You will not split the mast if you use the top hole of your extension. They are designed to be used like that. I have found that Chinook skinny extensions are very prone to bending if used much over 20cm. It is a bummer because carbon ones are insanely expensive.
Good question ! When I use my big sail it reqires 30cm of extention and as Ive seen the extentions bend below the collar and the collar damage the rails of the board. I now leave the collar right at the bottom and put a 30cm sleeve of skinny mast (cut from an old broken one)on the base and the mast then sits on top of this.
i would check your warranty card (if its a new mast???) cos i'm kinda sure that some skinny brands (ie. powerex) wont replace your mast unless the extension is in a certain distance: 6cm sounds short?
That said, with regards to bending, have been using my chinook ali skinny extension for about two years now up to 30cm of extension, and having snapped a skinny, broken two sails, and a boom whilst using it its still straight as an arrow.
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