Gas tank/exhaust pipe clearances ?

[Donaldo]

Seems I am running the 3" exhaust under the tank - it is in the rear section of the bed. I have about 1.25 inches of clearance, if I place a heat shield mid way, the length of the tank, will that be enough to omit the bomb ?

[oldman3]

I did the same thing to mine, and it's been OK for over 10 years...Jim

[Denny55]

Originally Posted by Donaldo

Seems I am running the 3" exhaust under the tank - it is in the rear section of the bed. I have about 1.25 inches of clearance, if I place a heat shield mid way, the length of the tank, will that be enough to omit the bomb ?

Mine is also pretty close to the tank. Do I need a heat shield and where do I get them?

[LostMy65]

Did you ever find out the answer?

[joedoh]

gasoline has a self ignition point of around 500 degrees, so as long as you have a vented cap, you wont have much of a bomb. You may lose vapor through the cap though, this is part of the reason why cars got charcoal canisters by the way, not just for emission reduction but also because in hot summer 70s days the smell of gas next to cars was terrible.

a "heat sheild" is just a piece of sheetmetal insulating the heat radiating off the exhaust from radiating to the gas tank. Its insulation, with an r value of radiation transmissivity. First, you should be aware that air is a really poor conductor of heat energy. It can be more efficient with more humidity present, when the heat energy can be more easily passed to the solid water in the air. Second, a heat shield is less necessary when the gas tank is NEXT to the exhaust, because heat rises, a thermal image of an exhaust pipe would show a teardrop of heat around the pipe. This is why heat shields are necessary on wood bed floor bottoms where the exhaust is close. Third, the surface area of the exhaust pipe from the front to the back is a pretty good heat sink for exhaust temperatures, so the temperature at the end of the pipe will be less than up by the exhaust manifolds.

to make one, just bend some sheetmetal around the exhaust pipe and attach it in the most lackadaisical method possible. heat through conduction (physical contact) is more efficient than convection (air) so some thin brackets attached with rivets will work better than thick brackets welded to the pipe. the air gap is the key, leave an air gap between the pipe and the shield and the shield and the tank and it will work like a fin on a heat sink (which is what it is, literally) dissipating heat quickly.

in my advance design (not the trucks) engineering days I designed an ejector for a jet turbine exhaust, basically a larger diameter pipe around the turbine exhaust pipe, and the turbine outlet was inside the larger pipe so that the exhaust flow would draw ambient air across the exhaust pipe, cooling its heat radiation potential and also cooling the exhaust temperatures as they mixed with motive ambient air. You can see a similar system on modern school buses, it looks like they have an 8" exhaust pipe but if you look close the small diameter exhaust tube is inside and retracted from the end, this makes a safer touch area for kids and keeps the exhaust heat radiation from heating up the bumper. very slick idea.

[dsraven]

why not do a bit of both. use a shield welded onto that side of the pipe with an air gap and also a rivetted shield between that and the tank. you could also make the shield a bit scoop shaped so it would catch some air and force it between the tank and the shield. make sure to leave some room for frame and exhaust system flex.

[LostMy65]

That answers my concerns.
Thanks