68mm Bottom Bracket Shell, crank and bb needed!
 

So I read I should get the crank first then bb to accommodate..

As I was about to purchase a crankset, (Dura-Ace 7410 172.5) the seller mentioned its for 70mm bb shell. So I stopped and measured mines, which turned out a 68 (6.7-6.8mm). Are these not going to work? I never knew cranks were determined/labeled by 70mm or 68mm shell compatibility as well..

68 is the most common standard, spindle is the critical part and they are very plentiful but you need to get the right length over all. ;)

Yeah so Im trying to figure out how I can determine what lengths I need for the spindle which would accommodate a 2x8 Dura Ace 7410 crankset. Whats the equation here?

You need a JIS 103 mm BB.

Just to be clear the crank itself is not 68 or 70 mm specific, Just the bottom bracket that is used with it. If you like the crank, go ahead and get it, then shop for your bottom bracket, won’t be hard to find.

70 is part of an Italian standard BB spec.. thread is different too , on the bearing cups 36 mm , not 1.370 inches.

and Italy .. fixed cup is RH thread, BSC, British/Asian etc. is Left hand thread..






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got it, so anything 68mm english threaded bb along with a 103mm+ spindle should do fine?

FYI, there were 70mm, English threaded shells. They were used on some Japanese brands, including Sekine.

Based on the info we have...yes. If you are going with the Shimano crank, the BB spindle should have a JIS square taper interface.

What they said.

Sometimes finding the right spindle can be frustrating. The width of the BB shell dictates the distance between bearing races so most any British (non-Italian) width should do. But the required length depends on the shape of the crank arms and how far outboard the ring mounting is w.r.t. the spindle hole. Different cranks could be 10mm or even 15mm different. Your spindle can be a mm or even 2mm off from the spec as long as the small ring doesn't hit the frame. After all, you will run different gear combinations and only two combinations will have perfect chain line; all the other combinations will be offline to some degree. But you don't want it so far off on the combinations you stay in most, and you don't want to pair up extremes, as say, a 115mm spindle with a crank that wants 103mm.

The taper is another issue that could be problematic but usually isn't. Almost all road cranks are 4 degrees, but the width between the faces is different between JIS and ISO. (IIRC, ISO is slightly smaller.) It is a taper, so a smaller width means the spindle will fit further into the hole. That affects the chain line. The more significant issue is that if the spindle is too small the cranks arm could bump up against the shoulder of the taper at the inner end instead of sitting flush with the taper faces. Also if the spindle protrudes past the hole's edge enough the crank bolt or nut hits the spindle and not the crank arm. But if the spindle is too large the crank arm won't engage the taper fully before bottoming out. This latter case is more of a problem if you are really strong and heavy and mash the pedals hard, in which case it could stress the hole.