Front brake caliper

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Tinkley
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Re: Front brake caliper

Post by Tinkley » Wed Nov 26, 2014 6:21 am

That's fine Mat, but also related is the stress/strain/fatigue curve over time and cylical loading. I have a feeling the 'drop off' point will be a lot later for the 8.8s. The plating issue is important too as most calipers I have dealt with have been aluminium a pretty reactive material even when hard anodised in a salt water environment. Usually it is the bolt that has corroded and 'grown' making it hard to remove, after all the wretched salt seems to travel down the threads... :wink:

TBH I have never seen plated 12.9s and mostly that grade has been present in commercial production machinery such as injection mould machines etc far removed from outside atmosphere or at least inside a fully covered and lubricated enclosure ie and engine.

So it may be that the longer 'life' of the 8.8 combined with fine thread and plating ability gives a useful characteristic for the caliper application. I'm wary of very fine threads in aluminium, even hard grade as it can burr or get damaged too easily and then fail. Coarse and fine are OK. Steel is good for the ultra fine, even cast iron is a little too coarse grain wise for ultra fine threads.

I'd guess the upshot is you can use either grade but be especially aware of the plating on 10.9 plus maybe a 'shorter' working life. The old molybdenum steel alloy m/bike frames from the late 50s' used to wear out after 3-4 years.....as they hit the cycle end of life part of the stress/strain....and would then fracture!.

Here's some 'stuff' found on a fairly good (even if U.S.) site.

Metric bolts use their own numbering system as well, unmarked bolts are often used for light-duty automotive applications such as plastic retention or used with body retaining clips. Metric bolts have their own rating system based on Mega Pascals (MPas). A Mega Pascal is equivalents to about 145 PSI. Grade 8.8 bolts are manufactured from medium carbon steel that is quenched and tempered. Bolts (below 16 millimeters) have a yield strength of 640 MPa (92,800 PSI) and a tensile strength of 800 MPa (116,000 PSI). Heavier duty Grade 10.9 bolts are made of quenched and tempered alloy steel and have a yield strength of 940 MPA or 136,000 PSI and a tensile strength of 1040 MPa or 151,000 PSI. The premium metric bolts used in the most critical stress areas are Grade 12.9. They are also made of alloy steel that is quenched and tempered and have a yield strength of 1100 MPa or 160,000 PSI and a tensile strength of 1220 MPa or 177,000 PSI. Metrics will sometimes be made more identifiable by a red coloring, but finding the head mark on the bolt is the only way to be sure it is strong enough for your application.

Stainless steel bolts are usually made with a steel alloy that contains up to 19 percent chromium and 13 percent nickel. Standard thread stainless bolts are rated as 18-8 and have roughly the same yield and tensile strength as a Grade 5 bolt. Metric stainless bolts are rated as A-2 and are rated slightly under the standard metric 8.8 ratings.

When replacing nuts, washers or lock washers, it is essential that you use hardware of the same grade or higher to ensure the bolts can still meet the specified strengths for each grade. A Grade 8 bolt can be compromised when held in place with a grade 5 *US or nylon-insert style nut. When the original nut or bolt is missing, assume it is at least a Grade 5 *US or metric 8.8 when selecting replacement hardware.
- See more at: http://blog.hemmings.com/index.php/cate ... 0QKQH.dpuf

I also note that some companies are supplying stainless caliper bolts. Now A2 (304) is actually about 6.6 grade and A4 (316) a bit better. If that is strong enough? well, 10.9 is overkill. I also found references to 8.8 grade bolts relating to Bendix calipers for vehicles about the same age as, but not specifically the BX.

pwykes
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Re: Front brake caliper

Post by pwykes » Wed Nov 26, 2014 8:33 pm

Thanks Matt but i'am struggling a bit with that level of science, so I went back to basics -- setup one Citroen bolt alongside a standard 8.8 grade bolt in a vice I then drilled each one in turn for 10 seconds. I was able to penetrate twice as far into the 8.8 bolt than i could into the Citroen bolt, the shavings from the 8.8 bolt were clear and crisp whereas the shaving from the Citroen bolt more like the state of pepper dust.
My conclusion is that Citroen bolts are very hard indeed this is also backed by the 94 ft lbs torque applied when fixing.
Cheers Phil.

