Brake drum thickness measurement and with micrometer questions

[Flathead Fever]

Originally Posted by Juergen

This has been discussed many times, especially when someone claims that .060 is the legal limit. When asked to document, no one responds. Ford never spec'd the limit for these cars even though they did for trucks. And I have a GM brake drum that says the limit is .090. So much for never.

I'm not aware of any specs for the early ford drums either. I have a really neat step-by-step Early Ford Brake job manual, made for Ford dealership mechanics. No where in it does it mention the diameter of the drums and its an extensive repair manual. I think the drums were just meant to be replaced if they looked unsafe. Your job was to sell the customer a new brake drum. I doubt the Ford dealerships even had brake drum lathes in the 1930s. I guarantee the small repair shops did not have brake lathes in the 1930s.

I only said .060" over was the maximum diameter because that was what was stamped on all the drums I ever machined. When you took a brake licensing examination .060" was the answer they wanted. The drums I machined were from the 1970s or newer. Maybe you can go .100" over and still have a safe drum on an Early Ford, they probably are. But I don't think anybody is going to put that down in writing in a new repair manual. You can only reproduce the factory specifications and so far nobody has come up with any for the Early Fords. I have heard that number, ".100 over" used a lot by Ford restorers, including my dad. The problem is just like there are no specs saying .060" is the max there are no specs saying its safe to go to .100".

If somebody is tailgating a new BMW equipped with four-wheel ABS disc brakes in their Early Ford and that BMW does a panic stop the Early Ford is going to slam right into the back of it. Then I know what happens next. Many times when there was an accident on the fleet vehicles we worked on the drivers immediately blamed the mechanics. In thirty-year's, never once was an accident found to be our fault. To protect the mechanics wrecked vehicle were immediately locked up along with the maintenance records and nobody was allowed near them. An outside expert consultant was brought in. If the driver blamed the brakes the consultant would tear the entire brake system a part. If you were the lucky mechanic that did the last brake job you better have done everything correctly. You better have flushed the brake fluid and machined the drums and rotors and they better be a safe thickness.

You can get away with all kinds of stuff that's still perfectly safe when your working on your own car but when your working on cars for a living everything you touch better be perfect. I would not perform a brake job on an Early Ford for somebody and use a .100" over drum. They might be perfectly safe but I don't have anyway to prove it. If it goes to court the expert is going to say .060" over is the maximum allowable limit when refinishing a brake drum and .100" over is unsafe.

For several year's I was on a SoCal team of ten person's that wrote the questions used for licensing the California Smog Check and Repair Mechanics. The State had a room full of shelves and shelves of manuals, they spent a fortune on those repair manuals. Any of the questions we developed for the test we had to be able to backup the answers in one of those manuals. That was incase someone taking the test challenged one of our question's and answer's. That would be the same if I had been on the Brake Licensing Exam Team. I could not write a question, "What's the maximum safe limit on an Early Ford Brake Drum? (A) 0.60 over, (B) .060 over, (C) .100 over, (D) there is no limit, because Ford brake drums are so good they never wear out. There is no correct answer without a specification in a manual to back it up.

There were lots of aftermarket brake parts available back then. A Ford drum might be perfectly safe at .100" But an aftermarket drum might be paper thin at .100" over.

1935 Ford drums, as others have said, are really hard to find. I buy the rears when ever I find them and store them away so I can run hydraulic brakes with wire wheels on hot rods without using the adaptors. I have found several drums at swap meets that measured almost zero wear. There is nothing wrong with using wire wheel adaptors on the later hydraulic brake drums, they work great. But if your restoring a '35 Ford you need to have '35 Ford drums. No purist is going to except anything less and it better have a little Ford script logo on it too. I was raised by one of those guy's.

[rich b]

One thing I've learned; the typical guy turning your drums at the parts store is going to cut until there is not a groove or pit left. They don't have to be that perfect.

[George/Maine]

In the days after 1948 drums were marked max dia. The old ford drums had lot of meat on them. I think back then people would drive till the shoes were medal to medal.
The real reason i think is so you don,t pop a wheel cylinder by increasing the travel of piston. If you have a chance to die take it.