Hello,
I am rebuilding a 1951 bicycle. The pedals are old, rusted and white. From what I can determine they are M14 / 1.25 thread. The internet has very little about this size. Everything is listed as 9/16 inch instead of metric. I have a few questions for you:
1. How do I verify that these pedals are indeed 14 mm
2. Do you know where I could source (buy) metric pedals?
3. What is the difference between a 14 mm and a 9/16? Is there a lot of work to change them from one size to another if I cannot find the metric ones that I need?

Close up of existing pedal

Those have French threading. You should be able to verify this with a digital caliper and thread gauge. The most common threading is 9/16" x 20 tpi, which is a tiny bit bigger in diameter with a tiny bit wider thread pitch. I wouldn't think you'd have much problem finding a pair of French thread pedals but maybe so if you demand vintage correct. You can always re-tap the crank threads to fit 9/16" x 20 tpi pedals. Many of us have done this with complete success. Steel cranks will put up a bit more challenge than aluminum so use plenty of cutting oil. Whenever I use a pedal tap on a crank, I always insert it into the inside of the crank to ensure clean threads on the outer side. Also make sure you use the correct tap for the side you're doing. Left side pedals have reverse threading,

If you search ebay for “french thread pedals” or “pedales 14 125,” or variations on that, you should be able to find stuff available.

Here is one option: link. I know they're not white, but they are a similar style.

Now, as to removing the existing pedals — lots of penetrating oil and patience…

[MENTION=560593]donnyc[/MENTION] you don't have to go out and buy a metric thread gauge. Go to a hardware store and either bring the pedal or buy one M8 x 1.25 bolt. While the diameter is different, the thread pitch is the same. Position the bolt so that the tip of each thread on the bolt drops into the root of each thread on the pedal thread. If they all line up, then indeed, your pitch is 1.25 threads per mm. If they don't line up, find a 1/4" - 20 screw and do the same thing. If this lines up, you have a 20 thread per inch pitch.

I have an imperial size thread gauge, but hardly use it anymore, because this technique is quick and easy.

Very cool workaround, thanks for the hint