Can anyone tell me what the maximum safe inside diameter one can machine a set of original brake drums on a '62 out to? I can't seem to find that information anywhere. The 'new' inside diameter seems to be 11.0 inches.

Your help is appreciated.

Thanks

Whit

 

drums usually have the maximum diameter size cast into the outer surface of the drum.
Bill

 

or maybe a older machine shop that turns drums could look it up.

 

hi, maximum diameter 11.060", you can turn them 60 thou.

 

thank you Mecron. et al

Whit

 

I could be wrong, but I seem to remember .090" as the max. A manual should tell you whether it's .060" or .090". I believe it's an industry standard. Not a GM or Corvette standard.

BTW, that's how you tell the difference between original and replacement drums. Sometime after 1962, the Government mandated that this be stamped on brake drums. So originals have nothing written. The replacements are stamped something to the effect that the max diameter is 11.090.

 

hey chris, i just went out and looked at one of the factory drums off my 64. this is what is what is stamped along the outside face of the drum: O E DIA 11.00 DISCARD DIA 11.060 . ALSO- 720047(part #??) 0-2030 W-3.

i have turned a lot of discs and drums and you are correct the .060 is an industry standard, not just a GM thing.

 

I seem to recall in the old days there was a bevel on the edge of the drum. When the bevel was gone the drum was done for.
When I worked in shops one measured and turned the worst drum first, the turned the other to match. Then when disc brakes became popular they said there was no need to match the rotor size to each. Now days they just turn the drums and don't match them either. They look at me like I'm crazy when I instruct them to do it my way, they've never heard of it. Then I want them to turm my rotors to match as well. I guess I'm a dinosaur

 

Mechron....yes..thank you...pitting on drum, sanded with 120 grit emery..still can't see any indication...'64 C2....0.060 seems reasonable...thanks..

 

You are replying to a 13 year old post.

 

yea...he may be at ground temperature....good point, I was so excited to see a real number .....nice thing about math though...the rules don't change very often....thank you for your very astute observation regarding my mistake...i'll try to do better next time....

 

Mechron died about 10 years ago. He was a very very knowledgeable mechanic about our old cars. Excellent with factory AC.

RIP.

Larry

 

Larry,
Thank you for your reply. Sorry to hear about Mechron. Seems the torch passing is getting less frequent these days... doing my damndest but ....thanks again....

 

I have been turning drums and arc grinding shoes for over 50 year and still do my own and .060 over has always been the accepted standard, I have never seen one stamped .090 over.

Bill

 

The 1963 Corvette Shop Manual states; "A brake drum must not be rebored more than .060" over the maximum standard diameter..."

The labeling of brake drum max OD and disk minimum thickness was a FMVSS that went into effect IIRC on 1/1/1968. I've seen service drums where the data is die stamped and some where it is embedded in the casting. I believe the former were drums in stock that were reworked to comply with the FMVSS, and later the casting tooling was reworked to include the data in the casting.

I completely rebuilt my J-65 system in the mid seventies at 115K miles using new GM J-65 drums, shoes, and springs. The shoes still had plenty of material, but the drums were worn .060" on three and .080" on one. Compared to overhauling a C2 disk brake system the parts were inexpensive. I recall the max diameter is 11.090" (don't want to pull off a wheel to verify at this time). Maybe someone with the wheels off or a loose drum can check to see if they have the max diameter die stamped or embedded in the casting.

Drums that need turning are usually turned .030", which means they can be turned .030" twice to 11.060", but not again. So if a drum needs turning due to out of round or whatever and they measure nominally 11.070" they should be scrapped. Likewise, if during inspection they are found to be over 11.090" they should be scrapped even if performance in normal driving is still okay.

Duke

 

Thanks to all for the information....

 

I've done literally thousands of brake jobs in the past 40 plus years, and I have seen drums that were stamped 'max dia. .090"' on them. Not common, though. .o60" over has been the industry standard for decades. Unless stamped otherwise, .060" is the max you want to run.

 

There seems to be some confusion regarding the maximum overbore diameter and the maximum service diameter. The way I interpret is 11.060" is the maximum overbore and 11.090" is the maximum service diameter. So if you have drums bored to 11.060" they cannot be overbored any more, but are considered serviceable to 11.090".

My J-65s wore .060"in 115K miles that included several hundred miles of big track hot lapping. Non-metallic linings probably wouldn't wear even half as much in that amount of normal road driving.

Duke

 

If you need to restore your drums because they are rare or not reproduced etc...you can have them sleeved.
Don at https://www.jgrelining.com/ has been providing the service for quite some time.

He cleans the drums up, cuts the ID to a clean surface that true round, interference fits a sleeve, coats the sleeve and drum surfaces a high temp industrial expoxy glue, presses sleeve in, drills 2 holes in the drum/sleeve and stakes/pins the two in case the epoxy and interference fit fails, then turns the new ID to factory specs.
Don also has the ability to reline or even make new sets of shoes, complete with fitting them to your drums.

 

Many brake rotors and drums have a 'machine to' spec and a 'minimum thickness' (for rotors) or a 'maximum diameter' (for drums). If the stamp on the drum says 'max diameter .060", that is the service diameter, not the 'machine to' diameter. Many rotors have a 'machine to' thickness that is .020" or more than the 'minimum thickness'. You resurface to the 'machine to'. With a drum, if you resurface it to .060" and install it on the car or truck, you may get by, but it's not going to be optimal for long. I might do it on one of my old cars that is a cruiser, but never on a customer's car.