its sounds like a great idea if it is worth the time to you.

You will still need some sort of rubber interface at the upper pan though. A compression type with rubber washers on each side that get squeezed down by threads with a short nipple on top might be an idea. Talk to your local machine shop or furnace repair for leads on local suppliers of oil hoses and fittings.

I personally would not want to weld the upper pan.

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