Thank you for the fast help, guys. I was thinking about some kind of N.P.T. or something like that but absolutely couldn't be shure having no SAE experience. We here use to use the sort of pipe thread wich is for heating/water supply. The sizes called 1', 1-3/4 and so on but the actual measurements are away from real inches. SAE hardware is difficult to get here, especially taps and dies. Although when looking at all that with a hobby wiew that's a kind of an adventure again. As for the polishing it was a story I'm still out of my mind about. For the first when I took the tank off I found out the inside cross wall was laying on the bottom off it's welding. It was a surprize just on a flat place. Googled for a tank repair shop, brang it there. They wanted to cut off the end side. I stopped that nice idea and told them to make a cut-out on the back side. Couple of days later it was ready and I got it. By the word I had to transport it in my Mercedes - G. Next I said to the guy at my shop to send it with a hand electric machine. He started and said "no luck, it's better to work by hand". Next (next day) we resolved to buy sand paper disks and continue with a hand drill machine. And so on. To make a long story short he spent about 20 working days to grind off most of the corrosion cavits (on the face areas) and send it up with all numbers of paper. In the end I started to work with a paste I used for Alcoa wheels and got some strange result of black stains I could get off only by sanding. Probably the tank was made of a different kind of alloy. A friend of mine gave me a bit of 3M branded stuff for alu polishing and I finally came up with what you now see. The tank doesn't shine perfect still and I hope (a bit) to polish it up more right before the instalation. In the spring, not yet. Now all that is about 3 weeks back but I'm still under impression and trying to figure out what to do with the other one