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My winter bike is a Merlin with pressed in Campy Spindle. the left arm got damaged and would like to simply replace with a Suntour or Sugino that is available. How close is the JIL to the ISO Tapers?
 

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It'll be 4.5mm out of whack, per Sheldon Brown

Square Taper Bottom Bracket Interchangeability

This, of course, sets aside the issues of symmetrical versus asymmetrical spindles....which may further ruin your day.
What he said. Angle of the tapers is the same, but the width is different.

More specifically, per Sheldon's article, "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."

That might work for you, depending on the actual dimensions of the parts and how the bb fits the bike, or the crank might bottom out and prevent proper tightening of the bolt, or the crank might bottom out against the bb cup. Hard to tell without trying it.
 

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i've mixed jis and iso a couple times without issue.

i just rebuilt my paramount with a jis spindle and vintage campy crank. it works great
 

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i've mixed jis and iso a couple times without issue.

i just rebuilt my paramount with a jis spindle and vintage campy crank. it works great
Yes, but that's the "no problem" way because the crank is further out on the taper. OP has the opposite "there may be a problem" way where he could run out of taper or run into interference.
 
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