I just spent two weeks cleaning up a flamingo pink 1977 Schwinn Sierra which someone gave me. I am new to repairing bikes, and converting it to a single speed was something that I've been dying to learn how to do. (I did save all the parts, in case the purists freak out about the conversion). Today, I finished everything and was 5 minutes away from finally taking it out for a ride, when I realized that there is one major problem I never even thought to investigate: the diameter of the hole for the pedals on the crank arms is much smaller than that of the Speedplay Frog clipless pedals that I wanted to put on. And it's probably smaller than any pedals made since 1977.

Now I can't ride the bike!!!!!!!!!!!! I am on the verge of punching myself or crying, which ever comes first.

Is there any kind of adapter that I can find to attach to the crank arms so I can use my pedals with it? Unless I can find some, or unless it is possible to pay some metal shop to drill out the arms so they fit the size (which I don't think is possible), I cannot use the bike. I refuse to use platform pedals. The arms are not like modern arms that just have simple threading you can unscrew. They're housed in the bottom bracket.

Someone, please tell me that this problem is able to be solved!

Adapters are expensive, but can be found.

What you need is 1/2" pedals, common for BMX and other one piece cranks. I know of no clipless pedals for 1/2"

EDIT:

A pair of adapters are available here for $45 or so...

https://sheldonbrown.com/harris/pedals.html

Another EDIT:

There are also adapters available to replace the one piece crank BB with a more traditional 3 piece crank.

That's basically like a Continental with a one-piece crankset, right? You should be able to run down to Wal-Mart and get some 1/2" pedals. Don't know whether or not better-quality pedals are available in that size. Someone else knows, I'm sure.
The standard size for quality bike pedals is 9/16".

It's almost identical to the Continental, yes. I don't want platform pedals, so just finding 1/2" pedals isn't an option. I have bad knees, and the Speedplay Frogs work wonders, so they are the ones I need to use.

Have the Speedplay spindles machined and threaded to 1/2".

Wouldn't it be better to enlarge and tap the cranks to 9/16"?

MUCH easier as long as there is room to hold the threads. Compare the crank arms with more modern stuff. And if they are steel arms (I have no idea what sort of bike a Sierra is) then you should compare what drilling and tapping would do with a steel arm.

The big trick will be that you'll likely need to send the arms off to someone with the set of taps or get a set and do the job yourself.

1-piece cranks from that era usually don't have enough metal around the pedal hole to do that safely.

I think you should switch over with a BB converter, a threaded BB, and a 3-piece crank.

Another problem is the left hand thread on the NDS.
That can limit your options about WHO does the mods.

I got adaptor rings that were DMR make from ebay for £1.
They push into the BB shell and are threaded for a set of english BB cups.

Ive got a Raleigh MTB that has a square taper axle in a Push in american BB.

Sure theres ones available for low cost.

Between the holes in the arms and the rounded edge of the arms, there is not a lot of space. Even if I could find someone to enlarge the holes to fit my Speedplays, I don't think it would be possible. I am, as I said, completely unknowledgeable about bike repair beyond the very basics.

$45 is a whole lot of money for two little adaptors, but at least I know that there is an easy solution to my problem. Would that be cheaper than switching out the cranks (which I don't know how to do)?