Are we talking 6" or 8" on the pipe?
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Sanding out the ID of the fitting is a PIMA. You might have an easier time sanding down the OD of pipe itself. It doesn't take alot of stock removal to get a fit that slides freely.
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Yes, you will likely increase the potential for more air. I gotta tell ya, I see the rubber coupling as the fix-all, end-all. Those things are very stout. The rubber for the 6" coupling is easily .312" thick. To me, the caulk thing is a quick n' cheezy temporary fix until you can do it correctly.
