Hey see the pictures below regarding how mine is drilled.
I made the measurements as well.
The holes are 20mm. I think we(OK my Dad as he is more accurate than me) did them with a hole saw.
I use a short offcut of a 3/4inch OD rubber hose that I think has a 1/2inch ID to adjust the fronts.
I just basically shove them down the hole and twist with pressure on the adjusters.
You can feel the clicks.
The car actually came the with Purples fitted brand new from the previous owner. Just no holes in the strut bar.
I am happy with them.
They work really well on track.
I thought with the front springs being more than double the stiffness of the rears the car would understeer, but it does not.
For the first time ever I actually adjusted them to full soft yesterday as my Dad wants to drive it to work every now and then since we can no longer do track days for the foreseeable future.
It does not feel like there are 11kg springs up front now at all.
And I have never hit the bump stops on these even when hitting ripples strips on track.
The factory Bilstein's I had on our first NC1 LE practically hit the bump stops all the time.
Then again standard NC springs are the softest of any MX-5 at 2kg/mm front and 1.5km/mm rear.
Also mine are not setup as per MCA's recommendation.
They say to have the rear a little lower than the front.
I have the rear a little higher than the front.
I have no idea why they would say to set it the way they do as that will likely cause understeer.
I think from factory they are a little lower in the rear than front once a driver is in it but that is with a completely different spring ratio of 4/3 where this is now more than 2/1.
And as I said before the height is very easy to adjust because it only has the one lock ring with a grub screw for the spring perch.
Plus it has tandem/helper springs. So it is very easy to go higher/lower and keep the springs seated.
The helper/tandem springs also allow these units to have more droop then a lot of other coilovers. So you tyres stay connected to the road when cornering hard on track over undulations and bumps.