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The threads on the right side of my crankset have been stripped and now only about half the threads remain (see picture below).

I can still insert my pedal all the way into the crank. Would it be OK to ride the bike like this whilst I wait for a new crank? Will it damage the pedal, or is there any chance the pedal could fall out?

Also is there a particular reason this would have happened in the first place? Under load, the pedal just popped off and took the missing threads with it. (The pedal is definitely the right sided pedal.)

remaining threads

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I would not ride with that crank as photographed, for anything more than a slow and careful "get home" temporary ride, with myself not rising from the saddle at all.

The pedal thread is only 2/3 to 5/8 there, and the thread that is missing is the side nearest your pedal, which is the fulcrum of the lever.

Stresses on the remaining threads would be much higher, making it more likely to fail quickly with no warning.

While replacement of the crankset is a valid fix, its not cheap and you may have compatibility issues. That looks like a Shimano RSX crank from the 90s through to some of the 8 speed sora cranks. Also depends whether you value the originality and period-correctness.

Instead, I'd look at a pedal thread repair, often called a Helicoil. A bike shop will have the special tools for this, which look like this : enter image description here

Its a replacement steel thread which gets permanently installed inside your bore, and the pedal goes into that. IMO it leaves a better connection than a steel thread inside an aluminium threadded bore.

Good instructions are at https://www.parktool.com/blog/repair-help/pedal-bushing-repair-kit-procedure

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    From a cost only perspective, a used Square taper crank set will probably be cheaper and easier, especially if the OP does the work himself.
    – mattnz
    Dec 18, 2019 at 19:59
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If you re-install the pedal and and ride with it the pedal may definitely become detached again. It did it before after all! I'd be suspicious that the pedal axle is not engaging all the threads in the crank, and that's why some are left relatively undamaged after the axle pulled out.

If the pedal axle can really engage the the remaining threads there is a high change it will rapidly deform them and strip them out. As Criggie says I wouldn't rely on the bike in this state for regular riding.

The good news is that the pedal axle should not be damaged. Axles are made of steel which is much harder then the aluminum crank arm. This is the reason that loose pedals destroy crank arm threads.

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