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If you have ever ridden a sand track with twin air skin setup used as instructed you will understand why people use them. The point is that the skin is not oiled, nor is the layer that is next to it. So first you have the normal filter oiled as usual. Then you have the dry thin coarse filter I...

I have a prediction for you, 20 years from now you will write here in Vital how that two stroke 125 was the best bike you ever had and they don't make bikes like that anymore

You are thinking backwards. Adding preload extends the shock and therefore increases the ride height, i.e. less sag. If you have weak spring then initially your ride height will be too low (too much sag) and therefore you need to compensate that with more preload. But it will still bottom out sooner than with correct spring. Always choose correct spring...

At least the Yamaha catalog I use for our 2018 show same part number for 2019. Best to confirm from dealer though.

I guess it even shows in your picture but I still want to confirm that you had the o-ring under that sleeve that you are holding in your hand? Did you oil the crank seal or that sleeve before assembly? Starting it dry may destroy the seal. And the seal itself was correctly placed in the case?

I reckon they are just basket cases.

There should be no need to grind anything. Check for example this video to make sure you have assembled it correctly. https://www.youtube.com/watch?v=zWfgXzlpHM4

Have a separate race bike with engine tuned for peak power at high revs by famous tuner and let everybody know that. However always practice with stock bike and then be mad at the kid as he / she does not keep the revs at the top when racing.

I think it is #33 in OP bike's countershaft diagram. Check the surface of the bushing that goes inside the seal and if it has worn then buy the repair kit #99.

WD-40, 5-56 or similar may at least temporarily make it look better.

I am sure there are several tricks for that roller bearing, a low tech one is to first pry the roller pins out then find large washer that just fits inside the outer ring. Drop that in there and then put the roller pins back, then you have a way to apply pressure from the other side. As for the...

Good old trick which is also the official procedure in KTM service manual: heat the case in oven to 150C and bearings fall out when you knock the case on wooden table. As for the clutch inner hub as it has visible notches from plates in addition to the damage you made I would replace it so that the plates...

Spill a bit of that and regular gas on paper and let dry, if there is oil it should leave a greasier stain. Edit: Oh I did not read that it is already gone. Well I guess you need to take dealer's word for it.

At least from the ones that I have bought from dealer on CD it is fairly easy to save them as a pdf file, I keep those in ipad and phone. Let me know if you need help how to do that.

I use these kind of spoons and the part where I tend to leave marks is the very last bit of putting the tire back on. The solution for that is more spoons as you can place them all in there before it gets tight.

At BDC exhaust port is of course wide open. To measure you can use a regular caliper or make a tool from something that you have in garage. It is not that critical to be exactly within 0.1 mm there is slack here and there but as long as it is close. A tool by Nihilo looks like this:

I think you have misunderstood something. You choose base gasket thickness so that you get correct piston height (squish, X-distance). Then after you have installed cylinder you check Z-distance to see that with those base gaskets that you used the position of the power valve flap is still in spec. If not, turn the adjuster until it is. Here is...

To be precise you asked "which cast pistons should I avoid for my 4-stroke" so the op bike was not the criteria Anyway we both talk about the same thing, modern 4-stroke mx bike pistons are almost always forged whereas 2-stroke pistons are either cast or forged depending on manufacturer. I know a link was posted above to a...

I can start the list, two can be found from the ProX web site "Cast genuine 4 stroke pistons are used in relatively low performing engines like the Honda XR400R or Yamaha XT600" http://www.pro-x.com/genuine-pistons/

This is getting completely sidetracked from the Hot Rods issue but let me explain the insurance saga a bit; this is not just mx it is all motorsports in Finland. With mx it is especially bad because of the small number of bikes vs lots of injuries. It all comes from the EU style vehicle insurance which is per vehicle...