1970 ct175b. Starter gear on shaft has flat spot with no teeth (see picture). When installing, how is flat spot aligned on shaft splines?  Thanks in advance. 

You have to use a 3rd party hosting service to post pictures right now...

That said, I'm pretty sure if starter gear is missing teeth it's broken?

My CT3 had a similar thing happen. The idler gear bearing had disintegrated which led to a domino effect including broken teeth on the starter gear and idler gear. The bearing recess is the aluminum center case. When the bearing spun it made the recess larger so a replacement bearing was a 32nd or so smaller than the recess. Later CTs had a larger OD bearing so I used one of those, peened the recess and installed with a special grey loctite designed for industrial use. It’s holding fine. Not saying this is your situation but you may wish to see if there is play in the idler gear.

On all those early models up to 175--You "might" get away with a tooth or three missing on the kick gear if you worked out how much of the circle was used in a full kick, as that % of the circle always stays the same, unless you've had that gear of the kick shaft & re-fitted it on a different one of the twisted splines. [think i'm thinking right]--umm. Never had to try it though in practice.

Thanks for response. The gear is not damaged. It is similar to the one used in early YDS2 # 152-15641-01. 
I'm replacing damaged starter shaft and must use this gear as it is different diameter from other ct175 gears that I have. I'm assuming blank area would be next to idler gear with kick lever pushed straight down. This would then disengage gears when motor started. I have had AT1 and CT1 apart and never seen this. 

I used google photos yesterday & looks like i've been there before but forgot.
www.google.com/photos/about/

Kick gears on the 125/175 were sometimes a weak spot...  they were changed and strengthened several times.  The first three years (69, 70, 71) had different size gears. 

Someone might have found an earlier gear fit...  but all the CT gear were full circle. 

If you find a manual for that YDS type, it will probably tell you how to index a gear that isn't full circle.

On my Ct1c parts book has a 26 tooth for the early case and a 27 tooth for the late case this is the idler gear I wish we could see which case you have but photos not working now

Not aware of any yamaha's with a gear like 'that'--well, ok, here's one but no trail bikes like that. Pic somehow would be good when pos. Thanks.
www.ebay.com/p/1624676428?iid=372652154590

Thanks for info. I installed with flat spot aligned with idler gear with kick lever at 6 o'clock position. S.W.A.G. method. It works great and starts easily as it should. I thought too this may have been a service fix some point in time to prevent kick back. Thanks again. 
 RIDE SAFE!!