OK first reply on this forum but here goes and I know its a little late,
We have had the pleasure and occasional pain of running several KTM 150`s we started with an 08
Our 13 150 sx stock squish measured 1.23mm on stator side 1.25mm on clutch side, with this clearance the bike was not much fun to ride strong mid aggressive hit very poor top end and very little over rev. The permanent marker from factory on our 13 cylinder stated 1.2 now confirmed by KTM at Dirt bike show that this is the squish as measured by them from factory. However the KTM engineer at show said all the 150 sx should now come with 1.7 to 1.8 marked on cylinder in permanent marker (although KTM would not directly confirm squish should be set at 1.7 - 1.8mm, more of nudge nudge wink wink, as this goes against whats in the manual?), when I said that that puts the piston approx 0.5 - 0.6mm below top of cylinder and not as per dimension X in specs. KTM guy shrugged and said, but its safer that way. Had a look around a few paddocks could not find another 13/14 150 sx with 1.2 marked on cylinder as ours was (however found 125 sx marked with 1.1 and 1.2 on cylinder) Motocross action mag on there 2013 150 sx has 1.8 marked on the cylinder, can be clearly seen in there 2013 125-150 sx shootout. (MXA test 2014 150sx has 1.7 marked up and the 2012 150sx tested has 1.6 on the cylinder)
So tried the new recommended squish setting basically set dimension X to zero (piston level with edge of cylinder) then put on another 0.5mm base gasket and got 1.73mm clearance. made a huge difference in top end over rev and smoothed out the hit (only slightly) but definitely more fun to ride. Makes sense as raising the cylinder 0.5mm advances the port timing, lowers compression and alters the squish velocity. Also had to jet up more revs more heat.
After raising the cylinder 0.5mm the exhaust port timing is still not as far advanced as the 07/08 150sx cylinder. The exhaust ports on those cylinders were cast 0.8 mm higher up in the cylinder. The 07/08 is still our favourite cylinder more linear power delivery, similar to the 125 sx, with more top end and over rev compared to later spec cylinders.
If you are happy with the performance of your bike especially if its new and never been stripped before and you do not have any issues then measure your piston to cylinder head (squish) clearance using the solder method prior to strip down.
. When rebuilding the engine you can remeasure the squish clearance and adjust gasket thickness to achieve the same measurement prior to strip down. The only thing to consider is excessive carbon build up which may affect measurement, our engines are stripped every 10 - 15 hrs so that's never a problem. Always a very good idea before installing gaskets to write on them with permanent marker what the gasket thickness is. Next time you come to a rebuild no need to guess which gaskets were used. From my experience with standard piston/vertex the difference in piston height when changing like with like is negligible, however always best to check. The only time I have had a difference with piston height is when trying a different piston make.