not permeate through the material and into the atmosphere. Down side is that low-perm material is more expensive. BUT the venting system is a little different in that it will allow air to come into the tank as the fuel is used but won't let pressurized air escape. This is why the tank swells. The cheapie (attwood, etc) tanks will swell up a LOT because the material is thin. I do know that an OEM Yamaha tank is a good bit thicker material and therefore doesn't swell nearly as much. Also, since the low-perm tanks don't vent internal pressure to the atmosphere, they won't allow evaporation, and don't allow condensation to form inside. I do not know of the other OEM tanks ('zuki, BRP, Merc, etc) so I cannot compare them. When I sold Merc (brunswick) they were all metal and a plastic tank was not even available yet. When we dumped Merc and went to Yamaha exclusively, we started seeing plastic tanks and thought that it was a move in the wrong direction. But there were no issues with them. We went to Yamaha for many reasons and the statement was made by a fellow tech that we would be working on more outboards. Nothing has been further from the truth. By Y2K they were mostly plastic. In 4/2011 the low-perm tanks started showing up. I thought they were going to be junk. But I was wrong. They are heavy and of a material that is pretty tough. Also, the new EPA compliant fuel lines are of better quality than the wal-mart lines are-they don't harden up. You pay more, but you get better quality. A new 6 gal tank and line is around $120. But figure in what an Attwood costs and an Attwood line costs, and multiply that by 2 or 3 times that it has to be replaced. They've gotten better, but still nowhere near the quality of the stuff that I have to deal with from Yamaha.
