Reference Manual

Chapter 4: Suspension: Front: Big Brake Conversion


As originally equipped, the G-body front disc brake rotors are 10.5" and use a smaller caliper than other RWD GM models of the time. This gives the G-body a lightweight braking system with adequate braking for most drivers needs.

For those that find the G-body brakes inadequate, there are alternatives. The disc brakes used by the 2nd generation F-body, the B-body and some C's and D's can be swapped into a G-body to improve it's braking capability. To do so the spindle must be swapped for a spindle from one of the above models. These spindles are taller than a G-body spindle, but are designed to use the same upper and lower balljoints as the G-body.

While it is possible to use the original G-body upper arms, doing so it not recommended. The increased height of the spindle when used with the G-body arms will make it very difficult, if not impossible, to properly align the front suspension. There are also suspension geometry issues if the G-body upper arms are used with the taller spindles. Global West and Hotchkis are two companies that make shorter upper control arms designed for this conversion.

There are two different basic spindles that can be used to increase a G-bodies braking capability. An 11" spindle and a 12", each has it's benefits and tradeoffs. Because the spindle locates the caliper the proper distance from the rotors center of rotation, rotors are not interchangable between the 11" and 12" spindles.

Once you have located a suitable donor, remove the rotor and bearings. Then inspect the bearing surface of the spindle for any damage. Unbolt the tie rod and the balljoints and inspect the balljoint holes for damage. If these areas are undamaged, you have found useable spindles.

11" Brakes

Many B-bodies from 77-85, the 74-79 X-body and the 70 1/2-81 F-body were originally equipped with 11" disc brakes using a 5 x 4-3/4" boltpattern. Any of these can be used as donors for an 11" conversion.

The 11" discs used a larger caliper than the G-body that is the same as the 12" disc caliper and the wheel boltpattern is identical to that of the G-body. This makes these appealing as a "budget" conversion.

If doing an 11" conversion, try to use those from a later model donor (post-78) with the "metric" calipers. Some have reported problems mixing the early calipers with the G's "metric" brake hoses.

12" Brakes

Some B-bodies from 77-85 and all B-bodies after 87 were originally equipped with 12" brake rotors. These can be used as donors to perform a 12" brake conversion. All B-body 12" rotors use a 5 x 5" wheel boltpattern. B-bodies with the 12" rotors also used larger 11" diameter rear drums, as opposed to the 9" drums used with 11" rotors and also for the G-body.

In order to maintain the same boltpattern front and rear, and use your original wheels, 12" rotors will need to be redrilled for a 5 x 4-3/4" boltpattern. Or 88-93 F-body "1LE" rotors (GM #18027886) can be used that already have the correct boltpattern.

Some 77's used a smaller diameter spindle measuring .750" at the outer bearing surface, avoid these. The spindle should measure .850" at the outer bearing surface.

Master Cylinder

For best results, the master cylinder should be replaced with a B-body master cylinder. The original G-body master cylinder will work, but you will have a low pedal. The larger caliper used with the 11" and 12" discs requires more fluid for proper operation and the B-body master cylinder supplies that as it has a 1-1/8" bore vs. the G-body master cylinders 1" bore.

Master Cylinders for 78 - 81

Manufacturer Cast Iron

GM 18008884
Bendix 11944
Wagner F101254

Master Cylinders for 82 - 88

Manufacturer Aluminum Cast Iron

GM 25509416
Bendix 12011
Raybestos JMC39328 MC39328
Wagner F104460

Aluminum (B and G) master cylinders are slightly larger than a G-body cast iron master cylinder, but bolt-in without any issues. 82 and later GM master cylinders use metric brake line fittings, adaptors can be found if needed.

Cast Iron and AL Master Cylinders

Wheel Studs

All the 11" and most 12" rotors were fitted with SAE thread wheel studs, while G-bodies used 12 x 1.5mm metric studs. Metric and SAE studs are not interchangable as they have a different knurl diameter. Be sure your machinist is aware of this when redrilling the discs. If doing a 12" conversion and using the 1LE rotors, the 1LE rotors are fitted with the correct metric studs.

If you would rather use SAE studs all around. Replace the rear axles of a 7.5" with those from a 78-80 A body, redrill the original axles or use aftermarket axles. KA offers axles for the 7.5 drilled for both SAE and metric studs. Because the G 8.5" axle only came from the factory with metric studs, redrilling the axles or aftermarket axles would the only options.

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