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1971 Opel GT
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Discussion Starter · #1 ·
In assembling new disc brake parts, I ran into a problem with the fresh pads being too thick to clear the new rotor. I already removed the thin (.030") steel shims glued to the backing plates. With out the calipers mounted, there is a few thou clearance and light drag, but when the calipers are mounted, there is rubbing when the bolts are snug, and pretty much locked when the bolts are torqued. It seems the easiest solution is thinner pads, Is it possible/advisable to put the pads on a milling machine and take .020" off each one? Given the small amount of driving the car will get, the pads will outlive me anyways.... Thanks.
 

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Über Genius
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Sounds like you got solid rotor pads and have vented rotors.
You don't need a milling machine but it won't hurt to take .020 off. A belt sander will work if you're careful.
 

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1971 Opel GT
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Discussion Starter · #3 ·
They are solid rotors, .500" thick. With the pads pressed back. the gap is just over .500". And there are no brake lines attached yet. I was thinking a thickness sander for lumber would work good too, but I don't have one of them either.
 

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Just Some Dude in Jersey
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Grinding down your pads sounds like a horrible idea!

In my experience, all disc brakes rub a little, especially when new. AFAIK, there are no springs to push/pull the pistons back in, the slight fluttering of the rotor pushes them back in when you take your foot off the brakes.

You may recall that I had an issue with the new aluminum caliper vented rotor big brake package. The calipers had nubs sticking out that rubbed against the rotors. I cut them off and all is well. I see that they offer the same kit with the aluminum calipers, but in the non-vented rotor style(see below). The caliper should be the same as mine, but without the expansion block in between the 2 halves of the caliper. You might have those nubs also and they may be what is binding the rotors. Is this the kit you bought?:

 

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Über Genius
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Because it would be very difficult, without some kind of a jig, to shave (or sand) them down PERFECTLY even. Even a few thousandths could cause some binding.
As cheap as pads are, it isn't worth the risk.
 

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Because it would be very difficult, without some kind of a jig, to shave (or sand) them down PERFECTLY even. Even a few thousandths could cause some binding.
As cheap as pads are, it isn't worth the risk.
Without being too rude, there's a difference between experience and theory.
In my experience, brake pads are going to settle, not bind.

You'd have to have some extreme angle to get a pad to bind.
 
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Super Moderator
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They are solid rotors, .500" thick.
What brake system do you believe you have, and where did you get them?

Stock Opel GT (and Kadett's and Manta/Ascona until 1974) rotors are 0.430" thick new, minimum 0.394", and are 238 mm diameter.

The "Big Brakes" from the '75 Manta/Ascona (same solid-rotor calipers from the first generation BMW 320i) and more recently sold by OGTS and others have rotors that are 0.500" thick new, minimum 0.465", and are 246 mm diameter.

The bigger rotors and early calipers are NOT compatible, nor vice versa. While the calipers might seem similar, the early pads are ~square and quite a bit smaller than the '75 pads, which are more rectangular.

A photo of the pads and of the rotor thickness will tell for sure.
 

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The other (obvious) thing is…are the caliper pistons fully retracted?

Doesn’t take much to cause an issue as you’ve described.
 

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Just Some Dude in Jersey
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Jay has the latest, newest, brake package with newly made aluminum calipers. His calipers also have the "nubs" sticking out, mentioned in one of my threads, that Gil and I agreed could be cut off if they cause a problem. Jay's kit is the normal width brake kit, not the wider vented rotor kit. Both the vented rotor and the normal solid rotor brake kits use the same aluminum calipers with the nubs, but the vented rotor kit has an expansion block between the two halves of the caliper to make them wider.

However, Jay has told me that his nubs are not rubbing against his rotors, so that's not his problem. I think he said or implied that his brake system is not hooked up to any lines and have no fluid in them and, therefore, there shouldn't be a problem with his pistons not being fully retractable. His problem remains to be determined. I suggest he take a micrometer to his pads and his rotors and then talk with Gil.

