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Old 08-23-2014, 07:00 AM   #1
Sabre170 OP
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Brake Disk Bolts

I'm in the process of upgrading to dual disks. I'm adding a second disk to my spoke style wheel on my '78 100/7 (40mm ATE Calipers).

I'm making the assumption that when I add the second disk, the 5 bolts that connect the disk to the wheel will no longer be long enough. I'm pretty sure the "correct" bolt I would need is 8mmx85mm. For the sake of ease and saving some cash, Is there a reason that I can't just go to Lowes and get longer bolts there?

Due to the fact that we Americans are still plagued with using SAE hardware, my calculations come to see that a 5/16" by 3.5" bolt would be a pretty close fit. Lowes carries this one.

Does anyone see a reason I can't do this???
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Old 08-23-2014, 08:38 AM   #2
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The OEM bolts are stronger. Not sure of the rating on them but I do think they have to be strong. The coating on the OEM part is pretty good. They don't rust after many years. That bolt you are thinking of using has no finish to speak of it will rust almost immediately. You could spray them with Galvanizing compound but that would eat up almost the entire ten dollars you saved?
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Old 08-23-2014, 08:44 AM   #3
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Rust is one reason. Strength is another.

A third reason would be every mechanic who has to work on the machine cursing your ancestors and descendants when the metric tools they have don't fit and they have to go scrounging for inch allens and spanners.

For me, the biggest reason would not be skimping on brake parts - the most dangerous place to try and save a few dollars.
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nothing screwed with this post 08-23-2014 at 10:17 AM
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Old 08-23-2014, 08:48 AM   #4
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Good points fellas.

I was figuring as far as strength goes I have no doubt in my mind those would be strong enough.....however, I didn't consider the rust factor. That would be an issue for all sorts of reasons.

I'll just get me some new bolts then.

Can someone confirm that going from 1 to 2 disks will in fact force me to need longer bolts?
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Old 08-23-2014, 09:14 AM   #5
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Quote:
Originally Posted by Sabre170 View Post
Can someone confirm that going from 1 to 2 disks will in fact force me to need longer bolts?

The fiche lists 85mm bolts for both single and dual disks. How long are your current bolts?

Source.
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Old 08-23-2014, 09:29 AM   #6
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I saw that on the fiche as well, but thought it to be an error. I don't know off hand the size of my current bolts....I'll have to take on off and measure it.
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Old 08-23-2014, 09:50 AM   #7
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I think the bolts are the same length. The second disc will be fitted in place of the plate which is fitted opposite the single disc if it is anything like my set-up. Looks like a straight replacement to me...
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Old 08-23-2014, 10:14 AM   #8
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Hucky lists 80mm for dual disc setups: http://www.bmwhucky.com/010034.html
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Old 08-23-2014, 04:41 PM   #9
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The grading on my twin disc bolts is 8.8
Structural grade.

Any decent bolt supply will have metric and imperial.
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Old 08-23-2014, 10:15 PM   #10
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You'll need longer bolts. I'll bet the fiche specs the longer ones for both single and dual. The stock single discs bolts are 'necked down' part of their length (special feature to save unsprung weight?) - the longer ones aren't.

I used metric stainless allen head bolts from the hardware store and they worked fine - but I go easy on brakes. Can't say how they'd hold up in rough duty.
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Old 08-24-2014, 01:24 AM   #11
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When I converted my monolever to twin discs I used the same bolts, there was a spacer instead of the second disc that I removed, probably not the same as yours though!
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Old 08-24-2014, 09:10 AM   #12
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So, it seems like the general consensus is that I probably would be just fine using a strong/normal bolt, but at the same stance.....also probably not worth the few bucks saved in doing it.

I'll just be using the BMW bolts myself.
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Old 08-24-2014, 09:14 AM   #13
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Stainless bolts are available and sometimes used (by me, for instance). I would figure they would come in as stronger than grade 2, but under 8.8 FWIW.
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Old 08-24-2014, 10:02 AM   #14
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I am pretty sure you already have the bolts you need since single or dual take the same bolt for the spacer. Bolts are necked down for strength. Our rod bolts, for example.
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Old 08-24-2014, 06:44 PM   #15
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On a /7, the original bolts are 75mm long. A dual disc arrangement requires an 85mm long bolt.

I've used stainless bolts to retain the brake rotors on a number of bikes, in spite of some reservations. So far, no problems.
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