This guide was written for the fitting of a BMW M3 front disc conversion. However, it can be applied fairly easily to the Apex big brake conversion, or just for replacing standard sized discs.
If you don't have anodised calliper spacers, I would recommend that you treat them well in advance (I painted mine with Hammerite high heat paint). (Figure 1)
Here's a look at how tiny the standard brake discs look. (Figure 2)
Loosen off the wheel nuts, jack up the front of the car and place it on axle stands. Remove one of the wheels and take a look at the setup. Unless you have virtually new brake pads, it will be necessary to change them at the same time as fitting new discs. Mine had not been on that long, so I kept them. (Figure 3)
Remove the clip holding the brake line to the suspension strut. You are going to need to move the calliper out of the way, so the line will need to be unrestricted. Remember that metal brake lines are quite fragile, so handle with care. (Figure 4)
You now need to undo the two 19mm calliper bolts on the rear (arrowed). These may need a soaking in WD40 beforehand and a breaker bar to loosen. Remember that you're your perspective facing the disc, these bolts loosen clockwise. Once the bolts have been removed it is a simple matter of lifting the calliper off the disc. Be careful to ensure the brake line is free of the suspension. Rest the calliper on a raised support of some kind to keep the weight off the brake line. (Figure 5)
You can now remove the old brake disc. This may be tricky depending on how long its been fitted. It'll be rusted-on and will probably need at least a couple of wacks from a rubber mallet to loosen (a cloth-wrapped hammer also works). If it still won't budge, you can try fitting a bolt into one of the two threaded holes in the centre of the disc. The bolt will apply pressure to the hub and force the disc away. (Figure 6 & 7)
If you are fitting bigger discs, you will probably need to either trim, remove or bend the brake guards back. I bent mine back.
The M3 kit's calliper brackets are fitted by lining-up the threaded holes on the plate with the original mount holes. The plate mounts behind the original with the long side running parallel to the hub. Use the short bolts and locking washers. Torque the bolts tight. (Figure 8)
You can now fit a new disc. The spigot ring need to be fitted with the angled edge facing the hub. The M3 discs are directional, so it is essential that you fit the correct disc to the matching hub. The interior cooling fins on the top edge of the disc should point towards the rear of the car. With the disc in place, use a few wheel nuts to hold it in position for refitting the calliper. Ensure there is sufficient clearance between the disc and guard if necessary. (Figure 9)
If you are changing pads, remove and replace the pads according to the guide here.
Carefully lift the calliper back over the disc so that it lines-up with the new mounting holes on the adapter plate. You may need to push the pads back (compress the calliper pistons) to get the calliper over the disc. The calliper will still need support. Pass a long bolt through the lower adapter hole from the rear. A bolt spacer needs to go on this bolt between mounting adapter and calliper. Tighten finger-tight. Repeat with the top bolt. Torque the bolts tight. (Figure 10)
Turn the disk to ensure it is unobstructed. Before refitting the wheel, pump the brake pedal several times to push the pads against the disc and check for fluid leaks.
If happy, remove the temporary wheel nuts and refit the wheel. (Figure 11)
Repeat for the other side!