short story: pkrasko, get the big puller described above, 33mm.
long story: as applied to 620/640 engines, not sure about the 400s; ignition identification by looking at the right side cover over the stator
Early lc4s all came with SEM ignitions, right? these engines have the timing aperture (the little screwed on cover)on the stator cover at the 7:30 position. these bikes all have the weak electrical output and are most likely kickstart only. ie pre 97 rxc, pre 99 sc etc, dirt only machines, race bikes etc. these take the 26mm flywheel puller
in 97, electric starter was introduced, so ktm upgraded to the kokusan 4k-2. these engines have a sight glass
on the stator cover at about 12:00. the new ignition gave much more out put for the lights, starter, bigger battery etc. most readers in this forum are going to have this one, it takes the 33mm flywheel puller. Most readers of this thread obviously have electric starters as well.
but the competition dirtbikes still had the crappy SEM until 99 when ktm started using the kokusan 4k-3 on these. these bikes are probably kickstart only units, the timing aperature is at 9:30 position on the stator cover. Since I haven't seen one of these flywheels, I am making an educated guess that these take the smaller puller as it looks small in the picture in the manual and because the torque for the flywheel nut is the same as for the small SEM unit.
Look at your stator cover, see where the timing aperture is located on the face of the cover. That should tell you what ignition you have. Select your puller accordingly. To triple check, you can remove the cover and measure the flywheel thread diameter with a vernier caliper.
If your bike doesn't have electric start, you aren't going to have oil around the ignition, if you do have e-start, you can lay your bike over and work on it on the ground. you might want to take your battery out but I didn't. Just dont drop anything as gravity will help it into the engine.
As you can see from my profile i am but a noob so feel free to correct any misinformation I may be disseminating here. I have read this entire thread many times and have not seen a definitive proclamation that a 26mm puller works on an e-start bike, if I'm wrong, let us all know so we can disregard what i've just typed.