Javier Quispe below, noted the following to which I totally agree with, excepting that I would just take the 230 mm / 25.4 as is the conversion to get the 9.055" value, however, there is more error than this and no one noticed it. The number stamped into the drum is not the dimension to which you can "machine to", it is the "discard" dimension. For brake drums the "machine to" dimension (or "rebore" dimension) is typically 0.030" less than the number stamped into the drum and this value is specified my many vehicle manufactures in service information. This extra 0.030" is meant to exist for the purpose of wearing out a full new set of brake shoes. Any removal of metal or wear of metal beyond the Maximum diameter, stamped into the drum, will affect dissipation of heat and may cause distortion of the drum. It truly is sad and scary that some one out there did not point this out before over some 30,000 views have taken place. It is scary to think of how many vehicles could be out there on the roads driving around on subpar brakes. Javier Quispe: You made 2 mistakes one is your drum spec which should be 9.055 and your micro reading. I don't know how you get 8.895 it should be 8.875 to this you add your 120. This should be 8.995.
He should have set the micrometer to 9 inch on each side the gauge would have read 9.055 his way is way to complecated you do not need all those extra calulations
So if I have a 250 point????? Brake drum. And I have a choice between a 200 mm????? Or 250 m.????? Spring rebuild kit. I should go with a 250 mm? Drum spring rebuild kit?
You made 2 mistakes one is your drum spec which should be 9.055 and your micro reading. I don't know how you get 8.895 it should be 8.875 to this you add your 120. This should be 8.995.
It is ridiculous the amount of calculations he did I could be done I 2 minutes if I took that long to measure a drum my boss would have wanted to know why it was taking so long he would get up me for wasting time
He has no idea how to use a drum mike if the drum is 9 inch you set to 9 on both sides of the micrometer and take a measurement 230 mm ÷ x 25.4 = 9 055 tho std his way is way to complecated if he is teaching his students that way i feel sorry for his students that is the most stupid way of doing it by the time he measured it i could have measured it machined it and put it back i used one of those ammco drum micrometers for 49 years as a brake specialist