Upgrading mountain bike crankset because worn teeth & chain slippage

It's quite unlikely you can't get enough top speed on that bike with those gears.

A 42/12 ratio with 29x2.2" tyres results in a 28mph top speed at 90rpm (most people could pedal faster). Since it is a mountain bike, you probably won't be going that fast except maybe down a steep hill, in which case not pedalling usually works out faster. On a lightweight road bike in a peloton this gearing might be insufficient. But for solo riding on an upright bike realistically you won't be pedalling very fast for very long because the air resistance will slow you down too much.

Your current chainset is non-replacable chainrings, so it's a fairly wasteful piece of design in that you must throw it away when you wear out a chainring. However since 7 or 8-speed (anything more requires a narrower chain and a total replacement of your entire drivetrain) is old, Shimano don't make a quality 8-speed chainset any more. So you might as well just replace with the same chainset you have now. It's heavy and disposable, but it works fine.

The other problem with your bike is the fork, which is a low quality one. Probably SR Suntour XCE or XCT. Upgrading this would definitely be more expensive than finding a used bike with slightly higher quality parts fitted (i.e. SR Suntour XCR coil at a minimum, but probably better an air fork).

At this point you can also consider whether you are not better off with no suspension, as this will make you faster and lighter work, typically.

Also there isn't anything specific to 'XT' as opposed to lower chainsets. Sometimes it means 'hollow crank arms', but sometimes you get that on 'SLX' as well. Usually the chainrings are lighter, but not necessarily more durable.

The advantages of XT are mostly in the shifters, which have 'instant release' and other features making them nicer to use, and in the rear derailleur which tends to be better quality.

You could definitely upgrade your bike to something more like the Marlin 7

https://www.trekbikes.com/us/en_US/bikes/mountain-bikes/cross-country-mountain-bikes/marlin/marlin-7/p/33148/?colorCode=orangelight_red

but this would need:

• new shifters (XT are the best choice)
• new rear derailleur (XT is the best choice)
• new front derailleur (Deore is fine)

on top of what you already need which is:

• crankset
• probably new BB
• cassette
• chain

And these parts would be more expensive ones as well.

Doing that is fine, the problem is having XT parts and that awful fork is kinda not a good match.

So you're likely better off looking for a used 3x10 speed bike if you wanted to go that route - it would have better parts all around.

BTW I'd recommend the X8 chain for higher durability rather than the lower Z series you have.

Contrary to other answers, there is a way to have everything you asked for: instead of buying new, find an old square taper XT crankset. At the time of writing this, it took 15 seconds to find one on eBay.

The old crankset will fit square taper bottom bracket (but you'll have to check that the length matches or is close enough). It will be compatible with your chain because 7 or 8 speeds were state of the art at the time. It will have the "XT" mark. At the time they were made MTB cranksets were always triples and it was common to have 46 or 48 tooth large ring (but as said in other answers, it's not going to make you faster, these were supposed to be used by racers on easy XC courses). There haven't been huge advances in cheap metallurgy since that time, so it's probably going to be better quality than current cheap square taper crank.

Thank all for your time in replying. This forum is awesome. Yes even though the bike is in low quality range i new this when i bought it. As i new what it would be used for. I didn't want anything amazing for this job. I realise the fork and all components are the cheapest you can prob get in that range. But i would rather not have to shell out for whole a new bike at the moment. I would rather upgrade with better parts when i need to, (although i will take advice from thelawnet and buy tools to check and replace the chain regularly so this never happens again) even though upgrading parts doesn't make any sense in re-selling the bike and losing money etc. But i have grown a little sentimental about the bike. And have also done a little bit of work to (this is nothing compared to what you all probably do daily) but the rear hub quick release spindle/axle snapped a while ago (prob due to weight of the baby) and so it took me days of research just to try and find a replacement sold spindle/axle that would fit and be strong enough not break again. So it's tiny mods like this i would rather keep it going rather than upgrading the whole bike.

Regarding wanting to be able to go faster, even though the bike is incredibly heavy with the bike seat / rack and 2x Kryptonite New York Fahgettaboudit d-locks. I ride over 20k every day. And in certain areas of flat roads, strava and apple watch tells me my max speed was 29mph and i spend 80% of the route in the hardest gears whish i wasn't having to pendal like crazy. And when i am hitting 28/29mph i'm still pedaling so fast and road bikes are passing me with legs hardly moving! So i really would like to enlarge the crankset if this is possible.

So from everyone who has posted this is what i've taken from this:

Forget about upgrading parts to XT on this bike its not worth the money

Because i have done no maintenance (all my own fault) i've left it to late and need to buy all these parts:

crankset
probably new BB
cassette
chain

 And these tools: (will definitely invest in tools as i want to fix the bike going forward)

chain gauge
Crank Extractor if the crank is not self extracting.
Cassette tool and chain whip to remove the cassette.
Hex keys. (got these)
Chain tool to shorten the supplied chain to size.
Big adjustable spanner. (got this)

Upgrade the chain to an X8

Or possibly buy an old XT crankset. This was the crankset i fell in love with as a kid but it was always on the top spec bike which i could never afford: https://www.ebay.co.uk/c/1823340930 But i would need to do a load of research to check this would fit? I guess i need to make sure the cranks arms are the same length as i have now as well? I'm worried i would get this wrong with my novice-ness. As this is 46 could it give me slightly more 'harder to pedal" in the hardest gear / slightly more speed?

Thanks again to everyone who took time to reply and help me