I've recently aquired some crossdrilled rotors from a guy here on the forum. He said they were for his 96 with the 13 inch brakes. He also said he bought them from a forum Vendor.

I mounted them on my 89 with the J55 brake option. The fronts fit fine but the problem is the rear. The new rotors appear smaller than my old rotors. I'm only getting bite on half the rotor. I have not put them up to each other side by side yet.

Two questions:
I'm wondering are the J55 rear rotors also bigger than the standard brakes?
Is there an adjustment, or could I have installed the rears wrong? I removed the entire caliper assembly and frame without taking it apart.

Here are some pics of the old rotors and the new.

 

It seems that the normal rears are 11.5 and the J55s got 12 inch rears. I don't think there's any adjustment either. Edit: I just checked: the 88-89 had different size rear brakes depending on if it had the J55 or not. The post-89 cars all had the same rear brakes.

 

It does appear that I'm missing a 1/2" of rotor. So all cars after 88-89 have 11.5 rotors?

 

No, all cars after 89 have 12 inch rear rotors. It was only 88-89 that the J55 gave you larger rear brakes. After 89 they all had the larger rear brakes. And before 88 there was no J55 option, they all had 11.5 inch rotors all around.

 

So the rotors should have been the correct size from the vendor. Hmm, I'm going to see what I can do to get the right size. I don't know how much luck I'm going to have since I bought these second hand. I sure hope the guy I bought them from didn't sell them to me knowing this.

 

To the best of my knowledge the J55 brakes or upgrade as its called only pertains to the front brakes. The rear brakes should stay stock or 12 inch rotors. You should not be changing the rotors on your rears with the stock caliper.

I have asked this question myself and have read other posts in regard to this and have never seen information on changing the rear brake system.

My rear 12’’ brakes on my 94 wears the same as your 3rd picture (rotor) that you show. Never noticed it till it was pointed out to me and realized not all the rotor area is used. As I have said before, what were they thinking, the engineering stinks.

My 280ZX and Jeep have rotors (where pad rides) 1 ¾ inch wide and all the surface is used. The vette is 1 ½ inch wide and not all area used on the rears. This may answer some questions why these brakes are poor.

Best of luck

 

I know not all the rotor surface is used but if you look closer to the crossdrilled, only half the surface is being used. I'm pretty sure these are 11.5" rotors. They are too small. Guess I'll have to put the old rotors back on until I can pick up a new set of 12" crossdrilled.

 

The cheaper cross drilled rotors are actually worse than stock
and they warp very easily
I was considering this swap also and was talked out of it
Save your stock ones to put back on when the pedal starts pulsing

 

94ZR1 is right on. Here is a link (to support 94ZR1) with some really good info in regard to the J55 upgrade and drilled rotors.

http://forums.corvetteforum.com/show....php?t=1249961

 

So where could I get a set of good slotted rotors?

 

Now I've been told that rear rotors on all C4's were 12" and there were no 11.5" Is this true?

 

why does the cal. and pads looks to small ?

 

Someone has now suggested maybe I have a stuck or bent caliper bolt. Unfortunately, I didn't have a chance to get under there last night and work on it but it's driving me crazy. Anyone else get caught up in there car as much as me?

 

Well, I went home for lunch and realized I'm an idiot. I looked closely and only half the pad is touching which means I did something wrong when reinstalling it. Hopefully I'll have a chance tonight to remove the calipers, reset the pistons and try it all over again. Thanks for the help everyone.

 

YAIC4BB (*).

79 280ZX: curb wt 2900 lb, 60-0 160 ft (Motor Trend, June 79)

94 C4 LT1: curb wt 3400 lb, 60-0 123 ft (Road and Track, Feb. 94)

Jeep.


The C4 in it's day had some of the best brakes in the world.
The Z/ZX brakes were crappy even in their own time.
Jeep - did I mention ?


C4 = crappy engineering? You bet. But not in the brake department.


(* yet another ignorant C4 brake bitch)

 

Well, got one to work properly. The other is still only grabbing part of the rotor. Guess I'll have to take it apart again and clean it up a little more. Thanks again for the help.

 

Okay, I took the other one apart. Cleaned it all with brake cleaner. Put it back together and still, only part of the rotor is getting grabbed by the pad. Should I go buy new pads? This is the only thing I can think of will help now. I'm getting a little frustrated because I'd like to drive my car! I don't want to drive it though until I konw the brakes are back to normal! GRRRR!