noobinsf 

This is a SR Royal crank from the early 70’s with unknown mileage (came on a used bike, I’ve never used it). I noticed what I worry is a crack, or maybe even two. Is this a common area for a crack to develop? Or is it possibly just a scratch? I ran my fingernail over it, and it definitely catches, so it’s not just a discoloration or blemish.

trailangel

Sure looks like it... or a bad forging that's been that way for years.
Take a file to it... see how far it goes.

Bill Kapaun

Or a light grinder.
My guess is it's just a surface defect(s)

sdmc530

Does the one closest to the center have a radial crack? does it go around the bend? Looks like a crack but might have been from not great quality control and probably not from damage.

I would do what already stated, file it a bit and see if its deep or just a light imperfection. If its been ridden like that for however many years I doubt it will be an issue now.

Andrew R Stewart 

Scratch a line at the end of the "cracks" 90* to the "crack". Monitor and note if the "crack" continues past your scribed line.

I will say that while I have seen quite a few cracked/broken crank arms (I have a Campy NR arm end in my desk 3ft away right now) I also have seen many crank arms with infant cracks which never have grown. Andy

davidad

I would file or grind to good metal. If it's shallow I would ride with it. If not, replace it.

Brocephus

The ones in the upper right circle look more like scratches, to my eye. But the bigger one in the lower left circle, really does look like a crack. Also, the one that runs off the "0" in the "170" looks like it may be some kinda stress fracture, as well.
I concur with the tests and remedies suggested above.

Bill Kapaun

Looking closer, it looks like you might have "fracture planes"???

FiftySix

That's what it looks like to me, too.

I'd probably use it as is.

noobinsf 

I really appreciate everyone's advice. I've decided to sell it with full disclosure on the uncertain imperfections -- it's posted in the C/V sales forum with a link to this thread. I've priced the entire crankset to reflect that this drive side arm is essentially an extra, and I will let the potential next owner decide whether s/he wants to use it as is.

dedhed

https://www.amazon.com/Dye-Penetrant.../dp/B00XLW4BRC

Bill Kapaun

You have to have a smooth surface to use dye penetrant. Those "indications" would pool the dye and show gross bleed through, proving nothing.
DP just shows an indication of a crack/porosity. The OP already has "indications".

CliffordK

Looking at the closeups, I think it is at least about 1/8" deep. But hard to tell if it is deeper.

Any engine (classic building) shop should be able to do the dye test, probably pretty cheaply if you can get them interested in helping you.

As mentioned, what appears to be a crack is deep enough that the dye test would likely fail, so probably not telling you much.

Crack or not, it would be a point where a fracture could originate.

I've been watching Forged by Fire a bit. A different field, but any small forging defect propagates, and gets big very quickly causing critical failures.

fietsbob

polish, file/sand it until you no longer see a crack .. to confirm its not a crack..

TireLever-07

If you have the skills to remove the BB Bracket & cranks, do it. It used to be easier,114MM ? Wide BB shell & only needed the crank remover, And a 3/8 " drive socket set with a few Metric Sockets. And a chain tool to replace the drivetrain. And be back on the road, taking another pounding. But if the as old as this crankset. Try a bike co-op. But if you're riding enough now. You might want a better lighter bike soon. Th drive rain takes a beating, lot of dirt hanging low to the pavement. Chains stretch Cog & Crankset teeth break.

rollalongnow

If I were buying it I’d find something else. If it’s not a crack, it sure as heck looks like some stressing of the metal as a minimum. Especially given there are two close together in parallel.