MGA Front Drum to Disc Brake Conversion
Tech Talk with Bob Shafto, Technical Editor
Converting MGA 1500 front drum brakes to 1600 front disc brakes is a straightforward job that will take about half a day. I will list the parts needed at the end of this article. The job is mostly a matter of removing the drum brake parts and bolting in the disc brake parts. However, there is one exception, and that is the transition from hard brake line to flex brake line uses different fittings and the bracket welded on the frame is in a different location.
The drum brake flex line mounts through a tab or bracket welded to the frame on the upper spring cup in the crossmember. The flex line has a 5/8-18 threaded end that goes through a hole in the bracket with a lock washer and nut on the other side. The end also has a 3/8-24 tapped hole for the standard male, hard brake line fitting.
The disc brake car also has a tab or bracket welded to the frame, but in a different location on the crossmember, closer to the frame rail. The flex line for the disc brake is a smaller 3/8-24 threaded end that goes through the bracket held in place by a lock washer and nut. This end is also a male, standard flare and the hard line is connected to the end with a female flare fitting.
On the 1600 disc brake cars the bracket is welded in the lower corner next to the longitudinal frame rail.
In the past, I converted cars that were being restored, so the frame was off the car. In that situation I cut the brake line frame mounting bracket off the spring cup and welded it to the disc brake position, near the frame rail. Each time I made a template from my 1600 disc brake car and used it to relocate the factory brake line bracket. The last conversion I completed was done on newly restored 1500 where I did not want to cut and weld on the powder-coated frame in tight spaces. As a solution, I opted to make an adapter bracket bolted to the original tab that extended over to the 1600 disc brake mounting position.
To complete the job, begin by removing everything on the front spindles. Remove, clean and inspect the bearings from the hub. They are often in good shape and can be reused. Inspect the spindle for wear, nicks and scratches. The bearing inner race is not supposed to rotate on the spindle, so the spindle should not show wear. Inspect the oil seal spacer outer surface for grooves, scratches and rust pits. This is the surface that the seal rides on so it must be flat and smooth; recondition if necessary, or buy new. Rebuild the replacement hubs with cleaned or new bearings, well packed with grease, and don’t forget the bearing spacer (small end toward small bearing). Bolt the new rotor to the hubs, install the new seal into the hub and oil the lip.
Bolt the adapter plate to the spindle back plate and slide the oil seal spacer over the spindle with the concave side against the spindle back plate. Then oil the outer surface where the seal rides.
Next, install the replacement hubs, outer washer and nut. Tighten the nut to 40 ft. lbs. and then forward to the nearest point that allows the cotter pin to be inserted. The point here is to keep pressure on the stackup of parts: washer, outer bearing, large spacer, inner bearing, oil seal spacer and spindle back plate. This essentially locks the bearing inner race to the spindle so the inner race can’t rotate. The outer race of the bearings are pressed into the hub, so all the rotation is by the balls rolling between the two races.
Now install the calipars (with pads installed), brake lines, flexline holding bracket and 1600 master cylinder cover. The reason for the larger volume cover is because the volume behind the four pistons is much greater than the drum wheel cylinders. As the pads wear, the piston rest position becomes deeper, storing more fluid behind the piston and less in the master cylinder. The master cylinder total volume is designed to have enough reserve fluid to account for total brake pad wear over the life of the pad. I know some have chosen not to use the larger volume cover, but then need to check and top off the fluid as the pads wear and then remove some fluid when new pads are installed.
When you have finished the assembly, pressure bleed the brakes. I recommend buying good used parts when you can because new parts can run around $1,000. Also, beware that many conversion kits on the market are not a truly complete set of conversion parts. They usually don’t include hubs, flexline holding bracket/lock plates or the master cylinder cover.
Full size drawings for both the welded location and the bracket can be found at: drive.google.com/file/d/1En26N_wKwC94W3YN3Dzx2g8GB5b21sIJ/view
