Bicycle Mechanics - 170 or 172.5 mm crank

I am putting together a frame that is size 51 cm and usually it would come with a 170 mm crank. Would a 172.5 work or would it cause a problem?

TIA

Shouldn't be any problem at all. The crank arm is probably exactly the same length, the hole is just 2.5mm farther out. I've put 172.5's and 175's on frames that came with 170 and had no problem at all. If you're making a fixed-gear bike, some might warn you about pedal strike in the corners.

Crank arm length really only affects your pedal stroke. A long crank means your leg has to flex more to rotate the cranks in a full circle. If you are using too long of cranks, your knees will be too flexed at the top of your stroke and your knees will hurt similarly to how they'd feel with your saddle too low. If you are using cranks that are too short, you are losing potential leverage to power the bike (Torque = force x distance: same force applied to a longer crank arm gives you more torque). The frame couldn't care less though. As reverborama notes, the only other thing somewhat affected by crank arm length is your cornering clearance.

If you Google "crank arm length" you'll likely find some articles talking about how to determine what crank lengths are most appropriate for your body. The measurement is based off your inseam.

I am putting together a frame that is size 51 cm and usually it would come with a 170 mm crank. Would a 172.5 work or would it cause a problem?

TIA

Crank length depends on so many different things that all anyone has are guidelines. I have very short legs and use a 167.5 crank. I am in love with that
size.

I an wondering why you want a longer crank.

Take a look at a ruler to see how short a distance 2.5mm is. Would seldom make a significant difference.