I am about to replace the sending unit in my 65 chevelle. Should I fill the gas tank or drain it before doing this? I don't want to blow up the car and myself. A mechanic friend of mine told me to fill the gas tank with water? That sounds insane. Should I drop the tank and completely drain it or am I being paranoid? If my car was originally a six cylinder, that would be a 5/16 sending unit right?

that sounds crazy. You want the tank empty to drop it to replace the sending unit. Gas is heavy. Safety issue.

good luck

Get as much gas as you can out of the tank BEFORE dropping it. Be it by siphoning, letting it idle until empty, or by driving it. The less weight the better. Disconnect the fuel lines. Use a good floor jack and a piece of wood (I use a length of 2x12), and support the tank. Take the straps off and lower the tank enough to disconnect the wiring harness from the sender. Then slowly and carefully lower the tank to the floor. You'll have to tap the retainer counterclockwise to remove the sender. Make sure you use a block of wood or a piece of brass to do this, as not to make sparks.

When I replaced mine I did what deejay stated but I used some plywood on pad of the floor jack.

Chris.

You don't need to drop the tank to replace the sending unit on a 65. It's easy to get to from under the car. You can drop the tank if you want, but it's not necessary. Remove enough gas to minimize a possible mess. An empty tank will explode easier than a full tank. The reason to fill it with water is if you weld on a gas tank. All the gas tank sending units are the same be it a V8 or a 6 cylinder for 65s. Maybe, just maybe the 350 hp L79 327 was different, but I doubt it.


My resto page(s) www.geocities.com/eosupersport65 (http://www.geocities.com/eosupersport65)

I can see that I don't need to drop the tank. However, if an empty tank will explode easier because of the fumes, shouldn't I have it at least half way full?

deejay has the right answer for you... empty the tank. If you don't you will wish that you had because even a half tank will weigh over 100 pounds (10 gal of gas + tank weight) and that is a bunch to try to work with under the car, worrying about fuel spillage, etc. etc.
I just removed mine and I used air pressure to get the tank empty ot about 2 gallons. I removed the fuel line connection at the fuel pump and placed the end in a plastic gas can, then just pressurized the tank with the air gun and out the gas came. Just be sure you don't use too much pressure or else you can possible split a seam or something. Worked for me.

'bear

But I don't have to drop the tank. Sending unit point right at the differential.

Just replaced mine on my 65. Run it low on gas, no need to drop the tank. Easier on a lift but doable by jacking it up.