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mat_fenwick
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Re: Front brake caliper

Post by mat_fenwick » Wed Nov 26, 2014 9:33 pm

I'm sure that either grade will be OK, but I'm surprised they seem to be stronger than 8.8 grade.
Tinkley wrote:That's fine Mat, but also related is the stress/strain/fatigue curve over time and cylical loading. I have a feeling the 'drop off' point will be a lot later for the 8.8s.
Actually (and it's a very complicated subject), the crack initiation part of the fatigue life for steels increases with tensile strength, although the crack propagation part of the fatigue life is pretty much unaffected by the strength. Usually, the initiation of a crack is the dominant factor so stronger steels have a longer fatigue life. If a component is poorly designed i.e. with sharp edges, then cracks will initiate more easily, crack growth may be the dominant factor and hence there may be little or no advantage to a higher strength steel.

BUT...steel has a fatigue limit which is typically around 50% of the tensile strength - so if you stress it below this limit then the fatigue life is effectively indefinite. Plus, as I mentioned before, if the clamping load is more than the working load, the bolt will see no cyclic stress and hence fatigue isn't an issue. This link explains it better than I can!
http://www.hobson.com.au/files/hobson-u ... vol007.pdf

I couldn't remember if plating 10.9 bolts is a no-no, but a bit of internet research suggests it's a bit of a grey area:
http://www.anochrome.com/technical/hydr ... ittlement/

Sorry to digress, but steel is a little bit of a pet subject for me...
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Tinkley
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Re: Front brake caliper

Post by Tinkley » Wed Nov 26, 2014 10:33 pm

You will find different bolts may be graded 8.8 yet be disimilar for drilling if the method of their creation is different. I would expect that they can be made several different ways, hot forging, cold forging and rolling, each one giving a slightly different characteristic because of the crystal and atomic structure is affected differently in each mode. So drilling two bolts may easily give an empirical difference, they will be different if made differently, yet still have enough 'strength' for the designed job.

My pet hate is the cheap stainless stuff from the far east whose heads burr at the sight of a screwdriver... :twisted: How about all those lovely Pozis the Japs used on crankcases on their m/cycles in the 70s? Nightmare to remove.

You know those have been made cheaply and do NOT conform to most European standards for fasteners. Let us not forget that Peugeot is primarily a steel company so is very likely to produce all of its own fasteners to a decent quality too. Maybe now in these days of global shoppping things are different but back in the 80s as much of a BX that could be French was. I have found that the '91 cars have bits from Eastern Europe whereas my '88 almost all was Fabrique en France..... :wink:

The crack initiation stuff is very very important and of course nice V notches are exactly what screw threads are. Yes I am aware of the actual dimensions and tolerances of fasteners and classes of fit etc. Certain steels like body shells get away with up to 35% elongation before failure hence are still so favoured for impact resistance on crash tests. Fewer sharp edges to dice pedestrians etc.

One other little tit bit is that most metals have useful real 'strength' up to approx one third of their melting point temperature, after that there is a fall off in their mechanical properties. This may or may not matter on a lot of stuff but one has to account for this on some designs.

pwykes
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Re: Front brake caliper

Post by pwykes » Mon Dec 08, 2014 11:39 am

Only able to find one supplier of 10.9 ht bolts (margnor fasteners) Bapps had none, Fitted the bolts and took into account the feel factor has described by Malcolm I then inched upwards on the torque wrench and tightened the bolts. The feel felt right when I it 75ft ft lbs i had a distinct urge not to pass this point and go on to the 93 ft lbs as in the Haynes manual. Marked bolts with white paint and used thread lock. Hopefully with this forum we have reached a safe conclusion to the correct replacement of the Bolts.
Thanks Phil.