Front brake caliper

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pwykes
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Front brake caliper

Post by pwykes » Wed Nov 19, 2014 6:53 pm

Good day all,
I have noticed that the front left hand side caliper as a bit of wear on the sliders and causing uneven brake pad wear, i see you can buy new for about £100 my question is this the bolts that connect the caliper look like they will be a dog to get out and i wanted to buy some replacements before starting the work in case i have to them drill it out, searched ebay and other suppliers to no avail, any ideas please.
Cheers Phil.

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mat_fenwick
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Re: Front brake caliper

Post by mat_fenwick » Thu Nov 20, 2014 7:09 am

If you mean the ones which connect it to the hub they are standard metric bolts, so available from any engineering supplier. Can't remember if they are M12 or M14 but measuring the head should tell you. The ones which connect the caliper halves are standard thread, but (if original) with a pentagonal socket; however you shouldn't need to undo these.
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pwykes
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Re: Front brake caliper

Post by pwykes » Thu Nov 20, 2014 9:26 am

Thanks Matt, I was'nt sure if they were shaped or made from special steel.

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Re: Front brake caliper

Post by citronut » Thu Nov 20, 2014 11:51 am

the ones holding the carrier to the hub have a 19mm spanner/sockect size,
and in all the years i have worked on and around BX's i have never had one of these shear/snap,

we are not talking the likes of the rear caliper bolts on the XANT's and C5's hear, as they are infamous for shearing/snapping off

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Re: Front brake caliper

Post by Dollywobbler » Sat Nov 22, 2014 1:06 pm

Seconded. I can't remember ever having to get brutal with front brakes on a BX. The 35mm hub nut, yes, but the caliper bolts always seem to undo. I do like to smack the heads with a hammer before starting.

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Re: Front brake caliper

Post by pwykes » Mon Nov 24, 2014 5:33 pm

I did manage to get the bolts (had to drill two to relieve pressure) out and i am going to replace them with Grade 10.9 bolts i presume that will be strong enough?, can anyone tell me the torque setting please.
Thanks Phil.

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mat_fenwick
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Re: Front brake caliper

Post by mat_fenwick » Mon Nov 24, 2014 11:17 pm

Would have thought that the originals would have been 8.8 grade, so 10.9 will be more than enough. Torque figures will be in the Haynes manual, although technically speaking will be slightly different with a different grade of steel...

TBH I've never torqued them up to a figure, just relied on feel.
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Tinkley
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Re: Front brake caliper

Post by Tinkley » Tue Nov 25, 2014 6:59 am

You should be careful changing the steel grade. The 10.9 are 'stronger' in direct shear along the bolt so would allow say greater cylinder head compression BUT because the steel is more brittle, less 'strong' across the bolt ie a transverse load. I would personally prefer an 8.8 grade because it will take more transverse repeated loads than a 10.9 and I believe this is exactly the kind of load the caliper mount bolts take. Effectively the 8.8 is a 'tougher' grade in this application, which is why it has been selected originally.

The 8.8 grade should be more widely available too. You should easily find the tensile strength tables on the web but do NOT fit 4.4 crap steel if you want to stop..... :wink:

I have known people change engine mounting bolts on m/cycles from 8.8 to 10.9 grade and they fractured from the vibration whereas the 8.8s (factory spec) worked fine - no failures. Certainly not the 12.9 extra high tensile strength ones which will be guaranteed to fail!.

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Re: Front brake caliper

Post by pwykes » Tue Nov 25, 2014 10:06 am

Thanks Tinkley that is a really good reply and i am with your way of thinking but the Haynes Manual say's tighten bolts to 94 foot pounds. The bolts in question are 1.5 pitch (metric fine) and not included
in the table below, the closest is M12x1.75 working backwards it seems to imply that grade 8.8 have a max torque setting of 65 foot pounds.
http://www.cncexpo.com/MetricBoltTorque.aspx

cheers Phil.

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Re: Front brake caliper

Post by Tinkley » Tue Nov 25, 2014 11:49 am

I have before now known wrong torque figures in manuals. When the stud/bolt snaps prior to the figure, you know something is wrong. Like Mat says, you can do a lot with 'feel' and the correct good ring spanner. In fact with experience I'd bet most of us could get within a gnat's of the specified figures.

