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Rebuilding a caliper


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I originally made this post on LRE, but some people quite rightly expressed concerns over a couple of things, so I removed it. That same post is on LandRoverweb. Same pictures here, but the explanation is different due to what could be incorrect method. I rebuilt these using my own experience, and have done this same job a few times.

No one has died, or suffered brake problems, but you must be aware of the risks involved in working on a braking sytem.

Issues raised then were:-

Bolt tightness

Actually splitting the caliper halves.

The O-rings used.

I personally don't think the job was botched or dangerous in any way, I would never botch brakes. The calipers in the picture are on a 1990 2.5TD90. I drove the vehicle several times at high speeds after the rebuild, and am confident they are safe.

I'm proud of this thread, so with the necessary warnings, here it is.

After checking the brakes on the 90 the pistons were badly corroded, some of the top steel rings that retain the upper seal were missing and the seals were out of their seating. No leaks, but they needed overhauling.

Mr Haynes says not to split the calipers, but I don't see why not. It would be very difficult to do this job without splitting them.

First picture is how you see them on the vehicle.

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Remove the pad retaining pins and the pads themselves. I used an old pad backplate to get the pistons out as far as I could without popping them.

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Clamp the flexi hose, remove the short brake pipe that goes to the bottom of the caliper body (there'll be a small amount of fluid loss). Undo the two bolts that hold the caliper to the hub assembly and lift the caliper away. Remove the bleed nipple and most of the fluid can be drained out.

You can do this job without a vice, but it makes it a lot easier to do if you have one. Use soft jaws though.

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Undo the 4 x bolts that hold the caliper halves together and it'll seperate with the pistons still in thier respective bores. Note the O-ring that assists the seal between the halves. The pistons can then be removed fairly easily and the remaining brake fluid can be discarded.

Note:- I was unable to find the correct torque for the bolts that hold the caliper halves together so I undid them with a torque wrench. It took 55ft/lbs, so this is the figure I'll use on reassembly. This might not be right

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Tools I used to clean the bare castings were :-

A wire cup brush in an angle grinder.

A flat wire brush in an angle grinder.

One of those pointy wire brushes in an electric drill.

Cellulose thinners to de-grease and clean.

The castings come up surprisingly well considering how corroded they were. Don't use any abrasive inside the bores, just use cloths and something to remove the black gloop that's normally in there.

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Each piston has two rubber seals and a thin steel ring that holds the upper seal in place. It's very easy to wreck the rings when fitting them, so I bought two sets just in case. I did ruin a couple, but the kits are very cheap.

The two seals are different - the upper one is solid and the lower one has a groove around the inside edge.

I used a suitable socket to fit the steel ring and have found this to be the best method of fitting them.

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Two new pistons, these are after market items, but you can get stainless ones.

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The other half of the casting - just a clean as the other. I want to paint the caliper, so I rinsed the castings in cellulose thinners to degrease and remove dust. The mating faces and the bores themselves mustn't be painted, so a bit of careful masking.

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The paint I used is '5-Wheel silver' It withstands engine temperatures, it comes in aerosol cans, and one can will easily do the job (about a fiver)

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Lubricate the bores with fresh brake fluid and press the new pistons all the way in nice and square. There's some resistance, but it should feel smooth

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I could'nt get the O-rings that go between the two halves, so I used some I already had (used in guns If I remember). You could use the old ones, or at least make sure that the new ones are resistant to brake fluid. They are clamped very tight when the caliper is assembled.

The 4-bolts are tightened to 55 ft/lbs as mentioned at the start of this thread. I also used thread locking compound. Final picture is one of the calipers all finished, it looks as good as new almost.

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You must take great care when repairing any part of the braking system, especially when doing work not covered in workshop manuals, your life depends on it.

It was well over a year that I did this particular rebuild, and no problems at all so far. I'm happy with the job, but you should be aware of the 'best guess' theme.

For those of you that pointed out the faults in the original thread - I hope this edited thread is ok.

Les. :)

Les.

