Originally Posted by Chris Nason
As I said, it WILL crack the discs. If you've done it and survived you've been lucky. New pads on new discs will take temperatures up to 800C before destroying the discs. But new pads on old discs (with quite normal surface grooving will only touch at the peaks. This gives you the temperature variance which can crack discs at a quarter of the normal temperature. If you add to that any possibility that the new pads are not aligned square to the face (ie the old pads had gradually tipped and therefore the disc face is not square to the hub) and your lucky that the first brake application is touching more than 10%.
Well that is why the smart and sensible move is to machine the discs before putting on new pads.
Of course with the original Alfa Discs and pads, they are both worn out at the same time
In reality the "correct" proceedure
is more along the lines of the following(which is similar to what Brembo suggests for their OEM replacement kits.
" All high performance after market discs and pads should come with both installation and break in instructions. The procedures are very similar between manufacturers. With respect to the pads, the bonding resins must be burned off relatively slowly to avoid both fade and uneven deposits. The procedure is several stops of increasing severity with a brief cooling period between them. After the last stop, the system should be allowed to cool to ambient temperature. Typically, a series of ten increasingly hard stops from 60mph to 5 mph with normal acceleration in between should get the job done for a high performance street pad. During pad or disc break-in, do not come to a complete stop, so plan where and when you do this procedure with care and concern for yourself and the safety of others. If you come to a complete stop before the break-in process is completed there is the chance for non-uniform pad material transfer or pad imprinting to take place and the results will be what the whole process is trying to avoid. Game over.
In terms of stop severity, an ABS active stop would typically be around 0.9 G’s and above, depending on the vehicle. What you want to do is stop at a rate around 0.7
to 0.9 G's. That is a deceleration rate near but below lock up or ABS intervention. You should begin to smell pads at the 5th to 7th stop and the smell should diminish before the last stop. A powdery gray area will become visible on the edge of the pad (actually the edge of the friction material in contact with the disc - not the backing plate) where the paint and resins of the pad are burning off. When the gray area on the edges of the pads are about 1/8" deep, the pad is bedded.