69 C20 Rear Brake Drum

[bollybib]

NOTE: This thread was written before I started my Project thread. It forms part of the BRAKES section (Posts #6 thru #12) in that thread.

I have a 1969 C20 with rear coil springs and an Eaton H052 rear end. The rear brake drums appear to be original equipment and are the nondemountable type. They operate with 11 5/32 x 2 ¾ shoes. They are heavily scored and at the wear limit.

The replacement drums that I purchased appear to be identical to the originals except for the bolt holes. The replacement drum bolt hole diameter is approximately .627 in. The bolt holes have been countersunk on the inside face of the drum. So have the drain holes.

The original drum bolt hole diameter is approximately .550 in. The splines of the wheel bolts fully engage the drum holes. There is no bolt-to-hole play. if you have access to one, Figure 52 in Section 4 of the 1969 service manual shows the hub and drum details.

The engagement of the wheel bolts with the replacement drum (I am reusing the original bolts) doesn't seem right to me. The bolt is loose in the hole and the splines are not engaged. The drum could potentially move relative to the hub within the constraints of the hole diameter unless the oil deflector (mounted inside the drum and secured by the wheel bolts) is deformed by the head of the wheel bolt into the countersink. Deforming the oil deflector with the wheel bolt heads into a countersunk hole is not mentioned in the service manual nor does it seem a secure fastening method.

I haven’t yet contacted the vendor (Centric part no. 122.66024). I’ve found and used a lot of good info from this forum so I thought I’d run this issue by y’all to see what you have to say.

[Jessechop]

Those splines you talk about being th eknurl on the wheel stud will never engage to the brake drum. Those will "lock" as you call it is in the hub, not the drum and those splines usually never even make it that far. As far as the drum being "loose" on the studs is normal on a great percentage of brake drums. After the wheel is tight the drom wont move

[bollybib]

If this were a demountable type drum, I would agree that the hole should be sized for bolt clearance and that the clamping force alone is sufficient to hold the drum in place. Demountable type drums are clamped between the hub flange and the wheel (similar to a front drum) whereas my nondemountable type is clamped between the hub flange and the oil deflector. The oil deflector looks to be made from 16 gauge sheet steel based on my measurement of its thickness at .050 in.

My old drums have 1968 casting codes on them so I'm pretty sure they are the original drums. Based on their construction it looks to me that the original design of the nondemountable drum’s bolt hole is for an interference fit with the bolt and the bolt provides shear resistance upon braking in addition to whatever clamping force is applied to the drum once the wheel is installed.

On 2/2/11 I talked with a tech rep at Centric who said the new drum’s holes should provide an interference fit with the bolt. Centric’s specifications for this drum call for a ".53 bolt size" which is quite a bit less than the .627 in. (approx) diameter of the bolt holes on the drums I received. He was at a loss to explain the countersinking. He recommended that I return the drums that I purchased. He also said it would be several months before Centric could sort out this issue with the fabricator. I called the vendor (racepages.com) and they authorized a return and paid for the shipping. It could take up to four weeks to credit my account for the purchase amount.

http://www.racepages.com/products/?D...let+brake+drum

I hope other C20 guys find this useful. I’m considering what to do next.

Hub and Drum Details (Nondemountable - Type Drum)
[illustration below taken from the 1969 Chassis Service Manual]
6. Wheel Bolt
7. Drum
8. Oil Deflector

[RichardJ]

You're over thinking this.
as long as the outside diameter of the hub is the same as the diameter of the hole in the drum, it is centered. The lugs just keep the wheel, hub and drum locked together.
If you look at aftermarket wheels, some are hub centric and some are lug centric. Hub centric uses the hub to locate the wheel and the lugs just hold it on. Same with your drum.
I've never seen a factory lug centric wheel and I would never imagine them doing it with a brake drum.

[Longhorn Man]

where did you locate drums?

[bollybib]

RichardJ, although what you say is correct, the issue I’m concerned with isn’t centering but with the rigidity of the drum/hub assembly. As far as overthinking goes, if a vendor provides me with something other than what they specify, I question it. My engineering background plays a role in this I suspect. Anyway, I sent the drums back and time (a lot of time, no doubt) will tell if I get my refund. [racepages.com credited my account for the full purchase/shipping price on 2/28/11]

Longhorn Man, google Centric 122.66024. There are quite a few vendors and eBayers that sell this drum but I found that all of them draw on the stock held in Centric’s City of Industry, CA warehouse like racepages.com (the guys I bought them from).

Before I shipped the drums back I cleaned up the old drums and took a pic of an original (on bottom) and a new drum back-to-back. You can really see the difference in the holes.

In the meantime, I had the old drums machined even though they ended up oversized. I need to be able to use the brakes and move the truck around without tearing up the shoes. I’m more interested in the engine/transmission situation but the brakes were so FUBABed I had no choice but to rehab them first.