Pictures of the nubs I mentioned:

Wheel Automotive tire Tire Automotive design Motor vehicle


Automotive tire Bumper Office equipment Gas Electronic device


Automotive lighting Automotive tire Audio equipment Gadget Automotive design


Automotive lighting Revolver Automotive design Motor vehicle Eyewear


Automotive tire Tread Stairs Automotive design Rim


Wheel Automotive tire Automotive lighting Tire Automotive design
 

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If there is no binding when the calipers are not bolted and binding when they are it means the mounting surface on the car is not flat and you are bending the caliper due to the force exerted on them by the bolts.
 

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1971 Opel GT
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Discussion Starter · #13 ·
I am continuing to track down the cause of the problem. Today I found that although there appeared to be a gap between the rotor and the 4 nubs, I could not get a .002" feeler gauge in. So I ground off the 4 nubs (as mentioned by SciFi Guy) and refit the caliper. It still locks up when fully torqued, so the nubs are not the entire problem. I also noticed that I could get a putty knife between the outside pad and the rotor, and as I torqued down the caliper, there became more clearance. Meaning the back side gets tighter, as is logical if you look at the assembly. I then sprayed a stripe of paint across the faces and edge of the rotor and reassembled it again. As it is tightened, the inside pad does rub hard on the rotor and eventually it does lock.
So it looks like the next step will be to remove .020" from the inner pad and try again...
 

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Just Some Dude in Jersey
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Hmmmm.......on mine it's the outer pads that are tight with no gap and the inner ones with plenty of clearance.........on one hub assembly. On the other hub assembly it's closer to 80/20. The opposite of your problem. YOU could insert thin washer shims between your steering knuckle and your calipers to fix your problem. I can't. I would have to file the bolting surface on the caliper or the knuckle to achieve perfection.

Shimming calipers is a common thing. I remember hearing a bunch of years ago about guys needing to shim these BMW-style-copy calipers when they installed them on their cars. I didn't need to on my red car. I've also heard of guys, who did other brake conversions, needing to do some shimming.

Someone also said recently, I think it was KillerTexasGT, that when replacing hub bearings it's good to drive them a short bit to get them fully seated and then redo the hub bearing adjustment, usually a bit tighter. This would not be a good thing for Jay, as it would cause the rotor to go even more inwards and press even harder on the inner brake pads. I don't know if Jay installed new axle bearings, but I did, and it would actually benefit my situation if my rotors shifted inwards slightly.

So, there's a couple of ways to go for Jay:
Shim the problem away
Grind brake pads
Wait until his system is all juiced up, bled, and the car running, then drive it around the neighborhood at low speed using the brakes frequently, then reinvestigate the pad gap issue and see if it has gone away.

Me? I'm going to wait until I'm up and running. I'll be doing short rides around town anyway at first. Then I'll check out how things look.....
 

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1971 Opel GT
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Discussion Starter · #15 ·
Thanks. I think I will make some .020" shims and see what the effect is. I thought about that but did not know if that was acceptable. The other side may be different. With the effect of manufacturing tolerances and the age/history of the various components, you can't expect perfection. I should also check the rotor runout for curiosity.
 

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1971 Opel GT
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Discussion Starter · #16 ·
I have found the problem!! The 2 bolts that attach the calipers are hitting the back side of the rotor during the last 1/4 turn of the bolts. They are the bolts and lock washers that were on the car. The drawings I see do not show any flat washers, but putting one on eliminated the problem. These bolts are much longer than they need to be. Originally I had observed that, but I got side tracked on other suspects. The ears on the new calipers seem to be slightly thinner, which would contribute to this. I plan to grind off about 030" from the end of the mounting bolts, and move on to other issues. Thanks for all the suggestions.
 

