1970 amc javelin disc brake conversion

The current Scarebird setup uses a Toyota Previa hat type rotor. The drum hubs need a little turned off the outer edge (of must hubs). It can be done on a brake lathe, only needs a little off.

TSM is building a setup similar to the Scarebird setup. Doesn't say what hat type rotor they are using (looks like it might be a 2WD Jeep XJ rotor?), but those are definitely GM calipers.

Willwood and Aerospace Components make a front disc setup. I prefer the Aerospace, but both are good from what I hear. They don't mention what spindle to use.

All except 75-78 AMCs use the same spindle dimensions for drum and disc brakes (same bearings and space between bearings). The difference in the spindles is the thickness of the base, holding the psindle further out from the rotor. There are at least three thicknesses: narrow drums (Americans and older six cylinder big cars, maybe six cylinder 60s Javelins -- 2" or narrower), wide drums (mostly V-8 cars, 2.25" or wider drums), and 1971-74/79-83 disc brakes. There may be more than one base for disc brakes.

The point is you can use drum spindles for disc brakes. You just need to install everything and determine how much further out you need the spindle, then use grade 8 washers (grade 8 washers are hard and won't deform under pressure, and are uniform thickness) and longer bolts (grade 5 are stock, use 5 or 8) to set the spindle out. Or make a spacer plate if you need 1/4" or more, though washers will work fine and be just as strong as a spacer. Once torqued down the washers act the same as a solid mass would.

The bigger brakes won't hurt, but the standard set is more than adequate even for a 390 car -- assuming normal street use (and even an occasional run down the strip). The only time you really need bigger brakes is if you are road racing or maybe live in a mountainous area where you're using the brakes a lot, or towing trailers without brakes. The stock 79-83 rotors are 10.82" in diameter, right between the Previa and Sportrac (four cylinder rotors were 10.27", but also thinner). That might be driving your decision for the 12" brakes, which is understandable. That and the hubs need to be turned down with the Sportrac rotors.

I've not used the TSM setup, but it's essentially the same as the Scarebird, they just use different parts.

Brakes aren't really all that complicated. They are just a hydraulic cylinder (the caliper piston) that squeezes a piece of steel (the rotor). The caliper bracket just has to center the caliper over the rotor and mount it out from the center the correct amount. Except for the bleeder screw the caliper can be mounted in any clock position around the rotor. Due to the way the mounts are made (and bleeder location) that essentially means it's either mounted in about the 2 o-clock or 10 o-clock positions -- in front or in back of the rotor. Which doesn't matter, leverage forces are the same (around the spindle). You can swap position easily -- just swap the bracket from the left side to the right side. I've done that using stock setups from a 79-83 Concord/Spirit on a 63 American.

SIZING everything on the hydraulic side gets a bit more complicated, as you need a master cylinder that supplies enough fluid at enough pressure for the size of the caliper. You need to move enough volume at enough pressure. You also need adequate pad area. A lot can be fudged in those areas, and a little more is better than not enough. The 10" rotor uses about the same pad area as the 12" -- the the main gain is that a larger rotor will dissipate heat faster than a smaller one.

When dealing with an entire brake system, there also needs to be some balance between the front and rear brakes. One mistake people make is upgrading the rear brakes when they upgrade the fronts. You don't need more rear braking power if you increase front braking power -- it's usually just the opposite. When you increase front brake power you also increase weight transfer to the front, so the rear brakes actually do a bit less work, and may lock up faster -- especially if you increase rear brake power. That's why proportioning valves are sometimes used, but those fell out of use with some OEMs as they learned to just install the correct size wheel cylinders to match brake bias front to rear. In some years the proportioning valve made more monetary sense. Same parts can be used for the rear in many cars -- the valve took care of balancing rather than brake size, or rather made sizing of the rears a bit more flexible.