Rotor Differences - Also, anyone do the 11.75-inch swap?

patrick · Post by **patrick** » Mon Oct 30, 2006 7:33 pm

Is there any difference between '71/72 and '73/74 front disc rotors? If so, what? I'm asking because everyone (Year One, etc.) seems to list rotors up to '72, but no further.

The REAL reason I'm asking is because I am trying to do the Mopar Action big-brake swap and am running into snags (or I'm simply too "anal") so I'm thinking of just keeping the stock 10.87-inch rotors, but need a source.

I'll go ahead and tell you my problem anyway. The caliper adaptor brackets I have for the bigger rotors are about an inch thick. The OEM ones on my '73 RR are about 1/2-inch thick, and about 1/8-inch of that is a recessed area where an equally thick washer sits flushed. The new brackets do not have this recessed area for the washer to sit in. I presume the car that used these larger rotors had a longer bolt to make up the difference (approx. 1/2-inch longer for a total of 2 1/4-inch length?)?

Also, should I use a washer and if so, what type? I presume not a 1/8-inch thick one, or does it even matter? Also, does the 3/4-inch bolt head matter?

The OEM bolt measurements are:

3/4-inch bolt head
1 5/8" shank diameter
1 3/4" length (w/o bolt head)
2 inches TOTAL length (with bolt head)
non-removable 1/8-inch thick washer (approx. 1-inch in diameter)
TPI = 20

patrick · Post by **patrick** » Thu Nov 02, 2006 9:18 pm

Ugh, not even Lowe's had the bolt. I am gonna try Ace tomorrow, but it's a real mom 'n pop looking store where I live. Basically, just need it about half an inch longer. Sigh...

patrick · Post by **patrick** » Sat Nov 04, 2006 10:39 pm

I'll answer my own question: '73 and later rotors are all one piece and all use the same internals whether 10.87" or 11.75".

Found the information in the August 2004 issue of Mopar Muscle and confirmed by going to popular auto parts stores and plugging in different vehicles.

The bolts I needed were from NAPA, but I had to ask them at the counter as they did not have them in the bolts 'n nuts aisle: Grade 8, 1/2"-20 (TPI), 2.0" long (versus OEM 1.75") to accomodate slightly thicker caliper adaptor bracket.

See ya!

road chicken · Post by **road chicken** » Mon Nov 06, 2006 1:41 pm

The 71/2 rotors have the small bearings, because the 71/2 spindle is "small" as compared tot he 73/4 units,( which used the same "big" bearings as the "C" bodies did.. If you are swaping to a Big "C" disk on a 71/2 car- you MUST find a 73/4 challenger/cuda/ dart/ and I think duster front spindles that use the same bearing size as the "C" bodies. 73/4 SAT/Charger spindles will not work because there were other suspension changes to those years that make them incompatable with the 71/2 bodies.

If you are using the 71/2 spindles withthe bigger caliper brakets- that may be part of the problem your seeing.

patrick · Post by **patrick** » Mon Nov 06, 2006 6:15 pm

Okay, well, I'm merely swapping a '73 Road Runner to 11.75-inchers, so all internals (oil seals, races, and bearings) will be the same, and no spindle change needed, but very, very good information nonetheless.

The caliper adaptor brackets I acquired to accommodate the larger rotors IS thicker and is the slider-type, non-HD variety, but found slightly longer bolts that will work. I'll keep everyone posted! (I originally had post-type calipers, but needed new calipers anyway, so what the heck...)

patrick · Post by **patrick** » Sat Nov 11, 2006 2:35 pm

Well I made real progress and got the 11.75-inch rotors and calipers all installed. Man, does it look nice. I don' know if I'll miss the pin-type setup, but these are bigger and new, so it should be a big difference. Plus, no more broken, stripped wheel studs. Also picked up new brake lines. Just gotta bleed the brakes and I should be good to go!