Chevy P30 Chassis 1973 brake rotor clearance.

I was somewhat surprised to find out that the maximum wear permitted on the 1.5" thick rotor was 85 thousandths and it was only .080" on my rotor. There was no way that I was going to get much more clearance by regrinding the rotor.

Of Interest?

You Betcha! Sounds like with some urging and persistence you're getting to the bottom of this thing and will end up with a proper job. That's great self satisfaction and getting to use the coach again. Even better when lives could be depending on it.

Good Work!

If it's of interest - the rotor I bought was wrong. It was about .3" too tall, and the rotor disk was 1.5" instead of 1.25" thick. I found a supplier who claimed to have the last 4 in existence for my 40 yr old antique RV, and priced them like gold - three times what NAPA sold almost the identical part for. After careful measurement, I found some through Amazon that have the identical dimensions, and are supposed to fit the Chevy 1 ton series. It was about $25 more than the rotor from NAPA ($145 instead of $120) and shipped free (not bad for 50 pounds of iron). Rockauto had the flexible brake hose on closeout for $10 each side. The local NAPA had the caliper for $18 (with $20 core). With luck, I'll get this back together next week.

The machine shop that claimed to be able to supply a rotor called their supplier and found that it was the same rotor being sold by NAPA - the wrong size.

It's pretty easy to turn a branch scraping on a window into The Boogie Man. This is what TEST DRIVES are all about. Low speed testing, to INSURE brake balance is even not only left to right, but also to INSURE the brake proportioning valve (which has nothing to do with this job) has gone bad, un-noticed.

Rotors that have flanges TOO THIN are the big worry. The thinness causes flexing and permanent warping.

With that job that was done "under the palms" had those rotors NOT BEEN TURNED at the machine shop, those new pads WOULD NEVER HAD FIT. not even using a jackhammer. As it was with the pistons collapsed 100% the caliper pads had to be "urged" to slip into place.

I've been following along here still...

I'm more concerned now than before about the balance between left and right braking...

Would it make sense to pull the other wheel and check for a casting number on the rotor you replaced a few years ago? Maybe that would help you find the right part-also, I've found lots of parts are not what they're supposed to be buying stuff at both the dealer and chains-make sure you got the right part-Your rotor may have been boxed up late Friday!!!

j-d wrote:
By how much is it too thick?

I'll be able to check it tonight. It was hard to estimate and DW picked up the old rotor today from the shop for comparison.

MEX suggested having it turned to get it true anyhow.

It wasn't needed. At least, not for truing. I put a dial gauge on it and it was nearly perfect.

There was discussion about what made it thicker, the two "platters" that have the braking surface, or the "honeycomb" in between the platters? If there's more "meat" in the platters (same honeycomb thickness as the one your're replacing) then cutting them down a little should turn out OK.

That's my thought, too. It's why I've got the old one back - to compare more closely. The honeycomb looked the same - the platters looked thicker, but I'll be able to measure accurately with the old one in my hands.

What about what I'll call "offset" where I mean how the rotor ends up inside the caliper gap? The offset I'm thinking of could measure from the bearing race to the center of the honeycomb.
I don't remember what the coach was, but somebody on this forum had to take a new rotor that otherwise fit but the Diameter was too big. Had the edge machined off of it, reducing diameter by a fraction of an inch. I don't remember if it was 1/8 or 1/4 or as much as 1/2 (inch) but everything else was OK and it worked after.

The diameter is fine, but the "offset" is something I wonder about, also. I did a rough measurement of offset as compared to the opposite side, but again, it will be easier to get this measured with the old one in my hands.

I hate to ask another question, but is the face of the braking surface wide enough (from edge to hub) for your pads?

Yes. I didn't think to check, but tonight I'll see if the old pads will fit from other side. I won't use them, but it should tell me a bit about offset, as well.

You could of course machine the Pads down, but that wouldn't be my first choice. For no other reason, all of a sudden you need new pads during a trip. You don't want to be crouched on a concrete sidewalk trying to work'em down.

