A-Body 66 Barracuda 8.75 backing plates

[Miranthis]

Which ones do I need? Is it the 10 x 1.75 ones. What should I be looking for and how do I tell the ones I need from the other 8.75 backing plates? Is there a year range i am looking for? Assume dart, duster, valiant, etc up until about 70. Is that about it?

 

[Professor Fate]

As far as I know it should be any A body 10x1.75 up to '72. All A body 8.75s had them (that was the only brake that ever came on them from the factory) as well as some 7.25s with HD brakes.
If you have a bunch of different backing plates to choose from and you're just trying to find the correct ones for your car, you need to lie them flat on a bench (the parking brake dimple will prevent this, so just hang that part over the edge of the bench) and measure up through the center hole. The ones you're looking for should measure about 1.25".
Diameter is about 1" larger than the brake it fits; ie a 10" backing plate is for 9" brakes, an 11" backing plate fits 10" brakes, etc...

 

[Miranthis]

So, if they are listed as 10 x 1.75 those are 9 inch and I need the 11 x 1.75 for 10 inch brakes?

 

[Professor Fate]

No, if they are listed as being for 10x1.75 brakes, the backing plates will actually be about 11" in diameter and 1.25" high as I described above.
Here's a link to a post with a picture showing how to measure the height:
Can someone tell me what these backing plates fit?
Brake size is determined by the drum's inner diameter and width (the friction surface of the drum), not the backing plate diameter or drum outside diameter.

 

[cudamark]

The actual contact surface width of the drum is about 1/4" wider than the shoe.

 

[Miranthis]

No, if they are listed as being for 10x1.75 brakes, the backing plates will actually be about 11" in diameter and 1.25" high as I described above.

I get that, but only if the person listing them actually knows what they are doing, which can be a stretch, especially once you wade through the scammers and fake shops etc.

The problem is trying to discern from an eBay or marketplace picture that is MARKED "10 x 1.75 A-Body" by the seller if that is MEASURED at 10" (and thus for 9 inch drums) or if it is FOR the 10 inch brakes and thus measures about 11 inches in diameter. Unfortunately, the folks that have them on eBay are not taking questions and don't put a ruler on the pic for scale ... :-( At least around her in KC there is no place ote go with a bunch of backing plates I cand leaf through with my tape measure... I wish there was

Now, since i was a boy the car has had a 7.25 rear that was put in it when the original 8.75 Sure Grip went bad in 1975-ish. So, I don't have the original to look at, but know that the car was a 4 wheel drum car and a Commando V8 with the 4 speed, so are the original rear drum brakes on the 8.75 rear 10 x 1.75 or the larger 10 x 2.5?

So many questions...

 

[cosgig]

Here’s a pic of the backing plate from the front, and one sitting in the drum. The outside circumference of the backing plate is 11” on the nose, and yes I’ll sell you these, with or without the drums.

 

[Professor Fate]

so are the original rear drum brakes on the 8.75 rear 10 x 1.75 or the larger 10 x 2.5?

As I stated previously, the only brake to ever come from the factory on an A body 8 3/4 rear was the 10x1.75 drum brake. Regardless of year.

 

[Miranthis]

As I stated previously, the only brake to ever come from the factory on an A body 8 3/4 rear was the 10x1.75 drum brake. Regardless of year.

Thanks! I keep seeing a reference on line to a towing package and "some folks say" that meant HD 10 x 2.5 rear brakes. Of course, "some people say" does not control. I have yet to see a document that says the larger ones were in the 1st gen. A-Bodies....

 

[cudamark]

Here’s a pic of the backing plate from the front, and one sitting in the drum. The outside circumference of the backing plate is 11” on the nose, and yes I’ll sell you these, with or without the drums.

View attachment 1716465942

View attachment 1716465943

Just for information, parts purchase, and re-assembly purposes, that upper washer needs to be flipped 180 so the ears are at the top. Also, the upper photo is the passenger side (RH), and the bottom photo is a driver's side (LH) off a '70-72 model.

 

[cudamark]

Thanks! I keep seeing a reference on line to a towing package and "some folks say" that meant HD 10 x 2.5 rear brakes. Of course, "some people say" does not control. I have yet to see a document that says the larger ones were in the 1st gen. A-Bodies....

The first A-bodies with 10 X 2 1/2" rear brakes started with the '73 models with front disc brakes.

 

[Professor Fate]

Thanks! I keep seeing a reference on line to a towing package and "some folks say" that meant HD 10 x 2.5 rear brakes. Of course, "some people say" does not control. I have yet to see a document that says the larger ones were in the 1st gen. A-Bodies....

