It's actually 4" components in the front with a 1" tweeter, but you can squeeze 5.25's in there with the right adapter. Remember you only have about 60mm depth to play with in the front and also you can't just replace the rears as they have an amp bolted to the left speaker, so you either need to extract the amp or fit another one (it's only 2 x 20w). Personally I wouldn't bother with the rears as whatever you fit will sound pretty awful due to the positioning. Spend more money on the fronts.
You'll need fitting adapters for all speakers.
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I mounted my Rainbow crossovers here, but they are slimmer than the Infinitys IIRC:
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Looking at the image of your door inner, how thick was the mdf you used and how did you attach it to the door panel?
I can see the screw form speaker to mdf but have never had mine off yet to inspect.
Also did you re-wire from amp into door or us existing wiring into the doors?
You use the original speaker mouinting posts but because you're stepping up in size you need 2 MDF rings. One 9mm ring cut to fit the 4" OEM opening, and fit over the OEM mounting posts (as Audi kindly don't have flush fit fixings), then a 6mm ring cut to fit the 5.25" new speaker size, with slight notches cut into it so you can get to the screws to the mounting posts. The 2 rings are then screwed & glued together to provide a semi solid mounting plate for the speaker. The job is slightly complicated by the fact that the 3 A4 mounting posts are at right angles to the door card, but the aperture is angled upwards, so to get a flush fit to the speaker hole the 9mm ring has to have slightly angled holes and the position of them is quite tricky to get right.
Well worth doing if you're handy with a jigsaw as the whole assembly does provide a more rigid construction than the plastic off the shelf ones that need a whole bunch of silicon to seal them properly and prevent the speaker flapping in free air.
I was lazy and didn't rewire the doors, tried to with a wire coathanger but gave up. If you do rewire the doors then you can mount the crosovers behind the glovebox.
NIce one. I'm OK with a jigsaw I suppose, but I am not 100% the previous owner hasn't already upgraded the fronts, althought they do flap when cranked with bassy stuff, but compared with my mates 1.8T golf mine already sounds miles better and i'd expected his to sound similar thinking being VAG they'd use the same speakers in the front doors.
Where does the CD changer lead run as i'm gonna change this some time soon too.