It looked like I'd need about 1/16" off the pads or inside surface of the rotor. Eventually the rotor will wear down to fit, but ....

If you weren't able to get the exact replacement rotor, then I'd say take your worn one and new one to an automotive machine shop and ask if they can make the New the same as Old started out.
I also suppose the later model chassis had a different caliper, but trying to find and install that could be another can of worms.

Yes. I've got a good shop that could handle it. That's where the brake rotor was when i started this. He called to say it wouldn't meet spec any more with the grooves machined off, and I figured I'd grab one at the local NAPA. Since then I've had trouble finding one anywhere for the '73.

By how much is it too thick? MEX suggested having it turned to get it true anyhow. There was discussion about what made it thicker, the two "platters" that have the braking surface, or the "honeycomb" in between the platters? If there's more "meat" in the platters (same honeycomb thickness as the one your're replacing) then cutting them down a little should turn out OK.
What about what I'll call "offset" where I mean how the rotor ends up inside the caliper gap? The offset I'm thinking of could measure from the bearing race to the center of the honeycomb.
I don't remember what the coach was, but somebody on this forum had to take a new rotor that otherwise fit but the Diameter was too big. Had the edge machined off of it, reducing diameter by a fraction of an inch. I don't remember if it was 1/8 or 1/4 or as much as 1/2 (inch) but everything else was OK and it worked after.
I hate to ask another question, but is the face of the braking surface wide enough (from edge to hub) for your pads?
You could of course machine the Pads down, but that wouldn't be my first choice. For no other reason, all of a sudden you need new pads during a trip. You don't want to be crouched on a concrete sidewalk trying to work'em down.
If you weren't able to get the exact replacement rotor, then I'd say take your worn one and new one to an automotive machine shop and ask if they can make the New the same as Old started out.
I also suppose the later model chassis had a different caliper, but trying to find and install that could be another can of worms.

FWIW, The rotor is too thick for the brake pads to go on. I know - I SHOULD HAVE CHECKED THIS FIRST! - I measured about 15 dimensions on the old rotor before buying this one - knowing that I was buying one marked '75, not '73. All dimensions matched up, except rotor thickness, which I didn't bother to worry about since I knew the old one had been ground. I can't tell if this rotor is really different - made with a thicker rotor - or if it's just manufacturing tolerances. A lot of searching hasn't really cleared it up. I have a Chevy chassis and engine manual, so I'm going to hunt through that. The shop (that was going to do the rotor regrind) said they could find the right part with my VIN. I hate grinding off a lot of valuable rotor surface just to make it fit, and I particularly don't want to do it if this rotor is different in some other way.

if anyone has any info on the P30 motorhome front rotor - dual real wheels - for '73 vs '75, please feel free to post it.

If you only knew the tolerances on new parts, you'd gag. Rotor manufacturers EXPECT professionals and consumers alike to take new rotors to a machine shop and have them turned. High speed machines at the foundry put a rough turn on cast rotor friction surfaces. Only the bearing bores are (hopefully) finished turned.

Wanna know what those two rotors sounded like when a machine shop chucked them up and made the first pass?

"Scrape.......scrape......scrape......scrape......scrape"

Perhaps (I do not know this for sure) only rotors sold at new cars dealerships are "Plug & Play". This has probably changed as well. Buy anything, and the inch-thick paperwork consists of 3 pages in English and a dozen pages devoted to disclaimers.

"New" rotors must be turned. Period.

bucky wrote:
There is a reason the parts place needs to know what you are working on. A better employee or a different parts chain could have sold you the correct parts without all this screwing around. The lock washers were on there for a reason. If that part was the only choice I would have had them turned rather than compromise the hardware and perhaps your safety.

I checked another reference and it said the same rotor was used for '75 and '73. AFAICT, it's not the wrong part. I could have had the rotor reground to thinner, but I'm not really concerned about the removal of the lock washers. The loctite will be fine. I'll put the washers back on with the next pads (after I've worn off some of the rotor) and I'll occasionally check the dust shield to make sure it doesn't loosen.