JohnDThompson 

Unless you're using French thread pedals, you want ISO 9/16" x 20tpi R&L hand taps.

SharpByCoop 

Oops. You are correct. Typing from memory. Thanks!

Breadfan

That's one way to skin a cat.

Garthr

If you really interested in a different perspective on crank lengths, read all of this. https://biketestreviews.com/cranklength/

I ride 150mm cranks with a mid-foot position on the pedal and and I'm 74" tall and proportionally long legs and foot size. To put it into words the best I can say is it feels very efficient, maximizing what power I do have. That certainly was never realized using 170, 175 or 185mm cranks. If anything efficiency was worse the longer I went. After refining my fit over the years it seems to me all that matters is the ability to push the pedals down and back towards the rear wheel, as fast as possible given my ability. More crank length forward is going the wrong way as it makes that more difficult, at least for myself it does. Having short cranks and a mid-foot position also has compelled me to move my saddle much more forward than I ever imagined(35mm on one bike, 25 on another), which in turns makes for a low aero position feel natural. Not something I could ever do before for very long. Now I'm totally comfortable riding the drops all the time.

How I got here is long tall tale, so don't expect to just radically change anything overnight !

I can't help with anyone of sources of power meter type cranks as I use a square taper and a 110 and 110/74 BCD. They're not hard to find using any search engine.

kcjc

I was using a 172.5 indoors and 170 outside. It was an older bike and dedicated setup for the indoor trainer. The longer cranks felt weird during the first few minutes of the first few indoor sessions but nothing at the end of the work or after a week or so. In the early part of winter, I'm still riding on weekends and the weird feel of the longer cranks didn't transfer to outdoor rides. I never felt it on the transition to outdoors after the weather forced me indoors exclusively.

VegasJen

I've considered trying this on my K2, which came equipped with 175mm crank arms. My only concern is there is only so much flat material before the arm is scalloped. I'm pretty confident it's enough to drill and tap the hole and still have enough meat that I'm not going to crack the arm. But this would be something I would certainly want to have a machine shop do just so I know everything is exactly square.

SharpByCoop 

VegasJen ^^^ Yeah, if you don't have access to a drill press, leave it to another. However, if you DO....

First I measured from the top of the threads to the other side. (It's 9/16 which is about 14 mm.)

Then I moved my ruler to the middle of the bore (the 7mm mark) and then scribed a spot down on the arm another 20mm farther. 175mm - 20mm = 155mm. That's where I center-punched a mark.

You can see the area just behind the original bore is flat. I raised it a bit with a piece of 1/4" material then clamped it hard there on drill press table. Leaving a bit of space under the spot where the new bore will exit.

Began with a small drill and grew up to the recommended tap size. If you are REALLY accurate you may leave it dirctly under the drill chuck and insert a center point drift.

Raise the chuck and lower the table to place the large tap in the hole, and bring the center point down into the BACK guide hole of the tap, to keep it perfectly square to the bore. Begin tapping with pressure on the chuck and it will remain perfectly square as you turn.

You CAN remove and do it by hand in a vise if you are careful. This is simply very best method.

Consider.