2017 tc250 stock base gasket

Usually ktm is conservative and uses the thickest one (.5mm)

You can move it around and try the thinner ones for more punch but loss of rev generally follows

I just tore the top end off my 16 because of a leaking base gasket. The 17 has the updated base gasket that has more material on the back of it so it doesn't pull back like previous years, so make sure you get the 17 base gasket. With that being said, I don't see any OEM parts available for it yet

EDIT: I'm positive that they come with two base gaskets, unless the 17 is different but I doubt it

You use a certain thickness gasket or a combination of gaskets to achieve the correct X dimension as listed in your manual, which is the distance from the crown of the piston to the top of the cylinder or the inner step depending on model. This sets the correct port timing and also comp ratio. As Derek mentioned, you can vary the X dimension a little to tailor the power some. Be careful though as you can also cause damage so be warned the standard dimension is the safest if you don't understand playing with port timing and compression pressures.

Here's a vid to give you some idea. The special tools are required to do the job properly. You can also measure the thickness of the old gasket/s and reuse the same although the used gaskets will have been compressed slightly from fitting. 0.5 is the usual go to if you don't have the tools or knowledge.

If you have the time and patience, you can play with varying the dimension a little and finding out where you like it best. The other thing you are also changing then is the Z dimension, which is the lower lip of the power valve to top of cylinder head. Have it too high and you loose bottom end. Have it too low and you loose top end. Get both dimensions right and the bike will go like KTM designed it to. Good luck with all that now.