Crack or Scratch?
 

I just picked up a clean Vintage TA Cyclotouriste crankset on EBay, but after polishing discovered what appears to be cracks on the non-drive-side arm near the top of the arm. Are these cracks or cosmetic scratches?
https://cimg4.ibsrv.net/gimg/bikefor...b73ee335a.jpeg
https://cimg5.ibsrv.net/gimg/bikefor...b6a7164c1.jpeg

Not sure what to see in the 1st photo so no opinion on it. The 2nd photo, my opinion is it is a scratch stretching left to right. The reason I say that is at the left end, it appears that a two "features" on the left merge at a very shallow angle, which I interpret as a short scratch merging into a longer scratch. But if you think they are cracks I would not suggest not using it; from what I hear a breaking a crankarm can lead to a serious crash. You could try to find a lab that can do an ultrasonic inspection to see if it is a crack.

Agree the first photo tells us nothing but that "scratch" shown in the second photo appears to indeed be a scratch. However, it's deep enough and in a high stress location that I would worry about it being a "stress raiser" and could lead to a real crack and failure. I personally wouldn't use that crank.

Thanks, guys. I appreciate the educated guesses. I will do some further inspection, and perhaps an ultrasound as you suggested.

Surface scratches can be filed/sanded/polished out to remove the stress risers. This is commonly done on airplane propellers to restore nicked and dinged surfaces to airworthiness. Information you may find useful starts on page 23 of this Advisory Circular: https://www.faa.gov/documentlibrary/.../ac_20-37e.pdf

Thanks for the tip. I might try to have it buffed out.

:)

Looks like a deep scratch to me, too.

I'd call that a gouge rather than a scratch. To use that unit, I'd definitely sand/file/grind it out so that it has a gradual taper to the main part of the unit. Take a piece of wood about the same size and try breaking it. Now put a deep gouge/scratch like that in the same area of the piece of wood and try breaking it. You'll most likely be able to break that piece of wood fairily easily and the break will happen on or close to where the gouge/scratch is. that's why it's called a stress riser; it concentrates stresses to that locale.

Cheers

Thanks for the recommendation. This seems to be the course of action that I need to take if I want it to retain its integrity.

You want the change from the bottom of that gouge/scratch to the top surface of the item to be as gradual as you can make it. It'll be less noticeable as well as stronger that way too.

Cheers

Thank you. I will definitely do this, as I don’t want it to be at all noticeable.

Cracks are not that straight, so like others have said, scratch or maybe better term gouge?

It's a gouge. Anyway we need a better pic of the whole part. I thought it was a dented rim looking at the pic. LOL
Is it CF ?? WTH.
I would DEMAND my money back from the bugger that sold it.

Thanks, GamblerGORD53. It’s a TA Cyclotouriste crankset. I got it for a song on EBay, so I’m not complaining.

Why not have it Tig welded********************?

i want the bike to look brand new. It’s about pure aesthetics.

You could have that Tig welded and polished out and NOBODY would know.

Oh, and I forgot the most important part: it’s a scratch, not a crack.

Doesn't make any difference, a good tig welder could clamp that up and make a quick pass over it and be done. I haven't tig welded in years but I know I could do it.

There's no size/scale reference in these photographs. Is this molecular, atomic or subatomic level?

I'm guessing on the width of the crank arm in the bottom photo that the scratch/gouge is not over 1mm wide.