What Size Brakes Do I Have?

[RR Fan Dan]

I’m trying to buy brake parts but my car is not here to measure. 74RR rear drum size. Stock. 10” or 11”?

 

[Don Frelier]

Ask Carnac the magnificent

 

[RR Fan Dan]

Ask Carnac the magnificent
View attachment 1952858

What they weren’t a standard size?

 

[Vaanth]

I’m trying to buy brake parts but my car is not here to measure. 74RR rear drum size. Stock. 10” or 11”?

Standard rear drum brake diameter on a 1974 Road Runner originally equipped with a 318 engine was 10"x2.5". 11"x2.5" rear drum diameter was optional. B28 on the production broadcast denotes rear 11"x2.5" drum brakes.

From the 1974 Salesman Guide: (S = standard, E = optional)

 

[mobileparts]

The “ general “ rule is :
Small Block is 10 “ x 2 1/2 “
Big Block is 11 “ x 2 1/2 “
However ,
It could have been upgraded from 10 “ to 11 “ in the car’s prior history….
And with MOPAR , “ Never Say Never “…

Bottom Line is : Measure and be 100 % correct / Perfect ….

And , if you want QUALITY parts , P.M. me , as per forum rules….
Or , you can go to a parts store or internet for Chineseum garbage and regret it…..

 

[WileE]

^^^ Ask Craig, ya can't go wrong !

 

[RR Fan Dan]

So, if they turn out to be 10” can I convert to 11” and still retain stock 14” rims? What do I need to replace? Drums backing plates and all the internal stuff? I can find drums but not backing plates yet.

 

[Vaanth]

So, if they turn out to be 10” can I convert to 11” and still retain stock 14” rims? What do I need to replace? Drums backing plates and all the internal stuff? I can find drums but not backing plates yet.

11" rear drum brakes work with stock 1974 14" wheels.

I prepared the following table to list part differences.

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Brake component differences between 10”x2.5” and 11”x2.5” rear drum brakes, 1969 - 1989.

​

Part	10” x 2.5” vs 11” x 2.5”
Drum	Different
Support (backing plate)	Different
Shoes	Different
Return springs, upper	Different
Plate (upper washer)	Different
Shoe retaining springs	Same
Shoe retaining pin	Different
Shoe spring retainers	Same
Adjuster/return spring, lower	Different
Adjuster	Same
Self adjuster cable	Different
Self adjuster cable guide	Same
Self adjuster cable lever	Same
Self adjuster cable lever pin	Same
Wheel cylinder	Same
Wheel cylinder rods	Same
Wheel cylinder bolts	Same
Wheel cylinder support seal	Same
Park brake strut	Different
Strut anti rattle spring	Same
Park brake lever	Different
Park brake lever lock	Same
Lever lock spring washer	Same
Park brake cable	Same

NOTES:

• 1962 - 1968 is similar with consideration for other differences due to manual adjusters or earlier self adjusters.
• 10”x2.5” comparison to 11”x2” or 11”x3” is similar with consideration for width differences such retaining pins, and early shoes (11”x3”) spring attachment differences.
• 1975 and later brakes use a different park brake lever to shoe attachment which interchanges with mating parts.
• 1973 and later 7-¼” rear axle 10”x2.5” brakes require additional swap considerations.

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[R413]

What do you have for front brakes? Disc or drum?

 

[toolmanmike]

And, are you having brake issues? Having a hard time stopping?

 

[RR Fan Dan]

What do you have for front brakes? Disc or drum?

Disc

And, are you having brake issues? Having a hard time stopping?

No. The car is completely stripped down under restoration. This is for when it goes back together. Everything is being rebuilt. I was getting brake parts for the rear today and that’s what started this conversation. If I end up having 10” in the rear I will look at upgrading to 11”. If the rear is 11” I will just buy all the parts. Wilwoods are replacing the front factory discs. New PTFE coated stainless steel hoses, stainless steel lines, ceramic brake shoes, and a Leeds polished master cylinder and vacuum booster.

 

[toolmanmike]

Disc

No. The car is completely stripped down under restoration. This is for when it goes back together. Everything is being rebuilt. I was getting brake parts for the rear today and that’s what started this conversation. If I end up having 10” in the rear I will look at upgrading to 11”. If the rear is 11” I will just buy all the parts. Wilwoods are replacing the front factory discs. New PTFE coated stainless steel hoses, stainless steel lines, ceramic brake shoes, and a Leeds polished master cylinder and vacuum booster.

Disc brakes provide 80-90% of the braking force in a combination system. You don't need bigger rear brakes.

 

[RR Fan Dan]

Disc brakes provide 80-90% of the braking force in a combination system. You don't need bigger rear brakes.

I realize this, but I’m buying new everything including drums and the difference in cost for parts is negligible so why not just go to 11’s. The only thing I wouldn’t be buying if I kept the 10” (assuming that’s what I have) is the backing plates. Which right now it looks like the backing plates would be the deal breaker from going to 11’s because it seems they are hard to come by.

 

[RemCharger]

And the 11" will reduce handling performance through unsprung weight as well as acceleration due to excessive rotating mass.

 

[RR Fan Dan]

And the 11" will reduce handling performance through unsprung weight as well as acceleration due to excessive rotating mass.

Now this changes everything.