Rear Drum Size question

[stonecoldtx]

Hi, all--

OK, so I've got a '70 Mustang coupe, and it's time to do a brake job on it.

It came with a 302 2-V and power front disks & rear drum brakes, with an 8" rear axle assembly.

I have found that there are two widths of these brakes--1-3/4" and 2".

I haven't been able to tell which size is used on which models.

I am guessing that the following get the 1-3/4" brakes:

1.) 250ci 6-cyl cars that have Power Front disk brakes/rear drum brakes
2.) All V-8 cars that come with Power Front disk brakes/rear drum brakes and have an 8" rear axle

While I am guessing that all of the cars that came with the 2" wide brakes had 9" rearends, and made more power than the 6-cylinder and 302-2-V engines, I'm not sure. It could be that some cars had "heavy duty" rear brakes on an 8" rearend, and would get the 2" width rather than the 1-3/4" width shoes and drums.

I have looked at the shoes for both sizes, and it seems to me that the major difference might only be that you need to match up the shoes, hardware (springs, etc.) and drums.

While 1/4" of added width might not seem like a lot, I am sure that it's substantial enough to make a difference, and my question is:

Am I correct in assuming that the backing plates between the 1-3/4" and 2", and that one only needs to select the matching shoes, hardware and drums, or are they different enough that you have to use what came on the car?

Anybody know for sure on this?

Any help would be greatly appreciated . . .

Thanks!

 

[mustangmike6996]

either the backing plate is set more inward (for the HD brakes ) or the axles stick out further for the standard brakes... im not really sure how you can tell... maybe find a VIN decoder on the internet and see if there is a certain letter that notifies HD brakes or something of that nature..

i have a 69 fastback 302 2V..C4 auto .... power steering.. manual 4 wheel drum brakes... i dont know if all of the V8 70's come with front disc let alone power brakes...

 

[D.Hearne]

Both likely use the same drums. You can buy drums locally at your parts house. Ranger drums will be the same. The difference in widths may also be in diameter too. They'll have 9" or 10" drums. Same on the Rangers.

 

[stonecoldtx]

either the backing plate is set more inward (for the HD brakes ) or the axles stick out further for the standard brakes... im not really sure how you can tell... maybe find a VIN decoder on the internet and see if there is a certain letter that notifies HD brakes or something of that nature..

i have a 69 fastback 302 2V..C4 auto .... power steering.. manual 4 wheel drum brakes... i dont know if all of the V8 70's come with front disc let alone power brakes...

Thanks for the information, but I was looking for an answer from someone who actually has checked into this; I do know for a fact that the axles don't stick out further (we would know about it, and there would be a lot of information regarding it when trying to swap rearends).

I also know that the drum backing plate doesn't have to be different--the difference in the width of the braking surface in the drum can be changed to accomodate wider shoes simply by alerting that part of the drum (i.e.: 1/4" wider, with a different drum casting).

All Mustangs came with drum brakes on the rear through at least '73; however, the low-powered 6-cylinders received 9" diameter drums (at least according to the parts book; I don't recall ever seeing a 4-bolt '69 - '73 Mustang).

Also according to the parts books, the 250ci 6-cylinder engine could get the 10" X 1-3/4" 5-bolt drums, while all V-8's most certainly got 10" diameter 5-bolt drums.

Now, the only question is if the difference between the 1-3/4" and the 2" brakes was due to the backing plates or the drums themselves (with the difference in the hardware, too).

I suspect that it's the drums that make the difference, and I think I'll try to take a look at a couple of drums today to see if that's true or not.

 

[stonecoldtx]

Both likely use the same drums. You can buy drums locally at your parts house. Ranger drums will be the same. The difference in widths may also be in diameter too. They'll have 9" or 10" drums. Same on the Rangers.

Thanks for replying, D.Hearne, but there most certainly is a difference between the 1-3/4" and 2" shoes AND drums.

As I mentioned in my previous post, I suspect the entire difference between the systems is in the drums themselves, and the attaching hardware for the shoes.

I'll try to go check the drums out today and see if I can determine a difference--if I can, I'll post back here as soon as I know something.

Thanks!

 

[D.Hearne]

1-3/4 & 2" can use the same drum. The difference in shoe width isn't enough to call for a deeper drum. The difference would be only in the shoes. The shoe "kits" (springs and hardware) are the same for all 9" shoes. Ditto for the 10" shoes, 11 inch shoes, etc. Each diameter shoe uses the same hardware for that diameter. It's not as complicated as you're trying to make it out to be. The only thing complicated is the way the whole setup is engineered (with all the B.S. springs, pins,etc) You can compare a set of hardware parts and shoes from a 1957 Ford with those from say a 1990's rear drums and you'll see all the same parts, the only difference is in regards to shoe and drum diameter.

