Step 1: Make Your Tees
You will need either two or four "tees". Two if you are putting your hose-in-hose system in series with your passenger heating system, four if you are putting it in parallel with it. Series means that coolant will flow through the hose-in-hose first and then the passenger heater core, parallel means coolant from the engine can take either route or both - hopefully it will choose both. Also, you can get 90-degree "elbows" if the routing of your hoses will be less kinky that way. Plan ahead!
You will need pieces of "half-inch" copper pipe - which is actually 5/8" O.D. You will need two-inch long pieces, because the Tees you have can't be used alone. Also, to merge two Tees (to make a four-way) you need a short piece of pipe. This will all become obvious when you look at the parts in your hand, and try to put the coolant hose onto one of the tees without first installing a bit of pipe. You can probably get the store to sell you short pieces of pipe, if they have it in a scrap bucket, since you only need two-inch long pieces.
While you're at the hardware store, get yourself the 3/8" aluminum tubing which will be your fuel line. It comes in a circular roll about 2-3 feet across, and you can straighten or bend it with your hands.
Most big hardware stores don't carry this stuff - but the small ones like Ace do, and they have copper tees and scrap buckets of pipe too.
The pieces in this image have already been soldered together. How to solder or "sweat" copper pipe is another instructable entirely!
Step 2: Measure and Cut Your Hoses and Tubing
Put electrical tape over the end of the aluminum tubing, which you have straightened enough to route it under the car, or truck or bus or whatever. You don't want to scoop up dirt with the open end, so TAPE it.
Route it under the vehicle, don't make any drastic bends just yet, but figure out how long it should be - err on the side of caution, it is much easier to cut something shorter than to make it longer!! You can snip it with tin shears and fix up the end later, or you can use your fancy tubing cutter now.
The tee which is shown on the tubing is for the back end of the car - where the coolant return hose connects. If you have a heater in your tank (which you should - see greasebenz.com) you might turn this part the other way, or leave the elbow off of it. Plan ahead!
Measure your distances and figure out how much 5/8" I.D. (inner diameter) coolant hose you will need to buy. You need enough to go the length of the hose-in-hose times two, because you need a return hose! Go to the auto parts store and get the hose, and get a few extra feet obviously. If you get too much, you can swing it around really fast and it makes a cool noise. Or save it in the trunk so you don't pop a hose under the hood.
While you're at the auto parts store, get a few pieces of 3/8" I.D. fuel hose! You will need a piece at each end of the system, at least two inches long. You can use 5/16" I.D. fuel hose for connecting the tubing to your filter, but you need 3/8" hose for the reason in the picture. Check the scrap bucket in the hose department, they might not charge you for three inch pieces of hose, especially if they are in your pocket by the time you get to the register.
Also, don't forget hose clamps!! You will need at least 12 of them for this job!
Step 3: Put It All Together
Take off the tape on the end of the aluminum tubing, and slip a piece of 3/8" I.D. fuel hose onto it - just about three inches worth. This is purely to keep the coolant from escaping. Use a separate hose for your fuel delivery! If you can't see an inch of the tubing, you can't be sure of where coolant will go if your hose clamps aren't tight enough. Now put the tape back onto the tubing and install the thing under the car.
This picture is what the tank end of my setup looks like - i have a tank heater, so my "return" hose goes to the tank before going back up toward the engine. You should have a tank heater too. The 90-degree elbow on this tee is optional - you just have to plan ahead so you don't wind up with kinked hoses.
On the other end it looks the same on mine, except I used the four-way piece. That's because I connected my hose-in-hose in parallel with my passenger heater. In order to make sure I get coolant flow through my system, I have an electric coolant pump stolen from a dead Mercedes Benz.
(they have it to make sure their rich passengers don't get too cold in the winter.)
Note: if you hook up your hose-in-hose in series with your passenger heater, watch out - some vehicles allow the passengers to shut off coolant flow to the heater if they aren't cold - and that will block flow to your hose-in-hose! Put it in parallel in these cars!
If you don't care about passenger heat, and/or if your heater core is leaking or missing, you can hook this system up instead of the heater core.
You will notice the hose clamp on the right is squeezing the big hose onto the little hose onto the aluminum tubing. This is what keeps the coolant from leaking out around the tubing! You might decide to use two clamps on that part of it, just to make sure it doesn't leak.
Aluminum tubing is the best choice for this setup because Copper will react with the fats and acids in your veggie oil, creating the evil green slime which clogs your shit. Also it will give your hose-in-hose a clogged artery because the reaction happens the most where it's hottest! So stick with aluminum.
Also, plan the direction of your coolant flow - the hottest coolant, coming FROM the engine, should go into the hose-in-hose on the end where the fuel comes OUT to your first filter. The return hose (from the back of the car) is returning the coolant to the system after its done its job. If you have a coolant-heated filter (you should) the coolant should go through that BEFORE it goes into the hose-in-hose.
good luck running veggie oil with this fancy fuel hose - it will add a bunch of degrees to your total heat and if it ever leaks, ordinary roadside tools will get you through!