Currently I have Gipiemme Sprint bottom bracket, italian threaded 70mm width and spindle of 115mm. I use Shimano 600 (6400) cranks 52T42T with them.

The bearings and the cups on the Gipiemme are a bit damaged and I want to replace the bottom bracket in the future, and I'm a bit confused about my options for upgrades.

Clearly I will need a bottom bracket that is 70mm width with italian threads (36x24tpi).
Shimano UN55 does strike as a replacement, since they make them in 115mm spindle, 70mm width ital threads, but they are a bit hard to find, though they are square tape spindle so I could use my Shimano 600 cranks with them.

What are other options for upgrade?
I noticed this while searching various shops:
https://www.planet-x-bikes.co.uk/i/q/...bottom_bracket

It's italian threaded and 70mm in width, though I have no clue about what kind of cranks with spindle do they take? Can anybody school me about what megaexo is and what kind of cranks and spindles can I use.

I guess I will need to keep somewhat similar chainline, so if anybody has any experiences with upgrading their bicycles with ital threaded BB's to something newer, I would appriciate any clues, infos or tricks.

Front derailer is 6400 too, and I use 8 speed cassette at the back with ergos+shiftmate to make it work, so the whole drivetrain thing should be usable with 8 or 9 speed chain (the two front chain rings are probably really close to each other in 10 speed setup, right?)

My current 52T 42T serves me well, but I would not have anything against 50T 36T or similar compact setup!

Thanks!

B.

The MegaExo BB works with MegaExo FSA cranksets, and would be a dramatic change from what you have now.
The UN55 IT is not so hard to find. I would replace with that and be done with it.

If you are really thinking about modernizing the cranks and BB, maybe consider a full new groupset. Depending on the level, it might not cost much more.

Is there anything similar to UN55 I should check out?

And how about standards? Since this bicycle is mish mash of components when I bought it and now I'm using an 8 speed cassette at the back, I have now idea of the correct spindle lenght.

I checked my crankset, they are 6207, and velobase says 116 or 119mm spindle lenght for 6207. I have 115mm currently.
Would using a 116mm or 119mm spindle with my 8 speed setup improve performance of the system?

Here's a good article on chainline.
Shimano wants 43.5mm. How does yours measure now?

I measured 46mm from the middle of downtube towards bigger chainring, so that puts me in the correct chainline width for double crankset I suppose. According to Sheldon's table.

Perfect. 70X115 it is. They're not hard to find.
Sticking with Shimano ensures the taper will be correct for your crank.