 

[stonecoldtx]

1-3/4 & 2" can use the same drum. The difference in shoe width isn't enough to call for a deeper drum. The difference would be only in the shoes.

Hi, D.Hearne--

With all due respect, they are indeed different, and you can't use a 2" shoe in a 1-3/4" drum; they are completely different drums altogether, and have difference sized friction surfaces.

But--don't take my word for it--you can look it up yourself in your favorite parts book, if you like.

Sure, you can use a 1-3/4" shoe in a 2" drum, but don't you think that would be kind of pointless and silly?

The whole point is that I would like to upgrade to the 2" drums if I've got the 1-3/4" brakes right now.

So, my quest is to find out, definitively, if the difference between the systems is in the DRUMS, or in the backing plates.

If it's the DRUMS, I'm home free--I just order the 2" drums, shoes and hardware kit, and install them.

However, if the difference is in the backing plates, and/or in both the backing plates AND the drums, then I have to wait until I pull it apart, and then see what I've got installed.

That means the car will be down for a week, while I'm waiting on the parts (Autozone, O'Reilly and NAPA all are too high priced for me, so I order my stuff from RockAuto online).

 

[D.Hearne]

Hi, D.Hearne--

With all due respect, they are indeed different, and you can't use a 2" shoe in a 1-3/4" drum; they are completely different drums altogether, and have difference sized friction surfaces.

But--don't take my word for it--you can look it up yourself in your favorite parts book, if you like.

Sure, you can use a 1-3/4" shoe in a 2" drum, but don't you think that would be kind of pointless and silly?

The whole point is that I would like to upgrade to the 2" drums if I've got the 1-3/4" brakes right now.

So, my quest is to find out, definitively, if the difference between the systems is in the DRUMS, or in the backing plates.

If it's the DRUMS, I'm home free--I just order the 2" drums, shoes and hardware kit, and install them.

However, if the difference is in the backing plates, and/or in both the backing plates AND the drums, then I have to wait until I pull it apart, and then see what I've got installed.

That means the car will be down for a week, while I'm waiting on the parts (Autozone, O'Reilly and NAPA all are too high priced for me, so I order my stuff from RockAuto online).

Yea, they do show different part numbers, but every drum I've worked with had more friction surface than the shoe used. So it's every bit possible to use a 2" drum on 1-3/4" brakes. The difference in part numbers from Ranger's to Mustangs could also be the center hole diameter, not just a difference in the drum. If you're in doubt as to what brakes you've got now, then you need to pull em apart and make sure you're ordering the right parts. And even then, you might not get what you thought you were. I've been doing this stuff for thirty years now and "been-there, done that" Don't be surprised to find you've got 2" drums with 1-3/4" brakes and find out they've been working all this time. After 30 something years, there's a good chance stuff's been mixed up on your car.

 

[1968notchback]

Thanks for replying, D.Hearne, but there most certainly is a difference between the 1-3/4" and 2" shoes AND drums.

As I mentioned in my previous post, I suspect the entire difference between the systems is in the drums themselves, and the attaching hardware for the shoes.

I'll try to go check the drums out today and see if I can determine a difference--if I can, I'll post back here as soon as I know something.

Thanks!

Did you get a chance to compare the 1 3/4" and 2" drums? The rear drums that came on my '68 (289 c4) are 2" wide and the brake shoes are 1 3/4" wide.

It seems that the 2" drum may fit in place of the 1 3/4" drum because the offset from the axle flange to the outside dimension of the brake surface may be less in the 2" drum.

I don't have a 2" drum so I can't check to see if it actually fits on a Mustang that came with 1 3/4" brakes.

 

[stonecoldtx]

Hi, All--

OK, I took a gamble and purchased the 2" brake drum, shoes and mounting hardware; I found that the difference in the drum was that the 2" drum had a friction surface that was 1/4" further outboard within the drum, making up for the difference in the shoe width.

Of course, since the center part of the back of the shoes is centered, there is a difference in the length of the pins that hold the shoes to the backing plate, so those parts are required at the very least.

So--to answer my own question, the difference is in the DRUMS, not the backing plates, and you need the mounting hardware for the wider shoes.

As a sidenote--you WILL REQUIRE the 2" drums with the 2" shoes, they WILL NOT FIT in a 1-3/4" drum.