No, pre-'73 A bodies never got anything larger than 10x1.75.
If I'm not mistaken, on a base model the towing package got you an 8.75 rear (and the 10" brakes by default- so they also put 10" on the front); HD braking could get you 10" brakes on a 7.25 rear (depending on other options) and 10" drums up front, and then there were the optional front disc brakes. There's a lot of possible combinations, but at no time were there ever any rear brakes larger than 10x1.75 prior to 1973.

 

[Professor Fate]

The first A-bodies with 10 X 2 1/2" rear brakes started with the '73 models.

And that was when the As went to the large bolt pattern; and dropped the 8.75 and went to the 8.25 as the HD rear end.
And although it was never an official option, there were some A body 8.25s that got 11" drum brakes- not as an upgrade, but just to "keep the assembly line moving" when the supply of 10x2.5s ran low... one of those things.

 

[cudamark]

And that was when the As went to the large bolt pattern; and dropped the 8.75 and went to the 8.25 as the HD rear end.
And although it was never an official option, there were some A body 8.25s that got 11" drum brakes- not as an upgrade, but just to "keep the assembly line moving" when the supply of 10x2.5s ran low... one of those things.

Yup, but the 4 1/2" bolt pattern was only on disc brake cars. The rear ends came in three flavors....9" brake 7 1/4 4" pattern, a 7 1/4 with 10 X 2 1/2 brakes with 4 1/2" bolt pattern, and the 8 1/4 with 10" brakes and 4 1/2" pattern......not withstanding any that might have come with 11" brakes due to supply issues.

 

[66Valiant528]

So on an early A 7.25 rear with 10 inch there must have been a 4 bolt backing plate or did they just use the 5 hole 8.75 backing plate and not use the bottom stud/nut?

 

[cudamark]

Different backing plate between the 7 1/4 and the 8 3/4.

 

[Professor Fate]

Different backing plate between the 7 1/4 and the 8 3/4.

Although I was always under the impression that the lower hole can be drilled to make it useable on an 8.75... I've got a 7.25 with 10s buried in a corner of the shop, but I just REALLY don't feel like digging it out and pulling it apart just to find out.

 

[cudamark]

IIRC, the center hole diameter is different too.

 

[66Valiant528]

IIRC, the center hole diameter is different too.

That'd be ugly. Ive got some 10 in plates off a 66 8.75 that I was planning on putting on the wife's 64 7.25. I was under the impression that it'd fit and you just didn't have to use the 6 o'clock bolt hole. So there are different backing plates for a 7.25 with 10 inch drums?

 

[Miranthis]

I have a 66 Barracuda, so all the options were 5 x 4 wheels/hubs in that year, and i think in all the 64, 65, and 66's.

 

[Miranthis]

Here’s a pic of the backing plate from the front, and one sitting in the drum. The outside circumference of the backing plate is 11” on the nose, and yes I’ll sell you these, with or without the drums.

Sent you an IM...

 

[cudamark]

That'd be ugly. Ive got some 10 in plates off a 66 8.75 that I was planning on putting on the wife's 64 7.25. I was under the impression that it'd fit and you just didn't have to use the 6 o'clock bolt hole. So there are different backing plates for a 7.25 with 10 inch drums?

Yes, a 7 1/4 with 10" brakes use a different backing plate than an 8 3/4 with 10" brakes. Bolt hole number for sure, but, I think the center hole is different too, since the axle bearings are different sizes. I'm doing this from fading memory since I got rid of almost all my 7 1/4's years ago.

 

[cudamark]

I have a 66 Barracuda, so all the options were 5 x 4 wheels/hubs in that year, and i think in all the 64, 65, and 66's.

Yes, all '60-72 Abodies used the 5 bolt 4" bolt pattern.....not counting the '68 Hemi cars which I'm unsure of....

 

[cosgig]

Just for information, parts purchase, and re-assembly purposes, that upper washer needs to be flipped 180 so the ears are at the top. Also, the upper photo is the passenger side (RH), and the bottom photo is a driver's side (LH) off a '70-72 model.

These came from the “BadSport” collection just as you see them. I cleaned out his entire garage and helped Martha get his stuff taken care of. I bought all the Mopar stuff, and have been spreading it out as I see people who need stuff.

 

[Two7T3Dart]

Yup, but the 4 1/2" bolt pattern was only on disc brake cars. The rear ends came in three flavors....9" brake 7 1/4 4" pattern, a 7 1/4 with 10 X 2 1/2 brakes with 4 1/2" bolt pattern, and the 8 1/4 with 10" brakes and 4 1/2" pattern......not withstanding any that might have come with 11" brakes due to supply issues.

There was also 10" x 1.75", 7 1/4 rear, 4" bolt pattern, as those drums fit perfect on my A body 8 3/4.