Replacing brake rotors

[greatdane]

Question... I'm looking at buying new front rotors for my 84 GMC. There are two different sizes coming up on the website I'm looking at... 4.95" or 4.65" overall height? How do you measure this? I havent pulled a wheel yet was just wondering? It's a 1/2 ton swb that originally had a 305/350 combo in it. I am assuming from the center of the spindel to the top of the rotor???

[Irish_Alley]

5" overall. i dont think so thats way too little. if i understand it right the jb system is for 1/2 ton gas engine the jd is for diesel engines. now the 3 or 5 depends of the gvwr 3 being the smaller and 5 being the larger. being only a c10 with the 305.i would lean on it being only a jb3 system which mean its the basic of the two so the smaller size would be the one for you

[Irish_Alley]

heres some more info
JB3 : BRAKE, POWER, DISC/DRUM, 4600 LBS
JB5 : BRAKE, POWER, DISC/DRUM, 6400 LBS

JD3 : BRAKE, HYD POWER, DISC/DRUM
JD5 : BRAKE, DUAL POWER, DISC/DRUM, 6400 LBS

[bd]

JB3 uses a 1" thick rotor.
JB5 uses a 1.25" thick rotor.
As Irish said, JB3 is the base model brake system for a C10.

What website are you searching?  4.65" vs. 4.95" overall height doesn't make sense, out of context.  Are you looking for stock replacements or performance rotors?

[greatdane]

Yes stock replacement. I was on O'Reillys site. There are two different sizes listed.

[Irish_Alley]

oh they mean the depth of the hub the 4.95 is the hd one

[greatdane]

oh they mean the depth of the hub the 4.95 is the hd one

That's kinda what I figured but wasn't sure. Should be the 4.65 one on my 1/2 ton.

[Irish_Alley]

i do think so. the 4.96 look thicker

[nlauffer]

You should just pull them off and measure.  I made the mistake of assuming my 83 SWB 1/2 ton 2wd with 305 had the smaller JB3 brakes.  Turns out they are the JB5.  My truck is missing the build tag, so it was trial and error.  Meaning I tried and the error was, I tried on a weekend night after everything was closed.

[nlauffer]

Another thing to think about when replacing rotors.  I used to work at a Napa.  The rotors need to be stored laying flat and not upright.  It can cause them to warp.  don't skimp on your brakes.  It would also be a good idea to let a knowledgeable brake shop run the new rotors on a brake lathe to make sure they run true.

[Irish_Alley]

just one problem with what you said. about rotors need to be stored laying flat. the only thing is if there was proof of this being true the rotor manufacture would have this written all over the box. and the brake lathe just a little over kill but if you have the extra money to find rotors that have been stored on its back and never sat on its side during transit or storage and to get new rotors run true then hey go for it. but besides that yes we are pretty much guessing on what style brake you have. but just look at the vent gap on your rotors and compare them to the new ones

[Captkaos]

pull the wheel off and measure if you don't have the RPO sheet.  If it is the HD set, it will be obvious when you measure it.

[78 Chevyrado]

just one problem with what you said. about rotors need to be stored laying flat. the only thing is if there was proof of this being true the rotor manufacture would have this written all over the box. and the brake lathe just a little over kill but if you have the extra money to find rotors that have been stored on its back and never sat on its side during transit or storage and to get new rotors run true then hey go for it. but besides that yes we are pretty much guessing on what style brake you have. but just look at the vent gap on your rotors and compare them to the new ones

Thick as they are, i doubt gravity will bend them no matter how you store them.  This sounds like a fan clutch argument...


never seen depth of the hub measurments, only rotor thickness and shoe length....

[Irish_Alley]

yeah if you search O'Reillys they give every spec on the rotors. but like you said i dont see why you would measure the depth when the dia is much easier, silly irish people making it much harder than it has to be lol

[greatdane]

I would of course measure them before I buy new ones. I was wondering what to measure lol... The rotors I have on the truck must be warped, truck shimmies when you put the brakes on, not out of control yet but getting there!