Upgrading crankset on mountain bike - what are my options I have a cheap old rockrider 5.0 mountain bike  also known under the btwin brand. Bit by bit I'm trying to improve it's performance as a road commuter bike. So far I've changed the rear wheel to a cassette type and upgraded the cassette from 14-something T to an 11-28T and changed tyres from 2" semi-slicks to 1.5" slicks. This much improved it as a road bike but after 2000+ miles since the upgrade I'm noticing I often max the speed out on flats and would like a bit more gearing. 11T is pretty much the best I can do on the rear cassette so I'm considering upgrading the crankset from a 42T to a 48T or bigger if I can. 

My current rear cassette is a Shimano Acera 7 Speed 11/28T Cassette  CS-HG41-7.
My chain is a KMC Z51.
The front derailleur is a  Shimano TZ30.

I'm aware that the front derailleur will possibly need to be adjustable to get the chain over the larger 48T chain ring. And possibly the chain may need to be lengthened. 

The crankset I have in mind is Shimano Alloy / Steel Triple Chainset FCM131 28 /38 /48 teeth ratio suitable for 5, 6, 7, or 8 speed.Is this likely to work ? Can I go bigger? And what compatability considerations do I need to take into account in this upgrade?

It's my first time upgrading a crankset so any advice would be much appreciated.
Probably you have already figured out an answer from the given comments, but here is my answer.

I have a bike with a very similar set up. It is an old MTB with rigid fork that I turned into a fast commuter. Like you, the first thing I did was changing the tires to a pair of "1.3 Continental Sport Contact. Later, I changed the crankset and the cassette, which happens to be the same as yours.

I am using a triple crankset Shimano FC-M171 (48-38-28). You can buy it new for around 20€.

About the required adaptations, I would say:


Don't change the front derailleur, probably yours will have enough room. You will have to move it up a bit. A rule of thumb is to leave 2 mm clearance between the bottom of the outer cage plate and the teeth of the large chain wheel, but you'll have to try to find the best fit. Sheldon Brown has a nice guide about derailleur adjustment.
Replace the chain, which is advisable when changing the crankset and/or the cassette (they all wear out at the same time). Also, the new crankset will require a longer chain. I'd say something between 112 and 118 links. It is preferable to buy it longer and then remove the links in excess.
Probably you will need a wider bottom bracket (larger axle length). This is a requirement for the new crankset, otherwise it may happen that the smaller chainring will touch the frame. E.g., in the case of the FC-M171, it requires an axle length of 122 mm. The diameter of the bottom bracket will be the same of your current one. A low-end Shimano squared bottom bracket (sealed) costs around 10€ new.


I must add that for me this amplitude is great for every occasion, I can climb reasonably well and I can also reach 50 kph with a reasonable cadence (not sure, probably something between 80 and 110 rpm).

Go for it.