So, I have a potentially stupid question. If I were to use a TA left-hand tandem crank (with left hand thread) as a drive side crank, would I run the constant risk of unthreading the pedal during a ride? This is more than a theoretical question given a situation in which I find myself...

A bit risky, Very risky if the cranks were French threaded and were tapped to 9/16 x20

I had a feeling that was the case. I bought a set of 165mm cranks, but I hastily reviewed the pics and missed some gouges that might make the drive side unusable. (Not the seller’s fault — he posted clear pics that I hastily skipped over.) There is a 165mm left side drive available from another seller, but it looks like I will find some other way to sort this out. Live and learn...

A lot has happened in metalurgy since then. Modern TA cranks are forged, machined and then polished. Especially the forging should lead to a stronger, lighter crank. No idea on what type of aluminum they were made out of in the old days and how much now though.

The adapter plate is cool, it would be even better if the projecting 'tabs' were trimmed.

Another advantage of the Stronglight 49D
 

Another advantage of the Stronglight 49D is that it is not anodized, making polishing easier and period correct

More Stronglight Pics:

https://live.staticflickr.com/3353/5...d9e3e4b5_b.jpg
Bertin Porteur by djk762, on Flickr

https://live.staticflickr.com/5714/2...293b8cb4_b.jpg
Stronglight by djk762, on Flickr

https://live.staticflickr.com/626/22...b726acbe_b.jpg
Stronglight by djk762, on Flickr

https://live.staticflickr.com/775/22...d2b98c5d_b.jpg
Stronglight 48T by djk762, on Flickr

https://live.staticflickr.com/5675/2...30a9abc2_b.jpg
Stronglight 48T by djk762, on Flickr

It's actually kind of geometric - the triangle cut lines are tangent to the central step. It was a long time ago and at that point

a) I knew I was better at straight lines than freehand-curves
b) all I had was a hacksaw and files

Nobody's pointed out the other aesthetic flaw.

I see that the 5-bolt pattern and the 3-bolt pattern aren't phased or clocked correctly with respect to each other, meaning the 6-spokes of the ring aren't symmetrical with the crank.

TA 5-vis outer rings always have 5 ways you can mount them onto the crank, and 4 of them are 'wrong' (if you value symmetry). Only one of the 5 ways makes the ring look right.

I'm thinking on your adapter, none of the 5 ways results in a symmetrical ring mounting, am I right?

Of course, function is 100% the same, this is only an OCD sort of concern.

Mark B in Seattle

Not exactly. They will line up, but that puts a chainring bolt under the arm.

A-ha, I couldn't quite tell from the photo whether there was one symmetric position.

I had a Pro 5-vis outer ring like that, drilled wrong at the 50.4 mm circle so that there was no way that was both symmetrical and didn't hide a bolt behind the crank.

I drilled another 5 holes between the existing 5 and called it drillium. ;) That ring also had its holes for the small ring at the Randonneur (116 mm) circle and I needed a cyclotouriste (80 mm) inner, so I drilled 6 new holes for that. Then I drilled 6 more holes at the Criterium (152 mm) circle, for the pants guard. It ain't pretty, but it went on a utility bike (grocery-getter), which is the opposite of a show-bike. It works!

https://cimg5.ibsrv.net/gimg/bikefor...9e6ea0d74a.jpg

Mark B