Real weird problem here.

If I search car part sites by REG for brake discs it tells me I need 256mm discs. Also eBay tells me this. Now the dealer tells me I need 260mm discs which some eBay sellers are also selling yet eBay tells me these discs don't fit my car.

So I measured a disc using a metal ruler, I leveled the ruler on an end and rotated it at the other side until I found the longest distance and I get it as 258mm every time byt I understand this method could be out by a mm or so but it should be pretty accurate.

Now there's no 258mm discs for my car that I can see.

So do brake discs get thinner from corrosion, could these discs really be 260mm but have corroded around the edges by 2mm?

  • The registration number can only tell you what size the manufacturer's original brake disks were. If a previous owner modified the car, all bets are off.
    – alephzero
    Commented Dec 17, 2020 at 20:43
  • If the circumference has thick rust on, then the diameter could also have increased.
    – HandyHowie
    Commented Dec 18, 2020 at 7:37

1 Answer 1


In a word? Yes. All it needs is 1mm of corrosion at the very outside to equate to 2mm worth of diameter (1mm from the entire circumference doubles because it's from both sides of the circle). The high heat which is generated in the rotor, compounded by the fact there's not wear action from the brake pads to wipe off any surface corrosion, could easily cause the rotor to flake off at the edges. I've found where it iron gets crusty like that, it'll easily flake off.

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