Rear drum brake backing plate questions.

[DAVEN1256]

I am going to have my differential gone through and while it's apart, sandblasted and painted. My backing plates are looking a little rough on the inside and I might be replacing them. I'll have a better idea of their condition after sandblasting.

Questions......

1) In all the photos I have seen of repro backing plates, none seem to have the 1" wide, 180 degree semi-circular plate spot welded to the rear lower half of the backing plate (outlined in red on my photo below.) Is it normal that repro plates do not come with this part? I wonder why they are not there? I wouldn't think the factory would have went through the effort of putting them there if they didn't serve a purpose. Maybe to help keep dirt and water out?

2) Every Camaro parts place that I have looked at repro plates, they cost about $63.00 each. That seems like a lot for what they are. I see you can get Dorman brand plates for '78 and later Camaros at the local auto parts store for $18.00 each. Dorman does not seem to have plates for 1st gen Camaros though. Does anyone know if 1st gen plates are out there more reasonable than $63.00 each?

3) Having almost no experience with rebuilding drum brakes, I have two questions about the areas on the backing plates that the shoes contact......There are three slightly raised areas on each side of the plate that the shoe contacts. (circled in red in the 2nd photo below).......As the shoes move back and forth, is it just metal to metal contact between the shoes and the plates or is something applied there to reduce friction?....These raised areas on my plates are a little rough and crusty right now. If sandblasting or sanding smooths them out, I would assume the plates would be OK to re-use. Any opinions on that?

Thanks........Dave

[janobyte]

Mine do not have the raised area you are refering to. They are original to the car. Smooth rear drum, if that means anything. If after blasting they look ok, might as well repaint and keep, imo.

Where the shoes make contact, I always smear a small amount of disc brake lube(slide pins,etc)...Permatax green.

[janobyte]

I'm waiting for my diff to come back from the shop. Called today cause I was out that way...mon or tues next week.

[Mike S]

 My guess is that plate is to help prevent water from entering the drum when driving through puddles. I checked both my '67's and they have that extra plate. One rear is a 3.07 open with an April 67 assembly date and the other is a 3.31 posi with a March 67 assembly date.

Mike

[bertfam]

All GM cars with drum brakes had the backing plate contact surfaces. They were there to keep the shoes in line with the drum contact surface and were to be lubricated with a thin coat of brake lube per figure 29 (see attached) of the Chassis Service Manual when a brake job was done.

Ed

[janobyte]

Sorry to confuse in my last reply, was talking about the raised ridge around the outer border not being on mine.

0424 12 bolt posi (68), Drum # 3841177

[janobyte]

I've read the inner surface received little if any paint. No plan and having judged, shot with Eastwood 2x satin.   

[janobyte]

Theses are actually in primer.

[janobyte]

My guess is that plate is to help prevent water from entering the drum when driving through puddles. I checked both my '67's and they have that extra plate. One rear is a 3.07 open with an April 67 assembly date and the other is a 3.31 posi with a March 67 assembly date.

Mike

Agree, my guess is removal was a cost saving measure. Where it's pennies x units produced. I will say these old plates are thicker then anything I've seen on newer vehicles...well 80's up. Don't know about the re-pops. Yours appear to be in pretty good shape.

[JKZ27]

Jano, in your backing plate rear view pic, it looks like there are dimples where the shield was spot welded on.

[janobyte]

Wow, good eyes. Went back out to check, and yes, evidence of the plates being present. Even a small grind scar by the weld. The who and why will remain a mystery.

[janobyte]

Pea gravel catchers.

[ko-lek-tor]

I did not read every reply completely, so if this has already been said, apologies. If I had backing plates that were not up to par, after blasting, then I would find another pair of GM backing plates. I have scads of them to dig through. Same plate as a 10 bolt and you can find those (10 bolt)rears subbing as jack stands they are so numerous. The thought of buying repop seems unnecessary.

[69Z28-RS]

I think the same backing plates and brakes (or very similar?) were used on the '72 and up 8.5" rears... which should be very common.

[bcmiller]

Yes I think the 8.5 axles used the same backing plates. I will try to check one this week. 