Yup, M12 ISO std coarse 1.75, fine 1.5 and ultra fine will be 1.25 pitch. More common on Euro stuff is M10 by 1.25 (fine) and the Japs tend to love the ultra fine threads more. Surprising really when in the somewhat mildly inferior recycled aluminium alloys they use.

I'd be surprised if the original Citroen/Pug bolts did not have the grade designation on the head though. Might even be worth a look at Bendix information. I can definitely say all my m/cycles have had 8.8 grade securing calipers to forks (opposed piston type) with both Brembo and Grimeca types, two pot and four.

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Re: Front brake caliper

Post by mat_fenwick » Tue Nov 25, 2014 12:37 pm

I'd have to disagree with the bit about shear strength for 10.9 grade bolts I'm afraid, I'll give my reasoning later and you can see whether I make sense! Remember that the friction between the two faces gives the resistance to shear, not the bolts themselves.
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pwykes
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Re: Front brake caliper

Post by pwykes » Tue Nov 25, 2014 1:59 pm

This is backing up the softer bolt theory but recommends a torque wrench setting of only 33 ft lbs for caliper to hub fixing, (please let me know if I am doing your heads in with this thread--mines buzzing) http://wiki.seloc.org/a/Torque_settings

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Re: Front brake caliper

Post by Defender110 » Tue Nov 25, 2014 2:45 pm

As Mat and Tinkley have implied I too think torqueing all bolts down with a torque wrench is way OTT. I have never torqued a brake caliper in my life and never had one come loose or bolt snap. Torqueing cylinder heads to ensure flat and prevent warping yes but the 2 bolts that hold the caliper just need to be correctly tightened. Obviously if you never work on cars and have never done much diy mechanicals then I suppose torque figures may be useful but 'anti roll bar caps' ..please!
The chart would indicate that HT bolts can be torqued down to a higher setting.
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Re: Front brake caliper

Post by Tinkley » Tue Nov 25, 2014 4:58 pm

That's OK Mat. If the faces are totally flat and the bolts take all the sideways shear, I'd most likely prefer the 8.8, application specific. If there are other faces that take the sideways shear ie joggled then I'd agree you could use the 10.9s' without a problem. As it happens all the m/cycle calipers have been flat faced on the joint and used plated standard ISO coarse bolts 8.8 grade. If I remember right M8 for the two caliper halves to each other and M10 for the caliper to fork mount(s). Obviously they were smooth along the alignment holes until the thread was needed to minimise load on the thread as a stress raiser and for better concentric alignment.

One thing often overlooked is that the 8.8s' tend to resist corrosion slightly better than the higher tensile grade steels on bolts. Worth bearing in mind, even if they are plated.

BTW I believe it may be possible to purchase NOS caliper mount bolts anyway, I certainly saw some for some similar age (to BX) Pug brakes on fleabay.

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Re: Front brake caliper

Post by mat_fenwick » Tue Nov 25, 2014 9:06 pm

Right, here is my logic/understanding - feel free to pick holes in it if you disagree.

The ultimate tensile strength (where they will snap in tension) of the two different types is 1000 and 800 MPa respectively; shear strength across the bolt is roughly 60% of this so the 10.9 grade is stronger in both shear and tension.

The yield strength is 900 and 640 MPa respectively, meaning that you you can put a higher clamping load through the joint with a 10.9 grade, before the material starts to yield (i.e. stretch). This means that the joint tension can be significantly higher than the normal applied load, so the joint rarely if ever sees any cyclic change in stress, and hence has a much higher fatigue life. Note that the joint can still be flat i.e. not stepped, and still rely on friction to resist movement - if it purely relied on the bolts they would need to be a very close fit otherwise the joint would move.

With 10.9s, the difference between the yield and UTS is less, so once you start stretching the material, you don't have as much of a 'window' before it will break, hence 8.8 bolts are tougher. But the point at which the 10.9 material starts to stretch is significantly higher.

As for corrosion resistance, you shouldn't plate 12.9 grade at all, and even plating 10.9 bolts is debatable - due to the risk of hydrogen embrittlement from the plating process.
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1993 1.9 TZD Turbo Estate
1996 3.9 V8 Discovery
1993 VW LT35 campervan
1985 Hyundai Stellar V8
2016 Hyundai iLoad