Just did my first disc change, 2005 Ghia but disc I got from motor factors was too big(300mm) needed 278? Even though I gave them my reg. So the old chestnut never trust anyone because they make mistakes. I'm getting more cynical as I get older WHY because I see life and people as what they are EVERYONES out to SCAM you REMEMBER THAT! and everyone on this Planet is trying to get your money It's terrible but true I actually don't believe that about 50% of people in life are trying to scam you but the other 50% are good like me Don't buy expensive piston pusher just get a clamp and keep old brake lad in situ (see photo) God is great

Giving the reg number to motor factors isn’t always the best way.

some will take the vin number from the reg and look it up on another system to check what is fitted on the car (even euros can do this).

but don’t automatically assume because it’s come up on the reg it is correct. 

When we have cars come in for a quote or we need to replace discs and pads through maintenance, we will always take a quick measure of the disc size as it can be a minefield. 

1 hour ago, spagball said:

 

Just did my first disc change, 2005 Ghia but disc I got from motor factors was too big(300mm) needed 278? Even though I gave them my reg. So the old chestnut never trust anyone because they make mistakes. I'm getting more cynical as I get older WHY because I see life and people as what they are EVERYONES out to SCAM you REMEMBER THAT! and everyone on this Planet is trying to get your money It's terrible but true I actually don't believe that about 50% of people in life are trying to scam you but the other 50% are good like me Don't buy expensive piston pusher just get a clamp and keep old brake lad in situ (see photo) God is great

When I bought them clamps off ebay I too didn't check the size and they were useless mini clamps. I just used pair of mole grips closed then opened them up in the space to move the piston that also works well. 

No one should buy brake discs without at least checking the diameter of their existing discs and seeing if they are vented or not as there are always variations for different years/engine size etc and with some cars being registered many many months after being made there will always be inaccuracies.  more likely the info the motor factor looked at was wrong (book or website of the disc maker/supplier) and not the motor factor's mistake, or the car was made before a change in spec but registered after that change. 

I use a valve spring compressor for pushing pistons back. Some people say that when doing this you should undo bleed screw otherwise you could flip the master cylinder seals over the wrong way (but never taken that precaution or had that problem myself)

300mm discs were generally fitted to 1.8 litre and above.  Smaller engines have 278mm.

But yes, never trust anyone, especially online!  Oh wait...  

 

 

Thanks for replays I actually think 70% of people are good online or off!

I know its a pain but always worth taking a wheel off and checking the measurement before ordering disks. Vehicles made at the end of a production run they sometimes stick on whatever is left - as long as its at or above the quoted spec. Unlikely in your case as they were actually smaller, but specs do change and cars can be made a long time before they get registered.

On 6/27/2020 at 4:15 PM, spagball said:

Don't buy expensive piston pusher just get a clamp and keep old brake lad in situ

They're not really expensive tbh. I paid under £20 for my set from ebay. It included both left and right hand threaded bar and all the bits you need for pretty much every piston out there.

G-Clamps are all well and good till you do a rear caliper that has a threaded piston (some are also left hand threads).