Odd question. I need to replace the sending unit in the rear tank. Went to siphon the gas out and in the siphon tube there are small clear crystals. Any ideas? I haven't gotten all of it siphoned yet, and have not seen the inside of the tank either.

 

I can't think of any reason there would be crystals in your fuel tank. Perhaps you are seeing pieces of the deteriorated filter sock that's on the end of the fuel sending unit...


Once you get the gasoline siphoned out of it you can take a look to see if there is any debris in the tank. Make sure you wear a respirator if you're cleaning the fuel tank though. Here is a good product for restoring your fuel tank:

POR 15 Fuel Tank Repair Kit

 

Well, I suppose if you had really old fuel turning to varnish in the tank ?

 

I'd consider replacing the tank since you have it out. I did it recently. They are not very expensive.

This is the one I bought: Replace FTK010056 - Ford F-100 Replacement 19 Gallon Fuel Tank Brand New | eBay