JBHoren

In his article "Square Taper Bottom Bracket Interchangeability", Sheldon Brown wrote:

[Conversely,] if you install a J.I.S. crank on an ISO spindle, it will wind up about 4.5 mm farther in than it would on a J.I.S spindle of the same length.

My question is: Is that "4.5mm farther in" overall, or "4.5mm farther in" per-side?

Specifics: A Sakae-Ringyo "SA" (road double) crankset requires a 122.5mm JIS/square-taper spindle; however, it seems they're rare as hen's-teeth for a 71mm Raleigh-threaded BB. But... a 132mm ISO/square-taper spindle (that will work with a 70mm BB shell) ought to work if the "4.5mm farther in" refers to each side of the spindle, where 132mm - (4.5mm x 2) = 123mm, which is close enough to 122.5mm.

Anyone know the answer? Is it 4.5mm overall, or per-side?

dabac

I'd say 4.5 mm in per side.
But I don't know if it's really that predictable.
Had some cranks and BBs I was mucking about with long ago, and they bottomed out against the BB cups.
I rarely see a square taper setup that's so tight that 2.25 mm further inboard would cause them to bind up.

Andrew R Stewart 

My experience is that the fit will be about 4mm per side. Did this many years ago to better set up a touring bike's left side pedal "Q" factor and had to shave off about 4mm from the inside of the arm's face at the tapered hole to clear the LH cup. Otherwise it fit well and I rode the bike this way for thousands of miles (over a number of years, not on the one tour...). Andy.

Reynolds 

I tried to use a Shimano crank on a Campagnolo 110mm BB and it didn't work because the inner chainring (42) contacted the chainstay way before the crank was tight. With the Shimano BB there was about 3mm clearance, so it could be 4.5mm per side.

fietsbob

It goes on Maybe half way (both are 2 degree tapers but the width differs on the faces.

.. JIS is More common ... so get a JIS BB for your JIS Crank

Or get a European Crankset to use your ISO BB.

(though some European cranks use the JIS taper )..

RrogerD

Hi,

Don't mean to hi-jack but my query's on the same point...

Is an SR Suntour chainset (cheap) JIS or ISO type?

Purchased a second-hand MTB and the tyre rubs on the front derailleur and chain when in the lowest gear. I'm aware that the tyre is slightly over-sized from what it should be but also, according to the manufacturers, the chain alignment seems to be 2 or 3mm narrower than it should be. This is all I need to get the clearance.

In other words and according to SheldonBrown, if it's a JIS crankset on an ISO BB then it is further in that it should be. Or, if they do match then I may be able to improve things by fitting the 'incompatible' type.

My spindle appears to be 113mm long with the longer tapers.

Any clues?

Thanks

dabac

If you still need to get another BB, why complicate things?
Just buy one of the same sort, but wider, to get a more predictable amount of change.

RrogerD

I don't actually need a new BB. I think that this is new as it is. My concern is that in not knowing what I've got at the moment.

I guess that what you're saying is, remove the existing one, hopefully it has what it is stamped on the side, and then purchase the next size up?

SkyDog75

JIS.

Even if it's not stamped, you can simply measure the length of the spindle to find out.

Instead of just buying the next size up, I'd suggest measuring your chainline (before disassembly) so you know how far you'll need to move the crank outboard. Then buy a new bottom bracket in the right size to move it over as far as need be.