Aha, a chance to give something back to the community from my own experience.
If you have a 4-into-1 system there will be one downpipe that crosses over the others to fit into the joining pipe. You need to 1) make sure it's properly seated all the way down into the joining pipe, and 2) use exhaust assembly paste (not the same stuff as exhaust repair paste). To make sure it's properly seated, feel and measure down the inside of the joining pipe, so that you know how far the downpipe is supposed to be seated. Then mark off that distance on the downpipe, and use a rubber mallet to knock the downpipe right into the joining section, making sure it goes in all the way.
If you're talking about where the downpipes meet the cylinder head, I have found that a certain UK exhaust manufacturer tends to supply its own single-piece exhaust collet which is significantly longer than the ones that came with the bike. These make certain pipes, though designed for a bike (and I've had this with Kawasaki and Yamahas), very difficult or impossible to fit to the head. You wonder why, until you decide to replace the rusting collets with OEM-spec half-collets, which are shorter. Hope this helps.
1̶9̶8̶3̶ ̶K̶a̶w̶a̶s̶a̶k̶i̶ ̶G̶T̶5̶5̶0̶ ̶(̶U̶K̶-̶o̶n̶l̶y̶ ̶m̶o̶d̶e̶l̶)̶,̶ ̶a̶l̶s̶o̶ ̶k̶n̶o̶w̶n̶ ̶a̶s̶ ̶t̶h̶e̶ ̶Z̶5̶5̶0̶ ̶G̶1̶,̶ ̶r̶e̶g̶i̶s̶t̶e̶r̶e̶d̶ ̶i̶n̶ ̶1̶9̶8̶4̶.̶
1981 Kawasaki Z440 (KZ440C1)