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I have found the problem!! The 2 bolts that attach the calipers are hitting the back side of the rotor during the last 1/4 turn of the bolts. They are the bolts and lock washers that were on the car. The drawings I see do not show any flat washers, but putting one on eliminated the problem. These bolts are much longer than they need to be. Originally I had observed that, but I got side tracked on other suspects. The ears on the new calipers seem to be slightly thinner, which would contribute to this. I plan to grind off about 030" from the end of the mounting bolts, and move on to other issues. Thanks for all the suggestions.
I had the same problem when I installed my BMW/ATE Big Brake calipers. I blamed it on these specific caliper mounts being thinner than the stock GT calipers, but it seems it is common to even the new OGTS calipers. But I knew as soon as I mounted the calipers and before I installed the pads, as the rotors were stuck as soon as I tightened the bolts. Actually made a small dent in the rotor.
 
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Just Some Dude in Jersey
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I wanted to make an addendum to my post saying that grinding down your brake pads to make them fit "sounds like a horrible idea". To which FirstOpel replied:

What makes you say that?
I respect FirstOpel a lot, but I thought he was crazy for disagreeing with me.

Well, this past week I finally finished my front suspension assembly and had to deal with my own problem of the brake pads not fitting into my vented rotor calipers. On one caliper I could slip one pad in easily, but couldn't even get the other one started. On my other caliper I couldn't get either of the pads to go in easily, although I probably could have tapped them in, but then they would lock up the rotor for sure. I did all the checks and made sure my caliper bolting surfaces were flat and free of rust, paint, etc. and I checked to make sure the mounting bolts weren't going through so far that they hit the rotor. I loosened the bleeders to make sure the calipers weren't somehow holding air pressure and stopping the pistons from going in all the way. My rotors were perfectly positioned and running straight and true down the center of the caliper.

Everything seemed okay and I was left with one inescapable conclusion: Some of the pads must be a tad too thick. I used to work for a machine shop that made the molds for Bendix brake pads and I would deliver 100 molds at a time once a month to their factory. They destroyed molds like crazy and they got warped and F'd up with regularity in the manufacturing process, plus brake pad material is extremely abrasive and all sorts of precise tolerances of the molds would get worn out. So, the possibility that the pads were an inaccurate thickness was a very real possibility. My comment that grinding them "sounds like a horrible idea" was due to me thinking that someone would take a bench top or angle grinder to them and do nasty sloppy grinding to them.

Well, I have a very nice bench top belt sander that I put various grits of metal grade paper on, with a nice 12" long flat area that is dead flat. It's so flat that you can put manifolds and other car parts that need to be milled dead flat on it and it does a fantastic job. So, even though I thought it was a horrible idea, I figured "Well, if I put a pad on the belt with LIGHT pressure for just 5 seconds and rotated it 180* halfway through, I couldn't possibly screw up the pad catastrophically." Just enough to start seeing some pad dust fly for a second or two. It worked! They all slid in with just a slight bind to the rotor. Today I had to swap cars on my lift and the one I just put the suspension with the big brakes on needed to come off the lift and get pushed back and forth in my narrow driveway in a K park fashion to spin it around 180* and get pushed back in my garage so that it was now under my lift and my red car on the lift. The slight rub of the brake pads didn't slow me down at all, so I guess the tiny bit of sanding down of the pads I did was just enough.

So, in summation, if you find that your big brake pads won't fit, a SLIGHT bit of rubbing them back and forth on some coarse metal grade sandpaper on a VERY flat surface might be an idea to consider. No, I am not suggesting doing this, it's a horrible idea, I'm just saying to consider it.

:)
 

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1971 Opel GT
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Discussion Starter · #20 ·
My first engineering job was at a long defunct Raybestos plant that made brake and clutch linings. All of them were sent thru a sanding/grinding machine to get them to the proper thickness. They were purposefully made too thick so that the outer skin would be ground down and the various bits of metal wires would be properly exposed on the friction surface and to the thickness specified.
 
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