I have recently brought a heatsheild for my zed, im looking to fit it soon and i know how too, i just wanted to know do i need to countersink the holes after i have the drilled them to allow the heatshield to fit better ???
If you countersink the holes through the plastic slightly it will let the heatshield fit tighter to the skirt. however, if you are only using self tapping screws, there will not be as much for them to grip so don't overdo it.
If you have the proper countersunk head stainless bolts you can countersink further and get a really close fit. (If you need some, try your local engineering supplier - they will be cheaper than MG dealers. I got mine from Lincolnshire Fasteners in Sleaford; 6 bolts with washers and nuts for £2.00)
Then you wont need to gain any access to the back as you would have had to do if they were nuts and bolts.
There is a fitting guide on this forum which should help. Just offer it up and work from the centre outwards. Use something like a 2mm drill to pilot drill the holes before screwing them in. Some people have advised using something like "No Nails" as well for added security but it is up to you.