Late 40s / Early 50s René Herse tip find

Clean the crank thoroughly with fine wet and dry then degrease. Do a dye penetration test. If not cracked and not really bent a gentle squeeze on two blocks under a press will remove it. Edit it does depend where and in what plane its bent before going down this route. I hope its not cracked. Good news is it's nds, a 49D is close. Best check both cranks for cracks though.
 
Oh no, that is really bad news 😢 Hope this can be verified that all is OK because the RH chain-set is such a centrepiece on this bike.

Moreso, it's got to be said this could all "escalate" quickly:

Let's say you are forced to look for a replacement, an original RH crank will be a nightmare to source and I dare not imagine the cost. Keeping it French and period wise correct means Stronglight and more tooling (what is to say the RH taper is compatible with Stronglight cranks?). There's also the pedal thread - Stronglight was available in 9/16" and French pedal thread. A move away from the French thread is not such a bad thing, but you have the original pedals already. Many early AL cranks (regardless of brand) suffered breakages and so what is available and in good condition can be very costly indeed - the time period here is sadly not helpful. A lot of stuff can be in poor badly man-handled condition too.

I remember going a little crazy on this subject and having a few PMs with @Guinessisgoodforyou and asking for advice elsewhere over what normally would have been a straight forward problem to fix after finding a crack in an early lightweight chainring.

If this was my project, I would seriously consider a complete steel cotter-pin chainset from a later mid-range donor French bike to get going. I'd give the TA Cyclotourist a good consideration mainly because of known robustness (even used on early MTB), no extra tooling and let's be honest would not be entirely out of place on a RH.

This is well worth reading too, because "underdog" brands like Nervar and Solida shouldn't be forgotten either if the budget is tight or the project not worthy ploughing in lots of money. https://velo-orange.blogspot.com/2006/05/what-about-nervar.html

It is looking like that DS BB cup may need to come out after-all. If you were to move away from a French chainset, I think I would go with an early JIS Sugino.
 
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I think a bit of measuring is in order to check the size and angle of the BB square taper and see what will fit. I was looking at 49D cranks the morning after. Visually they’re the closest match if I could find a single NDS crank.
 
I can post you a Stronglight spindle for comparison if needed. Edit I'd be inclined to change the chainset for Stronglight. If one crank is u/s the other won't be far behind, that and odd cranks is not great.
 

I did look at those. They actually sell single nds crank arms for $165:

https://www.renehersecycles.com/shop/components/cranks/rene-herse-crankarm-spare-part/
$165 aside, the main issue for me would be that it's too shiny and the wrong alloy. The 2014A alloy they've used is actually a quite similar make-up to dural aluminium, but I don't think it will ever look the same. And it's probably anodised.
 

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