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Vorshlag has long promoted that you can never have too much camber in your McPherson strut suspension. For the R56 Mini, which is front biased and puts over 800 pounds on each front tire, this is especially true.
When customers are buying wider wheels and grippy tires for track or competition use, that puts more load into the suspension, and the squishy bits (bushings) tend to move around more. This further drives up the need for more static camber, to -3.0 or even -3.5° up front for ideal tire wear and maximum grip.
Adding camber plates to a Mini is a real commitment, especially if you go straight to coilovers. If you are going to this much effort, you are going to want to run a lot of camber.
The problem is that the strut tower opening is very small on the R53 and R56 (see above) chassis. So even with our camber plates, you might not be able to get to the ideal camber or caster setting with some ride heights or strut brands, as the strut shaft will can into the opening at the strut tower.
This picture above is with an opened up tower on an R53. Cutting the tower opening larger helps you get to the alignment adjustments your tires might need. And if you want to run any additional positive caster, this way you won't lose camber adjustment.
Some people have really hacked up towers - with a Dremel or saws-all - or made some hole saw drilling fixture out of a block of wood, which is easy to get started wrong and end up with a hole that is off-center.
This is where the Vorshlag Strut Tower Cutting Fixture comes in. This tool bolts into the Focus strut tower from underneath, to help align a hole saw cut. These are CNC cut and welded together on a production fixture, at our facility. Bolt it in, line up your hole saw bit, insert a 3" hole saw (or bigger, if you like) with a 1/4" arbor bit into the fixture, and cut down towards the plate. The raised centering bushing lines up the cut so it is never off-axis. You should be able to use this tool dozens of times, on both sides of the chassis. The saw might scuff the jig from touching the plate, but it's 1/4" steel and can take a beating.
The fixture comes raw and unpainted - we are trying to keep costs low. You supply the top nuts, the hole saw, and use a 1/4" bit for the arbor. Don't worry, we've done this cut on dozens of cars for many years, and have not seen any weakness in the multi-layer towers on these cars.
This link shows how to use use this fixture in the S550 chassis. This same technique works on a number of cars (a McPherson strut design is pretty generic)