Stirrup bar rivets

[Kbeldam]

I’m looking to repair an older CWD with a wood tree - need to reattach the stirrup bar on one side, what size rivets will I need? 

[Mulesaw]

@Kbeldam I can't say for sure, but I am pretty sure that they are either #9 or #12. I am uncertain if the rivets are iron or copper, so you just have to substitute 1:1

You can measure the diameter of the hole in the stirrup bar, and that should be the diameter of the rivet. 
I would guess that you need a rivet that is at least 3/4" long, given that it has to go through the tree and the stirrup bar, but again that is something that you can measure. 

Without having seen your saddle, I am more worried about why it came loose in the first place? 
To my knowledge it isn't a common thing to come loose, so please check the tree very carefully to see that there isn't a crack of some other damage that has caused the stirrup bar to come loose. 
The stirrup bars carry a lot of weight, so if one fails e.g. during a jumping competition, someone might get hurt. 

Do you have a picture of the saddle with the tree exposed that you could post?

Brgds Jonas