This may have been discussed before but i can not find it in a search. Is it worth it to put an XR lower unit on a factory bravo 1 drive. I know obviously you are gaining the larger prop shaft. Am I going to just destroy the upper unit anyways?

I have been running XR lower gears in my Bravo 1 for 5 years now with very good luck.

The upper is the weak link in a Bravo drive.

+1

If anything I would say the stock 1 inch prop shaft would twist and give a little. That 1.25 shaft doesnt have any mercy on the upper gears.

Putting an XR lower on a B1, wont make the drive any stronger. You still have the limitations of the helical cut gears. But they seem to be alot better than a bad set of XR gears. You never know what you are getting these days with the XR upper gears. I have stopped recommending them for that reason.
Depending on your power level and the respect you drive the boat with, the B1 upper gears (with thick floors, X gears as I call them) can survive.
I have built several B1's with XR lowers and they have done well in light boats with larger power. It helps to have the upper setup properly and steel towers in the case and cap.
One thing about the smaller propshaft, it used to be, and I don't know if it is still true, but he B1 propshaft used to be a two piece shaft. Welded right at the taper where the thrust washer sits. Years ago the Cats would snap the shaft at that point and loose a prop. Not sure if it that way these days. I do see several B1 propshafts with twisted splines, mostly V bottoms. The Cats will run the larger shaft. If you have a B1 with an after market shaft that is a good route also.
Hope this helps.
Dick