Big Brake Drums

[Jaime G.]

In the Big Brake Finned Drum set up on C-1 cars, does anyone know if there
are published standards as to the maximum wear allowed in the drums before
they must be replaced? Using a Brake Mic , what is the maximum reading one
should get and still use the drums?
Thanks for your help.

[Joe L.]

Re: Big Brake Drums

In the Big Brake Finned Drum set up on C-1 cars, does anyone know if there
are published standards as to the maximum wear allowed in the drums before
they must be replaced? Using a Brake Mic , what is the maximum reading one
should get and still use the drums?
Thanks for your help.

Jaime-----

There are always standards for brake drum wear limits. Usually, the maximum that a brake drum can be machined is 0.060" over and the discard thickness is 0.090" over. So, for the HD brakes, the limits are in all probability 11.060" as the machining limit and 11.090" as the discard limit. Theoretically, a drum could be machined beyond the 0.060" limit as long as it didn't get to 11.090" (if one could find someone willing to do it). Of course, in service the car could then get beyond the discard wear limit. However, for a car with limited use this would probably not be a problem.

[Dennis C.]

Re: Big Brake Drums

Jamie,

I looked this up for my '61 with standard brakes and found some information in the Chevrolet Service Manual. The spec in the service manual was as Joe called out. I never specifically looked for big brake drums but don't see why it would be any different.

Dennis

[Mike M.]

Re: Big Brake Drums

it's been 10-15 years since i restored and duntov'd out 61 RPO corvette, with all 4 of its brake drums being original to the vet, but as i recall, the wall thickness of the fined drums was considerably thicker than those of a non-big brake vet. if i had the time and energy, i'd pull a drum off the old 61 rpo vet and confirm or deny my recollection. mike

[Bill M.]

Re: Big Brake Drums

it's been 10-15 years since i restored and duntov'd out 61 RPO corvette, with all 4 of its brake drums being original to the vet, but as i recall, the wall thickness of the fined drums was considerably thicker than those of a non-big brake vet. if i had the time and energy, i'd pull a drum off the old 61 rpo vet and confirm or deny my recollection. mike

Don't bother. I have one of each on a shelf. No question the finned drum is heavier (21.0 lb) than a regular '59 (11.5 lb). But the critical dimension is the OD of the wearing surface at the inner edge of the drum; this is where they will crack. These are basically the same based on my rough measurement.

Stock original '59 (replaced these when I rebuilt the brakes)



RPO 687:





Based on this, I would use the 0.060 spec for turning finned drums too.

I had a brake engineer tell me not to turn drums, just install new linings. They won't feel right at first, but will bed in. (I don't think a brake shop could do this because the brakes would not feel right when the customer tried the brakes the first time!) So I would use finned drums (very rare) worn beyond 0.060, but don't turn them. They will have less heat capacity so the brakes will fade sooner, but the edge won't be weakened by being too thin.

[Jaime G.]

Re: Big Brake Drums

thank you all.

[Larry B.]

Re: Big Brake Drums

90 is the limit but not much more no more than it willbe driven. THE DRUMS SHOULD ALWAYS be turned when new shoes are installed. The trick is finding someone who even knows about arcking the shoes to fit the drums.When accked the shoes fit the bigger than stock brake drum.

[Duke W.]

Re: Big Brake Drums

I've never turned a drum or rotor on any car I ever owned. That includes the front rotors on my Cosworth Vega that went through six sets of Delco OE brake pads in 5000 miles of track events, and they're still well above minimum thickness and true. (They are actually vented rotors and companion calipers off '76-up H-Specials as I knew the OE solid rotors would not survive hot lapping race tracks.)

As long as there is no sign of warpage or out of round, which can usually be detected by vibration at the brake pedal or steering under braking, why take off good metal?

As previously stated, it does take a judicious break-in, which might take a couple of hundred miles of normal driving. That's why brake shops always turn rotors and drums. The typical customer is not going to have a discipline to do a proper break in. They just leave the shop and jam on the brakes at the first stop sigh and expect them to work the same as before.

I've never found anyone who could arck or acck brake shoes, either, but there are still some shops out there that have the equipment to arc-in shoes though I don't think it's necessary in most cases if you do an easy break-in. Usually about 200 miles of around town driving will get the job done.

Duke