Why don't my brake caliper+pads fit on the rotor?

1992 Chevy K2500 with JB6 RPO code.

So far I took my fully compressed caliper and put the brake pads in it. When I try to put the caliper over the rotor it simply does not fit, maybe by a mm. I'm not sure what the issue is, as far as I can tell all these parts should fit together. Please tell me where I'm wrong so I can buy a new part.

Brake Pads: https://www.centricparts.com/disc-brake-pad-set-6829078

Rotor: https://www.centricparts.com/disc-brake-rotor-121-66009

Calipers: https://www.centricparts.com/disc-brake-caliper-6824068

All parts are new/remanufactured.

Sorry I'm a noob.

Behemoth said:

1992 Chevy K2500 with JB6 RPO code.

So far I took my fully compressed caliper and put the brake pads in it. When I try to put the caliper over the rotor it simply does not fit, maybe by a mm. I'm not sure what the issue is, as far as I can tell all these parts should fit together. Please tell me where I'm wrong so I can buy a new part.

Brake Pads: https://www.centricparts.com/disc-brake-pad-set-6829078

Rotor: https://www.centricparts.com/disc-brake-rotor-121-66009

Calipers: https://www.centricparts.com/disc-brake-caliper-6824068

All parts are new/remanufactured.

Sorry I'm a noob.

Click to expand...

I checked all your parts using your links, and they look OK upon casual observation; I then compared the rotors and pads to others on RockAuto, and the dimensional specs jibe:

Rotor 32.65mm width
Pad: 15.7mm (outer) and 16.5mm (inner)

Have you tried to compress the piston into the caliper bore, to make certain it's fully seated? You said it's "fully compressed" so it sounds like you did.

The pad thickness, rotor and caliper gap specs are given on the links you provided. You might confirm these specs by measuring yourself.

Evidently you've got a 6-lug K2500.

i'd make sure the caliper is compressed completely. i usually use a big pair of channel locks and it can get the piston below the surface of the caliper as mentioned

Have you tried to compress the piston into the caliper bore

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I used some large tongue-and-groove pliers to try and compress it. It didn't budge but they're new calipers.

I'll make sure to confirm the measurements, and yes it's a 6-lug K2500, thanks for adding that.

I want photos (several, different angles) of the pads as installed in the caliper.

In focus, nicely-cropped please.

Any chance the piston won't compress any farther because the master cylinder reservoir is now totally full?

Schurkey said:

Any chance the piston won't compress any farther because the master cylinder reservoir is now totally full?

Click to expand...

I was going to ask the same question......

glock20 said:

i'd make sure the caliper is compressed completely. i usually use a big pair of channel locks and it can get the piston below the surface of the caliper as mentioned

Click to expand...

I use a C clamp and a piece of wood, and I was also wondering if the master cylinder was full. OP will need to take some fluid out of the MC if that's the case. A baster will work, but don't use the one your significant other cooks with! They're not that expensive at Walmart....
Brake fluid will remove paint so you want to be careful where it goes, and not spill it.
The piston in the caliper should compress all the way into its housing, and the pad mounting surface should be flat. Also be sure the retainer clips on the pads are properly inserted into the back of the caliper piston; if not, the pads won't fit correctly.

Yes use a c clamp and stick the old pad back in while you compress it. It will take some force.

Sean Buick 76 said:

Yes use a c clamp and stick the old pad back in while you compress it. It will take some force.

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"Some" force? Yes.

A new or freshly-rebuilt caliper should take so little force that the C-clamp isn't really needed--the piston should slide back in by hand. In fact, they ought to come out-of-the-box with the piston(s) fully retracted.

Old, used calipers often take much more force, because the piston(s) are partially-seized on the square-cut seal(s).

High-mileage caliper from Treasure Yard, with remnants of horrible, crappy brake fluid, semi-seized piston, semi-seized caliper mounting sleeves.


Square-cut seal with "crust" partially-removed by scraping with fingernail.


Caliper piston and bore were cleaned-up, seals and O-rings cleaned and lubed, bleeder screw cleaned and lubed. Caliper mounting sleeves cleaned, slide back and forth with a finger on either side. New brake hose attached. Once the piston (and seals) are lubed with brake fluid and the piston is started in the dust seal and square-cut seal, it pushes the rest of the way in with two fingers.


Original pads used temporarily to scour rust off of Treasure Yard rotors. After several stops, fresh pads were installed.

Good info Shurkey