Brake drums

[57_BN4]

Hi Guys,

I have three questions about brakes:

1- What is the generally accepted maximum machining diameter for 11" drums? The factory manual doesn't mention anything, nor do the other manuals I have. My local brake supplier says generally 2mm increase in diameter is max, mine are 1.8mm over and are showing 0.1-0.2mm out-of-round when bolted up to the hubs.

2- What is the milled area for on the outside of the rear drums? The machining is about 80mm long only on one side of the drum. Both rear drums are very out of balance and this area falls to the lower side statically so I'm guessing it was an attempt at balancing. Further, does anyone balance drums?

3- Are replacement drums available at reasonable price/quality?

Thank you,
Andy.

 

[Healeysince59]

I've never seen a drum with that raised area. It looks very much like material was removed to balance the drum. I've balanced drums by adding weight (welded or JB welded). It almost always takes more weight than I am willing to carve off the drum.

I can't address your questions 1 and 3.

Marv J

 

[Duane_Rhynard]

Andy,

My BN6 parts book shows a drum like the one pictured with the area removed. I also have the drums pictured below. They all the same type drum with a small section removed, a larger section removed, and without anything removed.

Hendrix Wire wheel balances drums:
https://www.hendrixwirewheel.com/drums.html

Drums are available from: (search for brake drums)
https://www.cape-international.com

Duane
'58 BN6

 

[John Turney]

From John Sims' BN-6 website I see Hendrix Wire Wheel balances brake drums.

 

[57_BN4]

Thanks for the replies and links guys, much appreciated.

So from Duane's pic it looks like these drums were balanced by machining various amounts from that rather heavy front ring section. IMO they did an average job of it so if anyone is experiencing higher speed shakes then I'd bet a dollar the brake drums are nowhere near in balance.

I had a play around yesterday with spinning up the drum with the electric drill and adding various bolts taped to the inside until it ran true. Seemed easy enough so today I'll drill out metal from the cast ring until it balances up again with no ballast.

Possibly will still have to scrap the drums anyway if they don't clean up within oversize limit but at least I learned something interesting.

Cheers,
Andy.

 

[Bob Claffie]

Andy, when I first got my '62 Vet (in '75) I had a friend, a professional mechanic, turn my drums. I told him NOT to go below the factory minimum size and I would be responsible if there were any problems. Since the drums didn't clean up at the minimum he kept turning until they were significantly undersize and baby's ass smooth, much to my chagrin. However, after 33 years they are still on the car and still stopping straight. Maybe a few MM's too thin is really OK ? Bob

Did you get the books yet ?

 

[Bob_Spidell]

I believe the minimum thickness for drums and rotors is a safety consideration.

I tried to get the drums for my BJ8 turned but the shop couldn't find a spec so they refused to turn them. In California, a shop can be severely punished for turning thinner than the minimum spec.

 

[57_BN4]

Bob C- yes, this is what I'm thinking, I'll turn them until smooth and then check diameter. If they are only a squidgen over then given the 1000 gentle miles a year I'll be doing chances are there will be no issues.

Books arrived the other day, thanks very much.


Bob S- I wonder if there is no spec given deliberately to absolve BMC of any liability for failures...

Andy.

 

[Bob_Spidell]

Bob S- I wonder if there is no spec given deliberately to absolve BMC of any liability for failures...

I just attributed it to the shop not having the correct manuals, or maybe the info's just not available anywhere (I also checked the Net and other sources to no avail). I would think BMC would be more liable if they didn't publish any specs.

It's getting harder to find parts and info on even less elderly vehicles.

 

[Randy Forbes]

Bob C- yes, this is what I'm thinking, I'll turn them until smooth and then check diameter. If they are only a squidgen over then given the 1000 gentle miles a year I'll be doing chances are there will be no issues.

Books arrived the other day, thanks very much.


Bob S- <span style="font-weight: bold">I wonder if there is no spec given deliberately to absolve BMC of any liability for failures... </span>
Andy.

When those manuals were composed, we didn't live in such a litigious society, so I doubt they were shunning responsibility.

 

[big6]

I seem to recall reading somewhere that the Austin-Healey brake drum inner brake shoe contact surface is hardened and is not supposed to be machined. When the harden surface that contacts the brake shoe lining is machined away the brake drum contact surface is now soft and useless. This is probably why no specification exists for the maximum brake drum diameter.

 

[Randy Forbes]

I seem to recall reading somewhere that the Austin-Healey brake drum inner brake shoe contact surface is hardened and is not supposed to be machined. When the harden surface that contacts the brake shoe lining is machined away the brake drum contact surface is now soft and useless. This is probably why no specification exists for the maximum brake drum diameter.

If that's the case, and I'm not saying it is or isn't, any decent brake-lathe operator would know, as soon as the cutter touches the surface.

I don't recall ever hearing the drums were hardened, but I can't say that I've ever looked into it either.

I will say though, I've never worked on a Healey where the drums, either front or rear, ever were damaged or needed to be turned. Maybe they are surface treated?

A quick and easy test, is to pass a file on the surface; you'll know right away if you can cut it or not!