Trouble Installing 18 spline A-833 through dual disc clutch assy.

If you go to the Brewer's "pivot" page and "fork" page, which are currently installed?

http://www.brewersperformance.com/products.php?cat=103
http://www.brewersperformance.com/products.php?cat=102
FWIW, I've never dealt with this type of setup, so I'll stab a bit on it.
As far as the input to disc engagement, I think I'd look at the splines for possible burrs that may interfere with engagement. This is not to say add chamfers, but make sure all edges are clean. Same for the input.
Maybe try to bias the flywheel side disc upward on the alignment tool when securing the PP.

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