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use electrical conduit, cheap and easy to bend up. make sure you drill all the burner holes in the very topp. i have put them in the sides and bottom and all this does is gets the pipe hot and eventually caues the tube to distorte and you gain nothing. so drill all the holes right in he top, remeber heat rises and if the burner is under the flame it can not get hot. go to the r.v. shop and buy an old propane oven or to the applaince shop and get a gas lp or ng stove and simply disassemble it and put t into your oven just like they have it installed. lengthen what needs to be lengthened. get your self a 3/8 or bigger plate to put over the burner for a shield so as not to expose the product to direct flame.
okay here is a trick i spent hours and weeks figuring out. where you run the gas line to the burner weld on to the conduit or thread on to it if you have away to thread it (does not matter) and connect the gas line. let the end of the burner run 8inches long before the first burner hole. install your gas fitting four inches from the first burner hole. now on the end of the burner, the other four inches out (same end as the gas is plumbed) install another fitting. by the way your running all the gas lines in the oven in copper tube, aluminum is a pain in the ass. now take this end fiting you just installed and plump it in copper flex tube to the outside and attach it to a 1/4 inch ball valve. on the other end of the ball valve i just screw in a fitting that my air compressor line will plug in too. this new gadget will do to things for you. 1 it will allow you to gat the maximum btu's out of you gas by feeding th air to the burner (just like a torch you now get control over the oxygen).2 it allows you to have a nice tight box with out the need for vents to feed air to the fire or to have your burner partiaolly on the outside so that it can draw in the air.
i just light the burner with all the stuff form the donor oven, use all the origanal stuff, if it is safe enough for yur kithchen with some care and understaing of what they are doing you should be just as safe in your work shop.
after you light the oven burner just look at the flame and start to crack open the ball valve and when the flame turns from orange to a blue just leave it thier and set your temp on your oven controls that you moved from your donor to the new unit.
i also use those long thermeter's that you use for deep frying turkeys, from the very bottom i drill a hole and install one right through the side of the oven and every foot up to the topp. you will be amzed at how this allows you to get a circulaton system installed that distributes even heat from topp to bottom, it works better than those temp guns after you use it a few times. yes you still need a fan.
a tip, drill the holes in your nw burner the same size and distance apart as the one in the oven you are using as the donor. just do this, i tried to re invent the whel here and nothing worked better. it takes alot of time and patience to build the burner so try to get it right the first time. trust me this is my most un favorite teedius part of the whole build. after you get done you will know what i mean.