Buying new crankset for old bike

[zed]

I've got a 2007 Giant SCR 2.0 and would like to upgrade the crankset to something like a Shimano 5750 105 10 speed (see below), but not sure if it would fit. I think the one on my bike at the moment is a FSA crankset (possibly model = CK-400SST) and it has 3 front cogs and I think a 9 speed cassette. So this would mean I'd also have to replace the cassette as well? The bottom bracket is cartridge. Will the front and rear derailleurs need replacing? They are Shimano Tiagra - not sure if the derailleurs are tied to a speed e.g 9 speed or they can be adjusted to be used with different cassettes? Apologies for the noob questions!

[Nobody]

The only things non-compatible are the bottom bracket (BB) and the LH triple shifter. The new cranks may come with a external BB. The front derailleur after adjustment may be able to lock out of the old small ring position by the limit screw. Therefore negating the need to buy another LH shifter as well. See how you go before spending.

[Duck!]

The crank will need a new BB. Although supposedly both 24mm diameter spindles, there's a poofteenth of a millimetre difference (I think the FSA is tighter). You'll need to purchase this separately, as Shimano road cranks do not come with a BB, due to the use of English (most common) and Italian (decreasingly common) threads or press-fit variants.

The shifter will work. Being a triple there's an extra shift point that you simply won't use (several of Shimano's left shifters were double/triple compatible with one lever) ; however you do need to be quite precise with setting the top limit and cable tension. If the limit is a little far out or the cable is a little too slack, you can start to shift into the extra gear position & damage the shifter guts. It's the main reason Shimano moved toward separate double or triple levers in more recent years.

You will need to replace the front derailleur with a double-specific one. Triple FDs have a deeper cage for the very small ring, and do not play nicely with doubles.

[zed]

Great, thanks for the replies.

So even if it's an old bike, a new bottom bracket should still fit? I think it's 68mm..

Basically I'd like to upgrade all the components to 105 and then eventually probably get a new frame & forks. I'm doing it this way, partly because I like to tinker with bikes, but also because I don't have the $$ to buy the 105 groupset in one hit. So I just need to make sure everything is compatible before I buy it. This is the groupset I plan on buying, bit by bit.

[Duck!]

The bottom bracket is a standard English thread, used in the vast majority of frames. A new screw-in bottom bracket appropriate to the crank will go straight in. The SCR was the overseas version of the OCR over here; same frame, but the various regional distributors varied the component spec a bit.

You could buy that groupset bit by bit, but you will have to wait until you've got a lot of it before fitting it to the bike. There is a lot less compatibility with 11-sp. gear than there is between 8, 9 & 10-sp. It will all fit on your frame OK, but most of it will not work mixed in with your existing bits. You will also need to change your rear wheel (or hub at the very least) to an 11-sp. compatible model, as the freehub body the cassette slides onto is wider than that for 8/9/10-sp.

[zero]

Great, thanks for the replies.

So even if it's an old bike, a new bottom bracket should still fit? I think it's 68mm..

The only issue that may arise with your bike is that the bottom bracket shell might need facing to ensure its perfectly square, as that dimension is more important to external bearing BBs. I had that done on my MTB when I changed from a square taper to external bearings.