Hi, everyone.

I'm an MTB rider, mechanically illiterate. My bicycle is a very old-fashioned one. Its rear derailleur, twist shifters and a cassette are all X0 9--speed parts. I am satisfied with my old MTB as it is now, though. But one of my friends offered me to give me a new X0 10-speed crank for free if I want it. Could I use the 10-speed crank with my other 9-speed parts? Some say there will be not a few problems and others say it would be okay. Try to use it or not? I would appreciate your experienced advice.

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To avoid bigger problems, I decided to give up the 10-speed crankset to another friend and buy a new 9-speed crankset. I thank all of you for participating in this debate and giving me useful feedback.

Did you ask them which problems will be not a few?

I don't understand every detail of their opinions. They say that chainrings of each speed have different gaps between their teeth, which will cause a main problem with either a chain or a sprocket.

^^^^^ +1; what's wrong with your existing crank?

To actually answer the question YES it will work fine. One of my bikes had x0 9 speed gripshifts and 10 speed triple shimano crank with the big ring replaced with a bash guard, shifted fine. Here is the thing that matters, the front 9 speed x0 gripshift isn't really indexed so it doesn't give crap what crank you have, what FD you have, or how many speeds you have. Now I say it isn't really indexed I mean it has a 1,2, and 3 on it but it has many trim spots between each gear

BB spec? and crank type/compatibility (1x/2x/3x) they are going to be potential issues.

What BB type does your current bike have, and is it the same as the new crank, if not, there are adapters, but it's often not worth the cost.

How many front chainrings does your bike current have vs the new crank? if changing from 3x to 1x there is a lot more to think about than just the crank.

With others, what do you expect to gain from installing a new crank?

I'm thinking the OP is doing pretty good given their location; English is probably not a first language.

OP, the crank will probably work (it's free so it can't hurt to try). I would compare the size of the chainrings, the large one especially, so you don't wind up with too much or too little chain. There are plenty of helpful people here if you decide to tackle the job yourself. Having enough posts to post photos is a big help (I think you need 10).

Which is EXACTLY why I used their words.
It was either/or, so I was asking what was the "or".

I'm going to pick up the 10-speed crank in about five days. My friend says the chainrings of 10 are the same size as mine, 22/32/44 (bcd 64-104). When I get the 10, I will ask for your help with photos. You encouraged me . Thank you.

BB would not be a problem, for it's not working well and I have to replace the BB anyway.

My bike is 3X9 and the new crank is for 3x10. The sizes of the chainrings are both 22/32/44, the same size.

Now my crank needs replacing BB and all of chainrings, and looks no good because of lots of scratches. But there's a new crank that has never been used! Wouldn't it be great if it was compatible?

Thank you for your advice.

"Speeds" is a cog count 10 in the back or 9 in the back matters little in front , there it's 2 or 3, or 1..

you continue to use 9 speed cassettes and chains...

Now 'speed/velocity' is about how much effort you put out.. enjoy those downhills,,

Should be just fine.
The different gaps are not between the teeth, but between the chainrings. The chainring spacing from 9-10 isn't a big deal. It's when you are using a 6-7-8 speed crank on 9-10-11 speed chains. The wider gap between lower speed crank chainrings allows the narrower chain to fall between the rings rather than landing on the next ring.
BB maybe different, but as you said you plan to replace that anyway.

In a case like this with limited (initial) info given, it could make a massive difference, The OP has now stated they have and are going from a to a triple, so it's a pretty simple swap (on the basis that the BB types are the same). Initially no specifics were provided, so if they were going to a single, they would need to look at removing the FD, front shifter, and getting a larger range cassette, maybe even new RD & rear a wheel if they needed to swap from a Shimano/Sram cassette to and XD cassette, so a lot to think about till full info is given.

My general philosophy is that, if I already own parts, I'll bolt up almost any combination and see if I can get it to work to my satisfaction. If I'm BUYING parts I'll only pay money for components that have been designed to work together. This is a free crankset. I don't know why you want to change it but, if it was my bike, I'd bolt that crankset up and see if I could make it work. I'm betting it will.

I can't see what you're gaining except extra work to do