Early GTA's were fitted with 305's, then later, from 2004 i think, they moved to 330's.
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Yep, because if you (like I did) drive your GTA like you stole it when you come to stand on the anchors you'll find the 305mm overheat and wrinkle up like ya nan's ****, so they went up to radial mount 330mm which didn't overheat so bad.
okay, so as someone who converted by 1.9jtd m-jet to Brembos you've got two options.
First (more expensive) option
You're going to need a set of GTA (156, 147 or GT3.2V6) uprights. The lug mounting holes are slightly wider on the 3.2V6s than a standard 156/147/GT and as the 305mm brembos as found on GTAs, GTVs, Coupe Turbos etc are lug mount then you need these uprights to fit them.
The holes on the uprights on a standard car vs the brembo car are not narrow/wide enough respectively that you can form a bracket for these, so uprights is your only option.
The uprights may cause issues with drive shafts and ABS sensors as the GTA sits slightly wider than the standard car, so this needs to be factored in as well.
all in all making this something of a no-go.
second option - the 166 way
The 166 V6s also had brembo calipers. These are lug mount also. Happily the lugs are so far spaced apart on the 166 calipers that you can fabricate a bracket to fit.
You need a stepped bracket ideally although I do know of people that had made these up out of plate and just stuck washers in there, not for me though!
you can see I started out with a plate set and used these as a template for the machined stepped bracket.
Those stepped brackets are solid steel, weight in at around 1kg each, as I didn't want aluminium (as it has no stress point) and I wanted something pretty beefy. They cost me about £80 to have machined from my template.
From there you need a set of 3.0 GTV V6 brake discs and to mount the lot up using 166 brembo brake calipers.
The calipers are a fair size, so depending on what wheels you've got depends on whether or not you're gonna need to space your alloys or not. That I can't help you with.
I should mention there is a 3rd "off the shelf" way - the Brembo GT kit (£££) which provides a radial mount 305 kit with brackets etc, but that will likely cost more than the cars are worth these days.
As for those saying they're not worth it, I disagree completely, I'll take a set of brembos with decent brake pads in over a single pot slider any day of the week - but the biggest factor is your friction material. you can have the 8 pots off a Ferrari bolted to the front of your car, but if you fit cheap ****ty brake pads they're gonna be no good!
edit to say "oh yeh, forgot..." also make sure you change your brake fluid at recommended intervals (or less) which I think is every 5 years. people never do this and will make all the difference in the world to your brakes. over time brake fluid takes on water. water boils at a low temp and causes gas in the system which compresses, meaning your brakes turn to mush when they get warm. fresh brake fluid feels also as good as smearing Cheryl Cole in baby oil, believe me!