[bcmiller]

Well I am not sure when they changed, but the 1974 Nova 8.5 ten bolt in my garage does NOT have the "rock guard" plates. 

Sorry, not a great pic.

[DAVEN1256]

Thanks for all of the replies and information so far.

Same plate as a 10 bolt and you can find those (10 bolt)rears subbing as jack stands they are so numerous. The thought of buying repop seems unnecessary.

I assume meaning numerous in junkyards. Can someone tell me up to what years and what kind of cars the rears with the same backing plates as mine were used?....The oldest cars in most of the junkyards I have been in recently are 90's, maybe a few 80's, and rarely anything older.

Thanks......Dave

[69Z28-RS]

Check out your local Craigs List ads for people selling differentials; check with them about selling you backing plates, but make sure you keep originals for comparison purposes before purchase.. 

A couple of weeks ago I saw an ad in a local neighborhood sales blurb for a '74 Nova; I called and asked about the rear...  and bought the 8.5" (60 inch rear) for $75...   It's the same width as the early 1st Camaros and the same as TriFive chevys (Just needs the spring perches changed for the Trifive), and is MUCH stronger wtih larger bearings than the older 10 bolt rears...   I will look and see if I have an extra pair of backing plates...

[DAVEN1256]

After having had my orignal backing plates sandblasted, the grooves worn into the raised areas (pads) that the brake shoes rest on were pretty clear. I have been told the new shoes can catch in those grooves and their movement can be impaired.

So I bought a new set of Right Stuff Detailing backing plates from Summit. They had issues!....So I tried a set off of Ebay that came from Inline Tube. They too had a couple of issues.

The one issue both brands have in commom is that the large hole in the center that fits over the axle tube is 1/8 inch larger than the original......On the original, there is almost zero clearance between this hole and the lip in the axle tube that it fits over. When the original backing plate is in position, it it centered and can't move.....In the repros, becasue of the oversized hole in the middle and the mounting bolts holes slightly larger than the bolts, the backing plate can wiggle an 1/8 inch back and forth in any direction. You would have to eyeball it to get it centered and then hope the bolts never come loose.
Is this acceptable? Up till now, I've had next to no experience working with these parts. I would think though, that having that backing plate perfectly centered around the axle would be critical to the brake shoes contacting the brake drum properly.

Just wondering what anyone else thinks about this.....if these repro plates are Ok to use or unacceptable.

Thanks......Dave

[BobH11]

The larger center hole backing plates are for non c-clip rears.

[DAVEN1256]

Pea gravel catchers.

Craigslist and Ebay have nothing as far as original GM backing plates. Checking the online inventory of the junkyards in my area for cars that might have used the same plates, most cars are 90's or newer, a few 80's, and 70's cars are almost non-existent.

Here's what worries me about using either of the repros. After you center the backing plate by eyeballing it, it can move a 1/16 of an inch in any direction. I know you are locking it in place by tightening the bolts but I wonder if it could shift after so many applications of the brakes and bumps jarring the rear end.

I have no idea how much clearance there is between the brake shoes and drums when the brakes are not applied but I wonder if one shoe moving closer to the drum and moving away could affect brake performance. I have no idea.

Dave

[BULLITT65]

Sorry if I missed this but did you post a picture of yours already blasted?

Unless they were submerged or were very flimsy, I would think your originals would be fine.

[DAVEN1256]

I've pretty much made up my mind not to use either of the repro plates with their oversized center holes. My gut tell me it's the wrong thing to do and I've learned in the past not to do something when I have that feeling.

Here are a couple of pictures of my originals. This is after sandblasting and painting. I took the paint off the pads that the brakes shoes sit on so the damage could be seen better. I circled in red the worst of the grooves worn into those pads. It was really hard to capture the grooves in a photo.

It maybe doesn't look like anything serious but I don't have the experience to be sure.

I got an offer from someone over at TC who is going to look through some extra plates they have and send me some if he has ones in better shape than mine.

Thanks.....Dave

[Mike S]

 If those were my plates and I could not find less worn ones, I would MIG weld (or flux core wire feed works as well) the valleys and file them down flush so as to fill those low areas.

Mike

[BULLITT65]

I would run those as is. I wouldn't lose any sleep over it.