I have a Giant Rapid 2 ( 2010) with an FSA OMEGA TRIPLE MEGAEXO crank set with an FSA external bottom bracket, chainline 46mm. I want to install a Shimano UN55 Square Taper Bottom Bracket with a FSA Vero Square Taper Triple Chainset 9sp crank set. The BB comes in a range of spindle lengths 107, 110, 113, 115, 118, 122.5, 127mm. Would anybody know which spindle length that I need ?. I checked the existing spindle but no markings on it. I can't measure it as the drive crank is fixed to the spindle. Any help appreciated ?.

You select the spindle length that the CRANK requires.

Provided thatt he chainline is reasonable and there are no mysterious shifting problems with the current bottom bracket, just get the same length spindle as whatever came on the bike. Measure the length of the spindle from end to end and it will probably be close to one of the available sizes you listed. Order that size.

Thanks Einstein, "You select the spindle length that the CRANK requires."

To the legit reply , I can't measure the existing spindle as the drive spindle side is attached to the crank.

I found the solution, I know that the existing spindle has a chainline of 46 mm, so I measured from the middle of the bottom bracket to the end of the spindle on the left, added them together and got my answer. QED.

Ironically, the response you thought was sarcastic was the correct one.

I thought you already had a square taper crank and you just wanted to replace the bottom bracket. However, you have an integrated spindle crankset (Mega-Exo in the name tells me that, but I missed it the first time). THe integrated spindle length has no relationship on the square taper spindle length needed for a different crankset.

Sorry about that, just goes to show you what I know about BB's. So, for a given BB, on a particular frame, how do you select the spindle length. I don't see anything on the Giant site or FSA site.

https://www.sheldonbrown.com/chainline.html
As Bill said, you select the spindle that the CRANK requires. What frame you have has no bearing on the spindle length, so Giant certainly will not give you any info.
FSA does list a spindle length for the double version of your crank. Perhaps you could use that number and assume the chainline has to be a few mm farther out and buy a BB accordingly. Remember that for a double, chainline is measured to the centre of the space between the rings, and with a triple is measured to the middle ring. Do the math. Get a spindle that is longer by twice the amount you need the rings to move out.

It just amazes me that it's so difficult to find an accurate figure. I thought each manufacturer would have value for each BB without resorting to maths or trial and error.

FSA gives recommended spindle lengths for their Vero double crank, but they no longer seem to offer the triple crank (can't find it on their website, anyway)... I'll bet when they did they had a recommended BB spindle length for it.

Just out of curiosity - why are you swapping to square taper cranks? Although I use square taper cranks on most of my bikes, even I admit that external bearings are an upgrade.

You are correct, the external bearings are supposed to be superior due to larger spindle and bearings. The bearings on my BB are grinding against other so it needs replacing. Two of the Chain rings are also worn so they need replacement. I don't have much spare cash at present. It seems that the square taper cranks/chainsets are being fazed out so much cheaper to buy. This will do me for a few months to a year until I'm back in the black again. At that stage, I'll got back to the external bearings. It's my commuter bike.

I eventually found out. For Double chainset, I need 110mm and 113mm for triple chainset.