As the title says:
Is it possible to bolt a WRX Rear brake back plate on STI knuckle?
I have search and haven't found a clear definitive answer to this being possible or not.
Can anyone shed some light on the question?

Thank you

They are complete different sizes - what are you trying to do? We might be able to make some suggestions.

Thank you for your reply.

I am installing a R180 diff.
Would like to keep my current brakes, use the R180 axles, R180 hubs and knuckles.

So you don't think the WRX back plate can be modified to bolt onto the STI knuckles? I haven't yet gathered all the parts so I am exploring different possibilities.
What I mean by modified, can the center hole be widen up and a new set of four holes drilled to secure the plate on the knuckle.

well, I guess you could, but I've never done it.

Has anyone ever tried this? is the reverse possible? meaning putting the backing plate from an R180 to a R160 knuckle.

I actually have both R160 and R180 backing plates in my shop and was looking at them today. So this is not hearsay. The R180 knuckles are about a half an inch larger in diameter than the R160. So the hole in the middle of the backing plate is a corresponding half inch larger. The bolt pattern for the 4 bolts that hold it on is larger, too. Also, and this is the real hangup, the parking brake is bigger on the R180. Meaning the shoes are larger diameter and all that. You could theoretically cut out the R160 backing plate and redrill the holes to fit the R180 knuckles but I don't think you would be able fit the parking brake shoes and hardware around the larger R180 bearing housing. For one thing the tube the parking brake cable comes through would have to be cut a bit and the cable would be right up against the bearing housing. Also, you wouldn't be able to fit the ABS sensor back in because that hole would get cut partially away as well. So the verdict is it mght be possible but you wouldn't have a parking brake or rear ABS sensors.

As for going the reverse way, R180 backing plates on R160 knuckles, why on earth would yo want to do that? It would be easier, though. Weld material into the center hole to make the holes smaller by half an inch and weld up the 4 bolt holes. Grind down the welds and redrill the holes to the R160 bolt pattern and voila! But what's the point?

Dave,
Thank you again, for taking the time and comparing the two parts. So conversion axles it is for now until I am ready to replace my brakes.