Cheap bike moan

My brother in law bought a Giant bike a while back for a couple of hundred quid and he had 28-42 tooth chain rings up front and he wanted to put some bigger rings on it. Because they were fixed I bought some new FSA cranks from CRC, square taper. Now I've gone to put them on and no matter how tight I tighten the cranks they won't sit far enough back on the taper. Im' going to try and find some old botom brackets for a comparison but I'm pretty sure that square tapers are one size only. Has any one come across this before.

Usually different axle lengths for different chainsets on square taper.

But the size if the taper shouldn't vary though should it. The other option is to get a shorter b/b as it was quite long anyway but trying to work out what's wrong with the current one

Yes!!!!!

Just had EXATCTLY this problem!!

Refit the original cranks fully, and measure the distance the outer ring is from a fixed point on the frame with an accurate method. I used a steel rule

Fit the new cranks again and measure the same thing.

Double the difference (remember it's a bit both sides) and order a BB that is that much shorter in the axle, or a bit less

A cheap BB is only about £8 on CRC and you don't have an option really….,

I had a similar problem with a Saracen years ago. Right from day one I could not get it to drop into the granny ring properly. It was a right faf. Swapped the chain set over to Deore and it shifted perfectly. There was a big difference between the two chain sets.

It's true. Chainsets have different chainlines. Well cheap SqT ones do anyway!

My bike came with a Suntour crank. To get the FSA one I bought to fit with the rings in the same place I had to go from a 122mm axle to a 113mm. That's 4.5mm difference a side. And that's enough to make the front mech unable to shift to the big ring

Cheapo chainset was likely running on a 122 or even 125mm bb. Your FSA is probably gonna want a 113mm. Square taper bbs vary,roughly from 103mm to 130mm – needs to be right.

There are two standards of taper – JIS and ISO. make sure you have the right taper for the bb/cranks.

hmmmf, I knew the bb length would come into play. These cranks were suntours and gone to FSA. Will have a measure tomorrow and get a new b/b. The drivetrain is going to be better than the bike itself!