Im working on rebuilding a 175 Yamaha and it looks like there is a piece broke can anyone tell me what this is

Probably threw a chain. The missing plastic cover isn't a big deal... the problem is the end of the shift drum shaft is broken off. There's a retainer and shim that rides against the case...

Is it shifting okay and not jumping out of gear? That's usually what happens with that broken or not assembled right.

Replacing the drum requires splitting the cases. Or RT325 might have a way to fix that he's done like drilling the stub that's left and using a bolt and washer to replace the missing part.

Not sure about the shifting this bike has been sitting in my garage for about 25yrs. I got it when i was about 10 and out riding it pretty hard when i was a kid and the frame broke. It cracked right in the center of the main support beam, been sitting ever since. Well now my son is about same age so figured id fix it up. Found a new frame at a salvage yard and started rebuilding it. Most likely that happen when around same time frame broke would be my guess

If you're splitting the cases anyway, it's an easy fix to replace the shift drum.

If not, hopefully RT325 will see this thread and comment... if anyone has a proven fix, it's him. He's in New Zealand where I think it's Monday morning so might be checking in here soon.

Yep, morning all. Without getting too much into it, she's a mess there & not broken off square. Hook maybe 4th gear would push the drum to that side & get as much through as possible. Then i've drilled a 6mm threaded hole & used a bolt or allen head screw with a washer. There's a very thin washer that sits on there first but might be pushing your luck to fit that as is broken on an angle. Might be able to square it up somehow but still need clearance to revolve when the screw & bigger washer are fitted to stop it moving sideways. Doesn't look like you'll be able to easily fit the three screw cover over there so might dribble a bit & keep the chain oiled 'sort of' lol. Without retaining it to the left 'bad things 'can' happen like picking up two gears at once but probably wouldn't happen while riding hopefully. Must say i've not done this for 30 odd years but was a common quick fix on farm bikes where i worked to avoid major expense. As i think MarkT said, to replace it is a total dismantle. I'll link a pic that'll show whats been broken off. 15-16-17 have gone & can see the groove where that C clip thing fits & where yours has broken off. Good luck & hope no damage already done.
www.partzilla.com/catalog/yamaha/motorcy...2/ct2/shift-cam-fork

I must have seen this kind of damage a dozen times back in the 70s. We ran the chains and sprockets until they were shot. A false economy as a worn chain falling off worn sprockets can cause so much damage. A friend had a Benelli with such a bad front sprocket it would skip if the chain wasn't really tight. Made a horrendous sound.

So if i dismantle it what part will i need to replace?

The shift drum 8 and 15, 16, 17.

If you decide to go that way I probably have some good used parts.

You might have fun retaining 17 but is necessary so do what you can with the 3,-- 6mm threaded holes. Just drill & thread the stump, she'll be good. What's the rest of it like.

How hard is it to replace? also what do you mean just drill & thread the stump