Those threads don't look damaged to me. Maybe a tolerances problem as stated above. I'd run a tap on the crank arm and see what happens.

If this were my pedal, I would grab a couple of steel crankarms from my cottered collection, douse the threads with anti-seize and forcefully run the pedal spindle in, using back-and-forth action.
The anti-seize is important, lest the metals gall (cold-weld) against each other, and which is still possible if high enough turning speed is used.
Back it out and clean/inspect if the turning starts to feel crunchy.

Hoping that this will re-shape the spindle threads enough to prevent the spindle threads from damaging the destination alloy crankarm.

I've had Shimano pedals which went in tight into older Campagnolo cranks, had to back-and-forth them in to assure that I wasn't going to shear threads or burst the pedal eye.

There are sizing differences between ostensibly identically-threaded parts like taps, pedals and crankarms, so running one or two different taps through the alloy pedal eye might ease the fit.

Now you tell me! :) I already ordered a new spindle. :)

Huh, maybe I stand corrected but I will say that when this comes up at the Drag strip, track or on the road where safety and 1000's of a second count, I will address threads like this with a very critical eye and take whatever steps necessary to make the next round or down the road.

I have never missed a round or didn't get down the road in my life, yet. ;)

You gotta do what you gotta do, but you also have to give a thread a chance to elicit the one idea that fits the need!

Not that patience is always my forte, and sorry to preach!

Yes, in this case, I just want to get some working pedals on the bike and ride. :)

The top photo appears to show that the blunted end threads are due to a factory bevel.

You've brought back memories from the drag strip of one round I survived even after breaking a selector shaft spring, having to dial-in at 18 seconds ET due to finding myself stuck in I believe fourth gear. Threw the opposing rider for a loop, him expecting a comparably-quick tree, yet I prevailed, only to go down in the semi (or was it the final?). <<<EDIT:, It was the final, long after dark, after being given a bye run>>>
Another time, due to a dragging clutch, I was unable to put the bike in gear after stalling it at the line and then re-starting it in neutral. So I had to push-start the bike in gear, which shouldn't have actually worked except for the fact that it was a quick-starting two-stroke and the heated tire magically found "reverse"-traction on the prepped surface as I dropped the clutch.

Destro = Right
Sinistro = Left

How about a photo of the offending pedal and threads.

If the threads are fowled with aluminum, then it may be able to be easily cleaned out with a knife or file. Look carefully at where the threads start.
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Ok, I looked at a few more posts and found the pedals. They look fine to me. Perhaps clean the threads in case there is any grit in the bottom of the threads.

I'd try the "S" (left) pedal on a different crank arm/bike. The problem could be with the crank.

Or does a different pedal work with that crank?

So do other pedals fit in that arm that the Campy won't thread in?

Yes, I took it to a shop to have the threads on the crank repaired (assumed it was the crank threads) but his test pedal went on the crank arm fine.

And there we have it, that pedal supposedly looked and seemed ok, yet is not, I would have worked those threads into compliance without hesitation. ;)

That test isn't definitive. Most pedals are made to a sloppier class of fit than Campy, specifically so they can go into cranks that are on the small end of their tolerance range.

Chasing the crank thread, perhaps to the larger end of the tolerance range for this specific pedal (like with my Hozan tapered taps) is a valid way to keep that pedal on the road. Yes, replacing the spindle is a valid way too, but I'm too impatient to wait for the replacement to arrive, then overhaul the pedal to replace it. I'd have my tap run through it in about 2 minutes and be out riding.

Assuming that worked! I know, not necessarily a given. But we know that pedal was ridden before, ergo it had to fit a crank at one point. It hasn't grown larger since it was taken out of a crank, and the pics are so good I think we can mostly rule out damage to the thread or aluminum cold-welded into the valleys. I think it's just a tolerance mismatch.

Mark B

I can mess with the threads on the spindle until the new one gets here. Worse case is I get it to work and have an extra spindle. :)

Just to provide some closure to the thread, I found a similar pair of Campy pedals at a swap meet today and they went on the crank with no issues. So, I guess it was the threads.