Yes, the friction plates are the dark-colored ones pictured. The material on the plates wears over time and the clutch can start to slip when the friction surface is worn, as well as the total thickness of all the friction plates is reduced overall which allows less tension from the springs due to the change in overall thickness of the set of plates. The steel plates are usually fine for the life of the bike unless they have been overheated and warped, but they will have the appearance of having been too hot and that will be easily identified (change in color, burned appearance). If there are a lot of miles (km) on your bike and you can't be sure if the friction plates have ever been changed, you should change them. You could change the springs as well, but it isn't usually necessary unless you're planning to race the bike or otherwise use it under extreme load situations.
I pulled the side cover, clutch and oil pump from my spare seized parts engine. The springs measure well, now just to go over the friction plates, I have no idea how many KM on this one. I assume you measure around the friction plate a few spots and if any are below spec it needs to be replaced? Also for long term storage do they need to be in oil or such?
I wouldn't worry about storage, engines sit for long periods and the oil (mostly) drains off clutch plates over time - and more often than not, things are just fine once it gets run again in the future. I personally wouldn't re-use old friction plates... you'll only spend the money once in the time you own the bike, so it's worth doing. Yes, you can measure the thickness to see if they meet specs, but when in doubt just replace them
friction plates rarely need replacing unless you've been running with an over tight clutch cable or you are a racer just learning how to launch from the start flag.
the 4 clutch springs are what will benefit from being replaced. measure the friction discs and if they are in spec run them
the main area I find wear on, on old friction plates are on the ears where they interface with the clutch basket. if run with low or dirty oil for long periods of time, they will wear grooves into the clutch basket and friction plate ears that cause funky clutch engagement...but if this is your problem your whole motor is likely trashed
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