I just finished rebuilding an old Raleigh Reliant that had standard threading on the left pedal (and matching crank arm threading, of course). I almost killed myself trying to remove the darn thing before deciding "What the heck, why don't I try the other way?" Much to my surprise, it worked. Indeed, upon inspecting the pedal, it was marked "L," but had standard threading on it. I've never seen or heard of such a thing.

Has anyone else ever encountered a standard threaded left pedal?

Never seen one before, but I know that it was the Wright brothers who invented the left threaded pedal

I'd heard the Wright brothers thing before, but it reminded me of an important detail: this bike is not an antique. I don't know the exact age of the bike, but it is a bike boom 10 speed. Without checking the BB, I think it was probably made in Malaysia.

It probably came off of a tandem, which had left-side drive.

Neal

That sounds probable but is disappointingly straightforward. So much for the rare and mysterious standard threaded left pedal!

How do you keep it from self-loosening while you ride?

When my friend built a tandem crankset out of three standard cranksets, he had to crossthread left-threaded pedals on the two left side drive cranks. When he tried putting clockwise threaded spindles into left pedals, they quickly came loose from the cranks, making him appreciate why we have left-threaded left-side pedals.

To tell the honest-to-goodness truth it is a Craigslist single speed conversion, which means the only riding it got from me is around the block a few times. In those few trips around the block, the pedal did not loosen.

Now that you mention it, I think I'll try to Loctite the threads and put a few more spins around the block on it just to be on the safe side.