When upgrading from a Claris to an Ultegra crank, the latter has a gap between BB and crankarm I have a Giant contend 2 2017, its been a great work horse but now it needs changing, I got an Ultegra groupset for cheaper than a 2nd hand Ultegra spec bike. I've started with swapping the crank over, but noticed a gap between the BB and crank arm on non drive side that wasn't there on the claris crank. I'm assuming I need spacers but none came with Ultegra crank.
What measurements do I need to take to know:

crank is in right position to too far one side or other.
what size spacers if needed and fitted which side,

Tried looking for build manual but so many different variants can't find what I need.
Thanks
Matt
Some Hollowtech II cranks have a spacer that's supposed to go under the left crank. You need to find the exploded view diagram for your crank model to confirm this. Here's the one for FC-6703 for example, the triple crank for the second generation of Ultegra ten.

If you don't have it, it will probably be okay because these spacers don't interact with the seals in any particularly important way, although they do act as a sort of shield for the seal area to a limited extent. You do need something in its place though, or else you won't be able to preload the bearings. If this is the kind of seal you're missing, the simple thing to do would be get a bunch of scuff washers or whatever other tuneable stack of 24mm spindle spacers you can get your hands on. The nature of Hollowtech II is that as long as the spline engagement is complete give or take some very small amount, the exact dimension of the distance between the cranks doesn't really matter, so you're free in this case to just find something that takes up the gap.