[AROWHED]

You can convert your 65-66 B body Pontiac to a factory 67-70 front disc brake system, but you must change out the spindles too which is a big hassle, plus a realignment may be required afterwards. But the drawback to that approach is it's about impossible to find new integral hub front disc brake rotors to fit the 67-70 Pontiac-only system. So sourcing a FLOATING disc brake rotor that could replace ONLY the drum, and leave the drum spindles and drum-removed hubs still in place would be ideal.
It's not easy to find disc brake rotor dimensions, but after a lot of searching, 2014-2024 Ram Promaster 1500 front disc rotors come very close.

Pros:
1. The Ram rotors are ALMOST a dimensional match to 67-70 Pontiac rotors, so they (can be made to) work, and are inexpensive through Rockauto.

2. There is no need to change out spindles, bearings, steering arms....so alignment is unaffected.

Cons:
1. You will need 67-70 B body disc caliper brackets, calipers, splash shields, and the special disc spindle anchor bolt that the caliper bracket requires. That bolt's thread does match and works with the drum spindle. And of course all the brake plumbing changes that any front disc conversion always requires: dual master, prop valve, etc.

2. The 65-66 drum hub will NOT fit inside the rotor hat until you lathe turn roughly .100" off the hub outer diameter. There is plenty of meat there so it's no concern.

3. The Ram rotors are 5 on 130mm [5.118"] lug pattern. That is so close to 5 on 5.000" that the rotor will slide on (since the Ram rotor lug bolt holes are .700" diameter) but the lug bolts won't be centered in the holes. So obviously it is best if you have the rotors professionally redrilled for 5.000". Another reason to do so is....

4. The B body drum hub has a 3.050" register, the Ram rotor has a 3.150" register hole. So the rotor will not be axially centered that way, but it could be centered by carefully matching the reduced drum hub outer diameter to the Ram rotor hat inner diameter, since it is fully machined there. Or ensure your newly drilled lug bolt pattern is axially centered and very close match to lug bolt diameter.

5. The .275" thick disc rotor may require slightly longer lug bolts. And your front tires will gain about .200" distance between them. Wide-Track!

Pictures show lathe-turned 66 drum hub on 66 drum spindle, misalignment of 5 on 5" lug bolts on 5 on 130mm Ram rotor, 69 caliper bracket on 66 drum spindle, and all of it assembled. Brake pad contact and caliper postioning is perfect.

[AROWHED]

I came up with this by painstakingly reviewing every one of these drawings (link below) trying to find a rotor that matched the basic dimensions of a 67-70 disc rotor but also would have an inner diameter large enough for the 65-66 drum hub to fit inside it. Unfortunately I could not find one that had a 5 on 5" lug pattern, a 3.050 register hole and was roughly 2.90" in height. The Ram rotor came closest but its lug pattern is wrong and it requires lathe turning the drum hub.

If anyone is aware of a better rotor choice, the basic dimensions to match are: Outer diameter of 296-300 mm, Height of 74-76mm, 3.05" register, and has a 5 x 5.00" lug pattern. Friction surface to be somewhere around 1.2" thick, and vented, of course.
Additionally, an inner diameter of 6.4" would making lathe turning the drum hub unnecessary.

https://brakeperformance.com/images/.../brake-rotors/

[AROWHED]

This rotor might work, it has a 5 x 127 [5.000"] lug pattern, and the 10mm [.393"] lug bolt holes can be easily drilled to 1/2" or more. The outer diameter is good, it's nearly an inch thick across the vented friction surfaces, and the drum hub might even fit inside it without machining; for that need a hat inside diameter of 6.40" min, that dimension is not shown.

But the 70mm height is about a .250" short, which will cause the caliper piston to be slightly out of its hole even with new pads, as opposed to fully seated with a correct height rotor, which needs to be around 2.90".

Finally the 101.6mm [4.000"] register is way too big, but the lug pattern can act to keep the rotor centered.

This site doesn't list applications, but I think this a front rotor for a GM 4WD truck, not sure year range.

https://brakeperformance.com/images/.../brake-rotors/

[hgerhardt]

Using the hub to center the rotor is the best way, since in the future you might get rotors with that inner hat diameter different than what these are.

Since you state there's a 0.100" hub diameter difference (3.050 vs 3.150), you could get a pair of stainless spacers made which are 0.050 thick. Make them a slight interference fit on the hub and a slip fit in the rotor. Obviously only as wide as the rotor hat thickness, so that they don't interfere with the wheel hub hole.

[stevep]

Great information! Thank you!
I know of someone who may be reproducing the 69-70 caliper brackets in the near future so you can use single piston calipers.

[455HOGT37]

I used the late 70’s Cadillac Seville rear rotors on my 63 Cat hubs. Took a little machining to take some OD off, but the 5x5 pattern slipped right on.

[AROWHED]

That does appear to be a good candidate, I linked it below. How did the 2.20" overall height and 11.16" diameter work out? Any pics? What parts make up the rest of the system...calipers, brackets, etc? I am still on the hunt for a rotor that would be "bolt on"...no machining necessary, but after looking at specs for hundreds of rotors so far, beginning to think its a part that doesn't exist.

https://duragoparts.com/catalog/part....aspx?part=211

[455HOGT37]

Calipers were the “metric” late model smaller footprint versions used in the late 80’s on GM. I fabricated brackets from flat 3/8 aluminum plate. No forming or machining required aside from drilling a few holes. Brackets could be cut with water jet these days.

[AROWHED]

Ah I see now, you made custom brackets. I'm sticking with factory 67-70 bracket and matching D52 caliper, so that requires a rotor close to 11.75" diameter and preferably 2.50 - 2.9" in height.