Oil Tank Building Made Easy

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Fab Kevin
Fab Kevin

An oil bag for your Custom Chopper may be made from lots of different materials: aluminum, steel, stainless steel, even blown glass. You can buy just about any kind of pre-made tank imaginable from many different vendors. For those of you who would like to try making an oil tank yourself, I’ll show you a few do’s and don’ts that might make the job easier for you, unless you are a glass blower. I can’t even tell you where to start making a glass oil bag…

I will use a Big Twin style oil tank as the basis for this article. Sporties would take a similar tank, and most of the info will apply to tank building, regardless of what motor it is for. A round HD tank consists of the following components: a cylinder, ends to cap off the cylinder, a filler neck, (4) 1/8” NPT bungs for the oil fittings (feed, return, vent, and drain), and some way to mount it into the frame. We’ll start out with a little simple math, to help calculate the tank size. I’m not comfortable with a tank any smaller than 3 quarts, and I try to design for 4 quarts. The first step is determining how much room you have to t the tank into, and designing the tank to occupy the space available.

A US Quart equals 57.75 cubic inches. If you want 4 quarts, you will need a tank with an internal volume equal to 231 cubic inches, PLUS a little extra room. Since it isn’t practical to ll the tank entirely, you need to figure the volume of the tank to be slightly more, to allow for the unused space at the top of the tank – for the vent to work and what not. For this exercise, I am going to assemble one of my Stainless Steel Oil Tank Kits. They utilize an internal diameter of 5 1⁄2”. The formula for determining the volume of a given tank, is: pi (which is 3.1416) X the radius of the tank2 X length. So, for a tank with a 5 1⁄2” inside diameter, the formula would be 3.1416 X (2.75X 2.75) X 1. Well, I used 1 so that I could then apply the volume per inch to any given length, to come up with the total volume. Once we know the volume per every inch of tank length, all we need to do is make the tank long enough to reach our target volume. Sound difficult? Re-read it a few times, and you will see that it is simple.

3.1416 X (2.75 X 2.75) = 23.758. So, for every inch of tank length, we have 23.75 cubic inches of oil. 231 divided by 23.75, equals a tank length of 9 3/4” to reach 4 quarts. Allowing for some wiggle room, I’ll add an inch to that number, and I’ll want a tank that is 10 3⁄4” long.

I will be starting with a at sheet of 14 ga. Stainless steel, so now I have to calculate the sheet size to roll. I’ve already calculated the length to be 10 3⁄4”, so now I need to figure the rolled length, so we end up with a 5 1⁄2” inside diameter. That is pretty simple. The formula for developing the inner circumference is diameter X pi. But, I will need to allow some extra material, because the roll will shrink the material slightly as it is worked. The secret is to add one metal thickness to the diameter, so I will use 5.578 (5.5” plus 14 ga, which is .078”) X 3.1416, and get a developed length of 17.52”. I will start by shearing a piece of 14 ga stainless, 17.52” by 10.75”.

A trick that I tell a lot of my oil tank customers, is to cut some posterboard into those dimensions, roll it and tape it together, and hold it in the space allotted for the oil tank. That will give you a very clear view at how the tank will look when installed in the frame.

A slip roll makes easy work of rolling the sheet into a cylinder, so the basis of the tank is now finished. I use domed end caps, spun into shape by a metal spinner. They have a small flange that slips into the cylinders I roll to make it easy to weld together. Before we start welding anything, we need to finish boring all of the holes into  the tank, for the filler, mounting bungs, and oil fittings. I have seen many tank builders bore the holes after the tank has the ends welded on, but there is no way to clean 100 percent of the chips out of the tank that way. By boring the holes first, it is a simple matter to clean the tank before welding it all up. The next time one of your buddies is installing an el-cheapo chrome imported steel oil tank that he got a smoking deal on, have a look inside of it. You will be shocked!

You will need (3) “blind bungs”, that is, three bungs that will mount the tank to the frame, and the tapped holes in them cannot break through the bung – or oil would leak out. I use 1” diameter stainless steel, and I turn a step into the bung so I can locate it from the inside of the tank accurately. I thread them 5/16”-18. For the oil filler bungs, I buy 1/8” NPT 316 series stainless steel half-couplings. That is a lot of verbage, for a small fitting with a 1/8” female pipe thread in them. They’re available at most industrial plumbing supply houses, or even a number if on-line plumbing supply sellers.

O.K., so lets lay out where to bore the holes. We need (2) bungs on top of the tank to hang it from, and one on the bottom to triangulate it, and keep it from swinging in the frame. Without the third, triangulated bung, the tank will crack 100 percent of the time. We need a hole in the bottom right side for a feed line, and one on the bottom left for a drain (so it will all drain out while the bike is on the side stand). We also need (2) holes in the top of the tank: one for a return, and one for a vent. Last, is the filler neck. Stock HD oil tanks have utilized a push-in stopper for an oil cap for many years, and they push into a 1 1⁄4” opening. My local steel supplier sells me stainless steel tubing that is 1 1⁄4” I.D. by 1/16” wall thickness. That means I have to bore a 1 3/8” diameter hole for the filler. This hole can be in the top of the tank, or in the top right corner. My numnut friend from Chicago (or is that my friend, Numnutz from Chicago?) wanted me to make him a tank with the filler hole in the center top, so that’s what we’ll do.

Once the holes are laid out, you can bore the smaller ones with a uni-bit, or step drill bit. It’s best to clamp the tank in place, and drill the holes in a drill press – but use whatever technology you have available. I bore the bigger hole (for the filler neck) with a hole saw. Easy does it – spin the drill too fast, and you will burn up the saw in short order.

If you’ve followed along so far, you’re well past the most difficult part of the job. Once everything is figured out and sawn, rolled, drilled, and t – all that’s left is the easy part, welding it all up. See – even if you don’t have a welder, or know how to weld, you can still do 90 percent of building an oil tank yourself, and just pay a welder to finish it up. My Oil Tank Kits (as well as other manufacturers) come in this form, so all of the hard work is done for you. For those of you that do weld, here are a few tricks that seem to make it easier.

I start by tacking the seam together, all the way across. Holding a round tank can be awkward, so I use a 10” long piece of 3” aluminum channel to cradle the tank, while I weld on it. This little trick alone is worth the price of the magazine, if you’ve ever wrestled with a tank rolling around on your table! Next, I weld one end onto the tank, usually the right end – especially if I have to add the filler hole there. That way, you can bore the hole right away, and give yourself room to adequately clean all of the chips out before you weld the other end on. Tacking in all of the bungs is next, then the filler tube, and finally, I weld on the left end of the tank. Tapping in the second tank end isn’t always easy, as they t into the tank pretty tight. One tip, is to tap the end in so that one corner of the endcap is in the right spot, tack it, and then gently tap the opposite side of the cap into the tank. Easier said than done, but be patient and it will go in. Another tip, is to thread a long nipple into each of the bungs. This helps you hold the bung into position, as well as use the nipple to make sure the bung is in straight. The added length of the nipple will accentuate any misalignment of the bung.

With the ends welded on, and everything else tacked in place, I finish weld the seam across the bottom, and then attack each bung, one by one. Weld up a top NPT bung, then a lower one, then back to the top one on the opposite side of the tank as the first. This way, you will spread the heat around, and keep warpage to a minimum.

There you have it, your very own, handmade oil tank. A stainless steel oil tank that will never rust inside, and will supply CLEAN, chip-free oil to your motor right from the first drop, all because you took the time to keep it surgically clean inside. That kind of care cannot be bought from any offshore, sweat-shop, cut-rate oil tank maker.