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Thanks guys for your concern!

The Gringo: Down here we try to keep beautiful ladies around, but thanks anyway for your generous offer!

Again, I removed the old gaskets myself, in fact I removed the whole head to clean it and repaint it, so I´m preeeety sure about removing old gaskets. The aftermarket gasket kit I bought is made in Argentina by a known manufacturer, but the exhaust gaskets have a larger diameter than stock, so they sit on the outer lip of the recess, preventing the pipe headers to tuck in properly.

Sure there are many Kwackers down here, among them many 250 Ninjas. Following Gringo´s advice I checked kawasaki.com and the gaskets for the 250 and the 550 share the same part ID, so they ought to be the same. Now I can go to the parts shop and just ask for 4 Ninja 250 exhaust gaskets.

Wish me luck!

I think you've found the problem, if I understand you correctly. If the gaskets are sitting on the lip, not going into the head like they should, they will never work. They probably gave you the gaskets for a KZ1000 in your kit.

There are a LOT of junk pattern gaskets around. Z1 Enterprises has the right ones.