Hello everyone,
I知 upgrading my 72 Cutlass Supreme to Wilwood Disc Brakes all the way around. I知 trying to determine what bore size I need to order for the Wilwood MC. Any insight will be greatly appreciated and thank you in advance!

 

Have you talked to Wilwood about this? Seems as if they are selling you the parts then they should know which MC you need to be compatible with their calipers.

 

Power or manual? As said, Wilwood will happily give you recommendations.
You can tweak master bore size if you're really serious about tuning the brake system, but IMO at that point might as well get separate masters with a balance bar.

The "typical" power MC is 1 1/16".

 

There's nothing magic about selecting bore size. You want the smallest M/C that moves enough fluid to operate the wheel cylinders even when the linings and rotors are at minimum thickness. A smaller M/C bore gives you lower pedal pressure for a given braking force, or more braking force for a given pedal pressure, but trades this for longer pedal travel to move the same amount of fluid. As an example, I swapped the 1" bore OEM M/C on my 62 for a 7/8" bore dual M/C and the four wheel drum brakes are MUCH better now. I was concerned about pedal travel, but it is not excessive so long as you keep the shoes adjusted (62 was the last year for manual adjusters on the drums).

 

You need to determine the front caliper piston area and the rear caliper or wheel cylinder area before you pick the master cylinder bore size. Most Wilwood calipers have a smaller piston area than the original D52 caliper. The D52 caliper used on the Chevy Chevelle, Buick Skylarks / Grand Sports, and Pontiac A-body cars had a single piston 2.934" in diameter and a piston area of about 6.76". Joe_Padavano stated in another thread that the Olds A-body D52 used a 2.75" front piston.

Joe is right about you most likely needing a smaller master cylinder bore than an OEM power disc brake car.

Power brake cars used 1 1/8" master cylinder and manual brake cars used a 1" cylinder. If you have less front caliper piston area than the D52, the 1 1/8" master will be too large. Using the 1 1/8" master will probably result in a pedal that takes a lot of force to generate sufficient brake torque.

Again, people need to know what exactly you are planning on installing before they can make a recommendation. They also kneed to know if this is a power or manual disc brake setup you are installing.

 

My apologies. You are correct. The piston bore is 2 15/16" diameter, which is 2.9375" diameter. I would have sworn that I've read 2.75" in multiple places, but I would be wrong. Sorry for the misinformation.

 

I have a Wilwood set up, Disc all the way around, 6 piston front. 69 cutlass
Converted to manual for engine compartment room.
I am running a 7/8 bore for less foot pressure required.
They almost feel like power brakes.
No issues with fluid volume moment.
Would absolutely recommend and or do again.

 

Right, but the OP needs to know which exact caliper model he is getting. For example, the Wilwood Aero6 six piston caliper line can be ordered with piston areas of 4.04 square inches, 5.40 sq in, or 6.52 sq in.

Most of the off the shelf front Aero 6 based kits sold for 68 - 72 GM A-body cars use the 4.04 sq in caliper. If that is what you have, I can see the 7/8 bore master being a good choice. But, if the system had the 5.40 or 6.52 calipers, it would probably be a little small and a 1" - 1 1/8" would be more suitable depending on if the car is a manual or power setup.

 

For ref, size unknown

Front Kit 140-10510 Caliper #120-10122/23
Rear kit 140-7179 Caliper # 120-6806

 

The fronts have a 5.06 in^2 piston area. The rears are 3.0 in^2.