I’ve only found Centric drums available for my truck, p/n 122.66024 at a reasonable cost, say under $100 each. There may be others. I haven’t tried calling people like Raybestos and Bendix but I found nothing online for my truck made by them. Maybe someone else has some information they can share on this.

Another thing I wondered about…can you change the hubs on my axle to accept demountable drums for the same brakes without any modifications ? I looked at this a little but haven’t come up with much so far. I think I’ll keep this on my list of things to investigate on a rainy day.

[bollybib]

I called Centric today and the tech rep said that 186 drums are on the 30-to-60 day production schedule. So I'll check with them again in a couple of months and when the new batch of drums shows up, I'll order a pair and give it another shot.

Update 4/17/11: I talked to Centric again last week. The drums are still unavailable. Maybe another 30 to 60 days. Put it on the calendar.

Update 6/16/11: I called Centric today. Looks like we're still 30 days out or so on a new batch of drums. Put it on the calendar and call in a month.

Update 7/20/11: I called Centric today. Apparently they have a problem communicating the fabrication issue to the factory because the factory keeps manufacturing the same mistake. Looks like another 30 - 60 day cycle of waiting for a new shipment. Put it on the calendar.

Update 10/4/11: So OK. Looks like another batch of drums may show up end of this month or early next so we'll see then...

[Stricklinator]

Subscribed; I am going to need new rear drums for mine too! Jim, I was curious, did you check Autozone? They list drums for a reasonable price, but I don't know if they fit for sure.

http://www.autozone.com/autozone/par...37684_0_0_5488

Oops, now that I look at it they don't look the same!
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[treveiger]

Interested
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[custom69camper]

The reason (I believe) you have slop in the holes on your drum is because the drum is for the 71-72 9/16's studs, if you check yours I'll bet they're 1/2 inch which was used up to 1970 on the 3/4 ton's.

[bollybib]

Originally Posted by custom69camper

The reason (I believe) you have slop in the holes on your drum is because the drum is for the 71-72 9/16's studs, if you check yours I'll bet they're 1/2 inch which was used up to 1970 on the 3/4 ton's.

The spec for the holes on Centric part number 122.66024 is .53 dia. The ones I received had .63 dia. which made them a 122.66040 for 71-72s like you say.

For Sale Info added on 2/3/13:
The drums with the .63 diameter holes are for sale here - - - > http://67-72chevytrucks.com/vboard/s...d.php?t=561651

[bollybib]

After many months of waiting, today I finally received the correct drums with the .53 in. diameter bolt holes.

If you are ordering these for your truck, make sure that you have them drop shipped directly from the Centric warehouse in City of Industry, CA. As of this writing, several vendors still have stock of their own that have the larger .63 in. diameter holes.

By referral from Centric, I ordered mine from http://store.zeckhausen.com/catalog/ in order to assure the drop shipment.

[Stricklinator]

Thanks for the info Jim. I may need these soon myself. I am trying to reuse mine for now. I got the rear bearings and races out today, but now will have to put new ones in by hand (no press). Can you provide any tips on this procedure? Do i need to pack the rear bearings like i did the front? I wasn's sure since the rear ones get lubed with the diff oil. I am also going to be challenged getting the shoes and hardware back on as it's been 15 years since I assembled drum brake assemblies. I left one side intact, so hopefully I can use it as a guide for putting the other side back together. Next will be all new stainless lines and a new master cylinder. How much are those drums by the way and are they the same for front and back?

Thanks again,

Paul
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[bollybib]

Hi Paul,

You could drive the races in with a punch if you go slowly, keep them square in the bore, fully seat them, and don't damage the surface of the race. The bearings get lubricated by the gear oil so no need to pack them. The drums front to rear are definitely not the same. However, the brake shoe hardware arrangement is pretty much the same front to rear except for the e-brake. The drums will run you about $80 apiece.


Here are pics of freshly reassembled shoes on the rear driver side you can use for reference.

[Stricklinator]

Thanks for the help Jim. I ended up hitting the cage of one of the inner bearings with the punch and ruined it. Then I couldn't get the race back out no matter how hard I hit it! The job was then passed off to a machine shop where they report neither drum is turnable. On the bright side, at least I know where to get the replacement drums thanks to you. This truck has now cost me double what I paid for it in repairs. Oh well, it's for fun, not for an investment.

Thanks again,

Paul
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[Stricklinator]

Jim, do those replacement drums have the knocked-out slots to access the adjuster wheel? My Dad took my hubs in for me and when I got them back, the two new drums look different. One has an outline for where the slot would go and the other has a weight welded to it and no knock or hole of any kind. I wish I had taken them in myself, but my Dad was trying to be helpful so I don't want to complain too much.

Thanks,

Paul
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[bollybib]

Service Manual Info

[RoughKuntry]

Jim, can you pm me so I can get some help on putting the brakes back on a 73 GMC Super Custom 3/4 ton?

[treveiger]

Jim your Awesome I'm doing this myself but don't know if ill need new drums. Thanks for the info!
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[KevinK7]

I know it's an old thread, but subscribing for future reference ; )