Moparmonster
Well-Known Member
Okay, first, am I measuring this correctly? And, provided that I am, am I correct that I need 11" drums?
Need New Drums
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Moparmonster
Well-Known Member
Hmm, I guess not. There are three variants; 2, 2.5, and 3.
mobileparts
Well-Known Member
" A " Body Mopars never had an 11 " Brake Drum from the factory.... someone could have done an upgrade to your vehicle....
I have every Mopar Brake Drum --- N.O.S. U.S.A. made at this moment in time....
Craig....
I have every Mopar Brake Drum --- N.O.S. U.S.A. made at this moment in time....
Craig....
Moparmonster
Well-Known Member
I never claimed that it was original; it's a narrowed B body axle that had done duty in a Barracuda before I bought it sans center section.
mobileparts
Well-Known Member
So, the magic question is what is the width of the Brake shoe.....
The Brake Drum is 1/4 " deeper than the Brake Shoe, because the brake shoe does not go lip - to - lip of the Brake drum....
Such that if the Brake Shoe is 2 1/2" wide, the Brake Drum is called 11 " x 2 1/2 " ....
( And that would have been common in a big block 1970 - 1972 Barracuda / Cuda Rear )
I don’t know why it is bold print all of a sudden --- I must have fat-fingered something?!?!
Craig....
The Brake Drum is 1/4 " deeper than the Brake Shoe, because the brake shoe does not go lip - to - lip of the Brake drum....
Such that if the Brake Shoe is 2 1/2" wide, the Brake Drum is called 11 " x 2 1/2 " ....
( And that would have been common in a big block 1970 - 1972 Barracuda / Cuda Rear )
I don’t know why it is bold print all of a sudden --- I must have fat-fingered something?!?!
Craig....
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Josh owen
Bang em till it breaks
Here's some I just pulled off a 1977 8.25 rear end this weekend. May help with a visual. These will bolt into 8.75 if you enlarge the bottom hole with a step bit to open up your options on better brakes
The measurement of a drum diameter is done to the inside diameter of the drum — the braking surface. Measuring from the shoes is approximate at best, since it depends on wear. However, it is probably close enough to tell the difference between 10 and 11 inches — there are no 1/2-inch sizes.
The width measurement, however, refers to the width of the shoe, NOT the width of the drum. A lot of people get confused by this.
The idea is that the combined measurements (indirectly) describe the brake swept area — circumference X width.
The width measurement, however, refers to the width of the shoe, NOT the width of the drum. A lot of people get confused by this.
The idea is that the combined measurements (indirectly) describe the brake swept area — circumference X width.
The backing plates are basically 1" larger in diameter than the drum size. So, if you have 11" diameter backing plates, they're for 10" drums. If you have 12" diameter backing plates, you have 11" drums.
If there aren't shoes or drums on the backing plates, you can measure the backing plate offset to determine the shoe size, since the backing plates are not interchangeable. The method for measuring the offset and the measurements for a bunch of the different brake variations are listed in the link...
Converting rear drum brakes from SBP to BBP
If there aren't shoes or drums on the backing plates, you can measure the backing plate offset to determine the shoe size, since the backing plates are not interchangeable. The method for measuring the offset and the measurements for a bunch of the different brake variations are listed in the link...
Converting rear drum brakes from SBP to BBP
Moparmonster
Well-Known Member
Okay, so, based on one of the pictures I already posted, this axle has 10" drums.
Also, thank you for the excellent info.
mobileparts
Well-Known Member
1963 - 1972 " A " Body used 10 " x 1 3/4 " on their Rears, &
1973 - 1976 " A " Body used 10 " x 2 1/2 " on their Rears....
Craig......
1973 - 1976 " A " Body used 10 " x 2 1/2 " on their Rears....
Craig......
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Yup, that’s what it looks like. And since it’s a narrowed B body 8 3/4 axle they’re probably the 10x2.5”, large bolt pattern (5x4.5”) version. Which are super common, most parts places will carry the drums and usually even a few different brake compounds for the shoes. For most kinds of street driving a good set of semi-metallic compound shoes will far outperform any of the OE offerings.
Moparmonster
Well-Known Member
I am, yes, but I don't know what year the axle is. Since we've narrowed it down, how do I tell the difference?
Also, if I wanna replace the hardware inside, I guess I'll need to know which to get. Everything looks okay, but for safety's sake, new cylinders are probably I order.
Yes, you're correct that the bolt pattern is 5 x 4.5. I have one of those wheel disk thingies that you put over the studs.
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If you follow the link in post #8 it shows how to measure the backing plate offset to determine which size brakes the backing plates are for.
But since you’ve got a B body rear axle housing and BBP axles my bet would be you have the 10x2.5” brakes, since B-bodies never had SBP axles or the 10x1.75” brakes.
mobileparts
Well-Known Member
The 10 “ x 2 1/2 “ Rear Brake Drum —- IS —- a 5 x 4 1/2…
The 10 “ x 1 3/4 “ Rear Brake Drum —- IS —- a 5 x 4 …..
Craig….
The 10 “ x 1 3/4 “ Rear Brake Drum —- IS —- a 5 x 4 …..
Craig….
I saw that. As per forum rules, you should respond through a pm only. Thanks
mobileparts
Well-Known Member
^^^ I did, toolmanmike ^^^
I P.M.'ed Moparmonster with additional information....
Craig.....
I P.M.'ed Moparmonster with additional information....
Craig.....
Please don't announce" I have some I would sell you". That's what I meant.
Not the whole story. There are aftermarket axles out there that have the BBP but the SBP axle flange offset and require redrilling the SBP drums.
Which means, just because the axles measure 5x4.5” doesn’t mean that the backing plates he has are for the 10x2.5” brakes. It looks like the OP has backing plates and not drums, which is why he was measuring backing plates to get the drum diameter
Since it’s a narrowed B-body rear axle the plates are most likely for the 10x2.5’s, but you can’t tell that just from the axle bolt pattern.
So it is safe to assume that drums didn't come with your rear. Did the axles come with it and can you slip them into the housing ?
You most likely are goin to rebuild the rear brakes anyway. Remove the backing plates and take them to the parts store to check drum fit.
You most likely are goin to rebuild the rear brakes anyway. Remove the backing plates and take them to the parts store to check drum fit.
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