Rotor thickness minimum 4-piston KH?

There are a couple of dangers in the rather heavy, "twin wall" old school rotors. One is that you get the web that connects the rotor proper to the hub too thin. You can actually check this by drilling a couple of say, 1/8" "check" holes in the web.

One other danger is that you get the rotor thin enough that the caliper pistons will lose their seal and dump brake fluid.

"In my early days" there was a private, 3rd party "ski bus" to Schweitzer mountain. This was a wide body van in the mid 70's with front disc, 1 ton. Someone brought them into the store and I refused them. The mech. gave me trouble so I actually took him back and DRILLED a check hole in the web. These rotors were WAY beyond use.

Months later someone brought that rotor back---in two pieces. They accused us of the trouble, and I recognized one of them from the 1/8 hole. Told them if they were going to sue, they better come up with an invoice.

One other time, someone brought in a pair of Chev pass car rotors, they were more than .130 below the "toss" figure. This was BEFORE I would have turned them, and was measuring "the high spots" with a common caliper. So the "low spots" were way worse!! I refused them, the mech ran them up to the N end to 'that other store' and those dummies turned them no problem. Then I "was the ***" according to him