AMMCO Brake Drum Micrometer 

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.

Thanks for your information, I even learn more some info I didn't have 💯 explaining and undersdable

Maybe I'm misunderstanding, but 7/8 is .875 not .895. Great and informative video though.

great video, completely get the point, math was a little off but it still helped me get the point of how to use this thing :)

Guyz you are cruel is explaining perfect 👌

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?

thanks for the video men, Good bless you

Thank you

Geez, are you still in the 70's? Get an electronic drum gauge like the Fowler 74-150-020.

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.

I still dont get it. You skipped a step

what??

and u get paid for all this calculations?

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

Or... just use the metric system like 90% of the world