I have an 80's Ross road bike. My left crank became loose so I ordered a replacement off of the net. From my research, I ordered a JIS style square taper. Problem is, the new crank I received has a narrower opening than the width of the taper on the shaft. Based on what I have read, a JIS should be very similar with an ISO taper. The difference between the new JIS I ordered compared to what is on my bike has got to be slightly more than a 1/16". The JIS crank doesn't even start over the taper of the shaft. What am I missing? Thanks in advance.

Sounds like you got an improperly-broached crankarm.

Your Ross spindle should measure 12.90mm across, measured 3mm from the end of the spindle. I'm sure that it does!
An ISO or Campag spindle will measure a very close 12.7 to 12.8mm.

Certainly an ISO or Campag crankarm might, at worst go on just 3mm short of full engagement to the spindle.

Here is how to measure, noting that the caliper jaws are exactly 3mm thick, and that this is a JIS crank spindle:

That's kinda the same thing I was thinking. I'll bring a caliper home from work tomorrow and measure. Based on what I found during my research, I thought the same thing. Even if I had ordered the incorrect taper standard, it should still go onto the spindle. I'll measure and provide follow-up - thanks.

Something doesn't add up here. My spindle measures 13.56 mm and the new crank arm I ordered is 13.34 mm. I've taken my readings same as shown in dddd post above. Any ideas?

I've never seen a larger square-taper standard than JIS, which is 12.9mm

Maybe time to pull the spindle out and look for any identifying markings, but firstly, what is the crankarm's designation, brand, model, etc.?

This sounds like a weirdo crankset, not that an oversized square taper wouldn't be a design improvement.