Bottom Bracket spindle length ?
 

I am looking to replace a 175mm triple crank on a 1985 Ritchey with a new 170mm crank. The original Shimano triple crank was on a bottom bracket with a 121.5mm spindle. A 124mm spindle is recommended for the new crank that I’m looking at. Will the 121.5mm spindle work (2.5mm difference), or should I put in a new BB?

You will have to adjust the front DR either way, so give it a shot.

Not all are symmetrical, so it could right on to NOT!
IF the FDER will move in that much, you "should" be good.
You may have to loosen the cable to allow that much inner travel.
Edit: add
Just meandering- the original crank might not be a "perfect" length, but the best compromise.

Not mentioned is whether the old crank and new one share the same "end of BB axle to center of the rings" dimension. Of course this isn't listed in any spec list I've ever seen. Chain line is the holy grail by many but ring/frame clearances and (as mentioned) ft der range also come into play. So my answer to the OP is "you'll have to try the best first choice and see how it all agrees with each other". Andy .

Thanks for the advice. I'll give the old BB a try and see how it works -- I guess I can always swap it out if there are problems.

In 1985 with a triple. I’m going wil Bill on this one that the BB spindle might be asymetrical; which means there is more spindle on the DS than the NDS. It effectively equals a 123mm, 124mm, 125mm symmetrical BB spindle,

John

The only other thing I think worth mentioning is watching the crank arms and the chain stays, to be sure the crank arms don't contact the stays (using the shorter-than-recommended spindle). It's only 2.5mm shy of ideal, which is only 1.25mm on each side (assuming symmetry, which can't always be assumed), so you're not likely to have issues here...but it is something to watch as you begin to reassemble the bike.

It's all about the chainline, which is arguably the most important thing to get correct on a bicycle drivetrain. Different cranksets may give you a different (and incorrect) chainline using an existing spindle. You can test-fit it and see, but you don't go by whether you have problems or not - you go by whether the chainline measurement is in spec. The good thing about test fitting on your existing spindle is you can measure, and then you'll know how much longer or shorter the new one needs to be.