The thing is the engine is going to move.
To a degree the pipes (if they're not bolted directly to the engine) are going to move seperately.

So if it's a roadbike (i.e. not apart every few hours of running) you need either top notch sealant or rubber rings on the inner flanges.

Charles is a clean freak and will be along shortly with pics of exactly what you need to keep the inner of your belly pan so spotless you can eat off it.
As for me I'll keep dropping the pipes and resealing them when they sound like they're blowing which in my experience is after less than one trackday.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread