When basic maintenance for very basic Suntour XCM/ XCC fork is needed?
Is it needed just in case if something wrong or once a year or after some amount of km?
By maintenance, I mean something like this:
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There is no 'XCC'. Possibly you mean XCT, XCE or XCR. XCR lacks slider sleeves, and IME doesn't need much maintenance. If you are referring to XCT or XCE, they are constructed the same but XCE has 28mm stanchions and current XCT has 30mm.
The diassembly visible there is of an old XCM fork without cartridge damper. There is a difference between 'XCM' and 'XCM HLO'; the former is a specific model, but 'XCM fork' could refer to an 'XCM' or an 'XCM HLO', where the former likely contains two springs as shown there, whereas XCM HLO will contain a spring and a sealed-for-life cartridge damper.
This is an XCM service kit ($30): https://www.srsuntour.us/products/xcm-rebuild-kit?variant=29808447747
these parts are sold separately
https://www.srsuntour.us/products/spanner-mtb-wrench?variant=27529158787 $2 for the wrench
https://www.srsuntour.us/products/xcm-wiper?variant=27529011651 $15 for the wipers (pair)
https://www.srsuntour.us/products/xcm-slider-sleeve?variant=27529024451 $9 for the slider sleeves
The service interval is annually/100 hours. On an XCR fork you still have the wipers, but no slider sleeves.
If you refer to the service manual
"Check function of fork / check torques of mountings screws and nuts on bottom of lowers (suggested tightening torque: bolt: 10Nm, nut: 8Nm) / check for scratches, dents, cracks, discoloration, signs of wear and signs of minor corrosion (maintain with oily cloth), or oil leaks." - every 50 hours
plus every 100 hours the same plus
"disassembly / cleaning the entire fork inside and out / cleaning and lubricating dust seals and slider sleeves / checking torques / adjusting to the riders liking. Before disassembly, check the slider sleeve play of the fork. To do so, apply the front wheel brake and gently push the bicycle back and forth at the handlebar stem shaft. Replace the slider sleeves if the play is excessive (more than 1 mm at the fork brace)."
In other words the slider sleeves are intended to be a service item, but you don't need the service kit - you do need tools and grease, and you WILL need to replace the slider sleeves (which are absent from XCR), but NOT every service. This is not a car and you are not replacing parts on a fixed interval, only lubing/inspecting on a fixed interval.
The cost of a service as shown in the video is a one-off purchase of tools and some grease. There are lots of brands of fork grease, some grease is too thick - it should liquify in use under heat. SR Suntour's own grease is called Conpanna and is $4 for 20g. https://www.srsuntour.us/products/conpanna-fork-grease?variant=27529154499 Possibly if you are stripping forks often you could buy a larger quantity more cheaply.
So you are looking at $6 for tools/grease and then if you had a 'slider sleeve' type fork you'd want to have those on hand because nobody wants to take a fork apart and see that you don't have the part. The wipers will perish at some point and would need replacing after several years but probably you'd not buy those right away.
With regards to the nuts, correctly torquing the nuts is important to stop the fork falling apart, but you wouldn't need to replace them often.
As far as the internals go, there is a LO damper cartridge; this is priced at $40. https://www.srsuntour.us/collections/xct-service-parts/products/xct-lo-cartridge?variant=27528603843
This cartridge has rebound damping and is better than other types such as HLO, or cartridgeless models. However it's also $40, which is not far off the value of an XCM fork.
In the US SR Suntour has a trade-in program which might be cheaper than buying a fork there. https://www.srsuntour.us/pages/upgrade-program
Here in Indonesia, a new SR Suntour Epixon fork is priced under $200 so perhaps those trade-in prices aren't that great. But anyway, $200 is roughly the price of a new fork which would be much better in that it would have air spring (adjustable for any bodyweight, unlike springs), a better quality damper, lighter weight (the Epixon is 1.7kg or so, certain XCM models are over 3.4kg), and wider, lighter (alloy rather than steel) stanchions.
That's to say that you would not service your cheap XC? fork in a developed country at a dealer because the labour cost would exceed the fork cost, so you'd likely just leave the fork till it failed and then replace it with a more expensive one, or even a new bike. Here in Indonesia labour would run around $3, so it would make sense, as does DIY, but essentially you are not going to do much with this fork beyond strip it, lube it, replace perished seals/sliders every few years, and eventually recycle it (nice lump of steel, weigh it in). A typical service might cost you 50 cents in grease, and then if you need to replace seals maybe $20. This is still much cheaper than any replacement, so it does make sense to service it if your labour is cheap.
According to SR Suntour's general fork manual (page 14), there are two maintenance intervals:
In my quick web search to find in the owner's manual, I noticed some links where this fork was for sale for less than $75 (US). Now the last thing I want to do here is bike shame anyone. But in my experience, forks such as these are more weight, maintenance, and cost burden than they are worth in performance benefits. This is because (just a guess) the seals, sleeves, and lubricant will cost you about $15 - $20 and the shop labor would cost around $40 - $50 to perform the 100-hr maintenance.
Their low performance comes from their weight, low-quality (high-friction & easily damaged) sliding surfaces, poor seals that leak oil and permit contaminants to damage the stanchions, poor damping, lack of spring rate adjustment, and lack of damping/rebound speed adjustment.
Selectively reading between the lines a bit, if you're not comfortable paying essentially the price of a replacement fork every 100 hrs of riding or sinking your own time into it, I would recommend finding a good rigid fork to pair with a high volume & supple tire.
These forks are often fitted to mass market bikes of a quality that frequently end up in bike hires etc. In this type of usage, where minimising costs and downtime is important, the forks are typically only considered in need of attention if play is detected between the upper and lower sections. A rebuild with parts, as others have noted, is more than the cost of a new fork (including labour) so the fork is typically replaced as a unit. The bike may see daily use on gravel tracks etc for a couple of seasons with no fork maintenance, the performance of the fork deteriorating slowly over that time. On the other hand, they are easy to disassemble and reassemble so you may wish to do this just to get a feel for how it works. (Experience from a 300 strong bike hire at a large holiday park)