What's the shortest Square Taper BB?

[FluffyChicken]

Yes, just past centre of the top ring and then it settles back (depending on the shifter)
(it should actually be easy to have the inside plate hit the top ring.)
Are you sure the crank is on properly, you're talking ~7mm out here and it woudl be hard to have a crank that badly fitted.

Something up with the mech..

Sorry 50mm (and 47.5mm) are the chain line. Retro setup use 47.5mm and later 90's and after started to move over to 50mm as the standard more
They are both valid chainlines

A chainline is the middle rings distance from the centre of the frame.
I thought there would have been a Sheldon Brown link by noe, so here have one https://sheldonbrown.com/chainline.html

It's the line the chain runs on, front and back. So if you had a ring front and one at the back, they would ideally be in line. Here it's the position where the chain work best over the range of cogs. It's not the be all and end all as the chain is flexible and we have lots of gears.

 

[jimmy5willy]

Re:

So, what you're saying is the middle ring should be 50mm from the frame? I can look into this aswell tomorrow. Might give some clues.

 

[My_Teenage_Self]

Not 50mm from the frame...

The centre of the middle ring should be 47.5-50mm from the centre of the seat-tube.

 

[Prodigal Son]

Crankset says what BB length you want.
47/50mm is the ball park chainline measured center of seattube to middle ring. I suggest measure from bottle cage bolts on seat tube till mid ring.