Anyone out there have any idea how long the mast extension needs to be for structural integrity? I have a 5.0m koncept that requires a 395 mast, but I only have a 400. My idea was to cut 5cms off the mast base so the 400 will do both the 5.0 and the 5.8m. However the np mxt 34 that I have is too tight inside the mast and I'm unwilling to force it. I thought I could cut down the mxt as well so it fits the mast without becoming jammed, but need to know how short some of the mast bases are. Thanks in advance.
There isnt an extension on the top strap to get a 400 in ? Had a North that was around the same mast needed, but top strap had abilty to extend so I could get the 400 in safely andf worked OK.
Pepe it will be fine. Some extensins are just the base with the pulleys. So cutting your one down will be fine. I have done the same to a few with no issues so far.
Cheers Ben, it's down to 27cms to fit. I put the extension on the lathe to make sure there were no nicks and all seems well so far. 
They also made a shorter MXT extension. (At least, they did because I have one) A 12cm I think, but I haven't seen where it is for a while, but it is relatively short. (Edit: I found it, its 14cm).
With the adjustment ring in the top hole, it leaves 95mm for the extension into the mast. If this is what Neil Pryde think is enough, then that's okay with me.
you want 2.5x the Internal Diameter of the mast inserted into the mast at all times. Mark this point on the sawnoff extension so you dont put the collar too high up on the extension.
This will have a good safety factor inbuilt :)
pepe make sure you put a rounded bevel after the cut to minamize your iner where on the mast and ive proberbly got a spare extension if you want it if that didnt make sence i will explain it in pepenese lol
HEY Pepe on a similar topic i;d imagine theres lots of people out there with 400 masts and the newer 5m Koncepts .
Let us know how it performs with the 5cm cut off and slightly different stiffness to the recommended 370 mast.
Hi Pepe,
If your 4m mast seems slightly too long for your 5m Koncept (2008 model?) don't shorten your mast, get a shorter base. If you remove the adjustment collar on most extensions you can gain the 4-5 cm you need. If that does not work, and you can find a mast extension that goes that short, try a stubby mast cup with no extension.
Dale,
The 2012 and 1013 5m KA Koncepts are designed for the 370 length mast. You would need to cut about 10-15cm from the bottom of a 4m to make it fit easily. With the 2012 model a soft'ish 4m SDM mast will probably work well.
The luff round was increased in the 2013 model and a cut down SDM 4m will probably feel pretty stiff and rigid unless the sailor is in the heavy range. A 370 SDM is recommended.
However, both sails will work very well on an RDM 4m cut down on the bottom and with RDM cams as the RDM masts feel softer and springier in use. I use a cut down 4m RDM in my 2013 for speed sailing which enables me to set it deeper with lots of top twist and stability when set super full. For a one hour attempt with the sail set quite flat in 30 knots of wind I use the 370 RDM which make the sail softer and more reactive in those conditions, easier to drive hard for an hour.
The main reason I use the RDM masts in the smaller speed sails is just that they survive the big loads and crashes. 
I've got the 2012 5.8m and have cut the mast down 5cms, hopefully a test tomorrow will see if it works.
