I was a lab technician, work in THEN Malaysia 3rd largest steel factory in 2001 ~ 2003 as British Standard/ American Standard/ Malaysia Sirim/ Japan International Standard product certification officer. Ayam the one and only person in the lab running all the sampling and testing for all the raw materials/finish product, including 25mm steel plate that use to cut as disc brake rotor. So, you may wanna consider my eversof***ing humble opinion Any of those so called "international" standard has a tolerance of 5%, some country(namely the 1 we're f***ing familiar with) allowed tolerance up to 10%. Hence, 25mm - 10% = 22.5mm. I set myself 21mm minimal thickness consider huge tolerance, otherwise I wouldn't be able to find a replacement rotor for the rest of my life. I think GaGa stock rotor only came with 19mm.

Hairline crack happened on the drilled area got 2 facts:
1) The drilled hole pointed at the leaf between the vents. As and when the disc rotor being heated up and expanded, the leaf takes longer to contract when it start cooling down, hence the surface of drilled area will have hairline crack because the surface cool down much faster than the leaf(inner part between the rotor vent).
2) Wrong brake pad temperature will cause every single drilled hole to emit hairline crack. Simple logic, hot expand cold shrink. When you have sohai temperature pad that rated higher than the rotor, ofcourse the rotor gets heated up until fire red, as and when it contract quickly, crack is unavoidable. Try a ceramic rotor, before the disc crack, the heat generated could probably set your absorber on fire already In the end, it boils down to the raw material of the disc rotor.

Would like to understand more on the above highlighted from your previous statement. Which drilled rotor does not drilled through? I'd like to further understand if such technology exist. On the other hand, you can drive lika moron in your Iswaga, you will never create serious crack your disc even if you use ceramic pads. Vehicle weight is the 1 that creates more friction from the pad to the rotor. These are my eversof***ing humble opinion. Over and out.