Rear Brake Drum Size

[MerganserMaster]

Any idea how to determine the correct wheel drum size for the rear wheel drums? I need to replace the two master cylinders on the rear drums and I can't tell if my drums are 11" or 10" as when I measure the inner diameter (as seen in the picture) I get between 10 and 11 inches (slightly more than 10 when measuring the full diameter). Which size should I go with. Its a 1975 302 V8 if that helps, but I am not confident that the brakes/drums were not replaced at one time. Maybe I'm taking the 10" measurement to literally lol

https://flic.kr/p/2puJWvT

 

[thegreatjustino]

What is the depth of the drum?

Stock choices are:

10 X 2.5
11 X 1.75
11 X 2.25

 

[DirtDonk]

And what does it say on the outside of the drum?
All of the originals should have a “maximum diameter” number listed.
I can’t say that all modern replacements and imported crap still have that, but they should.

 

[MerganserMaster]

And what does it say on the outside of the drum?
All of the originals should have a “maximum diameter” number listed.
I can’t say that all modern replacements and imported crap still have that, but they should.

Where would this number be? It says "FOMOCO" on it so I'm assuming its OEM. Also from what points to I measure the depth from, the reason being is that there are a few ridges. I'm assuming its 10" now because no matter how I orient the tape measurer the inner diameter of the drum does not exceed much past 10", and I'm guessing now that it might be due to wear on the drum.

 

[DirtDonk]

Sorry for the delay. I was actually looking for a photograph here on the forum that I just saw yesterday.
Clearly shows that the location of the stamping of the “maximum diameter” number was easily read on the outside of the drum.
If you are facing the drum’s exterior it would be printed along the diameter of the outer surface. On a rib just adjacent to where the wheel stud holes are.
Would be a big surprise to me if a an original Ford part did not have that.

 

[MerganserMaster]

Sorry for the delay. I was actually looking for a photograph here on the forum that I just saw yesterday.
Clearly shows that the location of the stamping of the “maximum diameter” number was easily read on the outside of the drum.
If you are facing the drum’s exterior it would be printed along the diameter of the outer surface. On a rib just adjacent to where the wheel stud holes are.
Would be a big surprise to me if a an original Ford part did not have that.

No worries, Its a bit rusty so I can't really make out any numbers, however I did find the 10" cylinders on amazon so if they don't fit/aren't the right ones I can just return them and I will know its an 11" drum.

 

[DirtDonk]

Not so fast. There were sometimes multiple cylinder designs, even among the same size brake. So just because they are for a 10 inch doesn’t mean they’re going to fit.
We’ve run into that a lot. Especially with 74 and 75 models.
My point is, just because they don’t fit your particular 10, doesn’t automatically mean they are for an 11.

 

[MerganserMaster]

Not so fast. There were sometimes multiple cylinder designs, even among the same size brake. So just because they are for a 10 inch doesn’t mean they’re going to fit.
We’ve run into that a lot. Especially with 74 and 75 models.
My point is, just because they don’t fit your particular 10, doesn’t automatically mean they are for an 11.

Good to know, I remember seeing this difference when ordering them now. Correct me if I am wrong but aren't there 4 styles (2 for each drum size) total that are based off of production years?

 

[DirtDonk]

Correct. Total of 8 different fitments, if you include the left and right sides.
There are the 66-73 10" and 74-75 10" rears. Then 66-75 11" and 76-79 11" rears.

Paul