They're on!
At 5 minutes past Simpsons tonight my Father and I went into the garage to embark on what could have been a fateful mission. I had in my possession a set of four AMG Mercedes wheels, of course set up for W202 fitment (10mm bolts). So, to get them on the A4 I would need to widen the holes, sticking to the 5x112 PCD and as accurately as possible.
And we did it. Perfectly.
I had played with various ways of getting the accuracy I needed, but we ended up using a modified table-top pillar drill, with the base plate turned upside down to sit flat on the wheel mounting surface. We would then manouever the bit until it was centred on the exsisting holes, gradually increasing the bit size.
This was the apparatus we used. I measured the original holes to be around 13mm (which seemed wide for a 10mm bolt) so, working with that sort of tolerance I figured we'd go with a 15mm hole, so we started the drilling with a 14mm bit.
Fortunately, assuming we lined up properly the bit pretty much auto-centred on the original hole as soon as it made contact with the the metal. After the first 5 holes were drilled, inspection of the prototype wheel (I chose the tattiest one for this) revealed our drilling to be absolutely bang on. Looking at the holes from the front, they are dead smack in the middle of the radiused parts of the original holes. I was chuffed.
Similarly chuffworthy was the fact that the new wheel bolts were a perfect fit in the new holes, so we didn't need to go up to 15mm. A test fit of the first wheel onto the car showed that the holes lined up perfectly with the hub, the bolts went straight in with no interference. And the same went for the next 3 wheels, and the next 15 holes.
Test drive revealed a bit of tyre noise (this should reduce after a bit of inflation) most noticable on deceleration. But nothing to fret about.
Pics coming when car is clean. I love suspense!