you put the rails in level with the stringers....then cut the plates to make the motor the angle you want it. its a tedious process of pulling the motor...grind/cut...put the motor back in, measure...repeat.
"shimming" the motor is only done if you screw up and cut too much off the plates....key is to measure a million times to prevent having to cut more then once.
of course, theres other ways to do this...but this is how ralph and i have done my last 2 rail kits
if you get serious about the rail kit, we can get it done in my garage no prob. let me know. it wont be like yesterday, i promise