Edited by Les Henson
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Guest diesel_jim

And, if "we're" not supposed to split the callipers, then how do we fit the vented spacer kits to them?

If they wern't designed to be split then LR shouldn't have made them in 2 halves! :D

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A very useful article Les, thanks.

Perhaps someone has the contacts that would let them source the correct type of o ring or at least identify them. They could post that info on here and it could be added to the article ?

Mo

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Good stuff Les. I have always thought that the technical side of these foræ is the best. I am not going to take my calipers apart, but it is nice to know that I can do so if I want.

I reckon you should put this in the 'Technical Archive' no?

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I have a fair bit of experience designing bolted connections and solving problems with fatigue failures of bolts.

The following are my views on tightening torques for bolts subjected to cyclic tensile loads, such as those clamping the 2 caliper halves together. The principles can be found in good mechanical engineering texts.

Fatigue occurs in the bolts due to the fluctuation in the bolt tension under the loading conditions.

In a well designed bolted connection, the connected plies are much stiffer than the bolt (can be in the order of 10 times more stiff). The caliper in this post rates as a well designed bolted connection.

The following comments only apply to metal to metal joints. Gaskets, paint, dirt etc. between the joint surfaces will significantly reduce the stiffness of the joint.

When a bolt is pre-tensioned (or preload), the bolt stretches (elongates) and the joint plies are compressed. The amount of bolt stretch and joint compression, is proportional to the bolt pre-tension and the stiffness of the bolt and joint.

It can be best to think of the bolt and joint as springs, a stiff compression spring (the joint plies) will not compress as much as the more flexible tension spring (the bolt) is stretched when the bolt pre-tension is applied.

An external load applied to the bolted joint (such as hydraulic pressure on the brake caliper pistons) causes the length between the nut and bolt head to increase.

Now if the load is not high enough to cause the joint surfaces to separate, the increase in length of the joint plies is less than they were compressed by the bolt pre-tensioning.

The sum of the change in bolt load plus the change in the joint load is equal to the external load.

Because the joint is stiffer, the change in load of the joint is greater than the change in load of the bolt.

Then the fluctuation in the bolt load (that which leads to fatigue failure) can be much less than the external load.

Two good rules of thumb for designing bolted joints subject to cyclic tensile loads are:

1. The bolt pre-tension should exceed the external load - 3 times is good, or 5 times for a greater safety margin.

2. For joints that must be dissasembled and bolts re-used (common case for us), the bolt pre-tension should be 65 to 75% of the bolt proof load (approx the yield point proof load and varies with bolt grade or property class).

However, it is becoming more common for bolts to be pre-tensioned to the yield point (eg. cylinder head bolts), but these bolts should not be re-used.

A good rule of thumb for calculating the tightening torque is Torque = 0.2 x bolt dia x pre-tension.

If bolt dia is in mm and pre-tension is in kilo Newtons, torque will be Newton metres. If bolt dia is in inches and pre-tension is lbf, the touque will be lbf inch.

Note: there is no difference for fine or coase thread pitches. This is for lightly oiled threads. I have compared the results against many published recommended values for tightening torques and there is little error using a pre-tension of 65% proof load.

Prescribed tightening torque is not accurate enough for pre-tensioning bolts to the yield point because of the friction between the components.

In conclusion:

The fatigue strength of bolts depends upon the joint stiffness/design and the pre-tension.

For the same bolt, the load capacity for fatigue, increase if the pre-tension is increased (up to the yield point).

Slotted or oversize holes, warped faces, paint, gaskets or dirt between the faces all reduce the joint stiffness and the load capacity.

If the holes are slotted or oversize, the use of a thick, large dia washer will help to increase the joint stiffness and load capacity.

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:blink: So lets assume that thread locking fluid (while still liquid and not set) has the same effect as "lightly oiled threads", no oil or grease under the bolt head to give false torques, using 8.8 bolts as they are, what torque do you recommend? :huh:
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:blink: So lets assume that thread locking fluid (while still liquid and not set) has the same effect as "lightly oiled threads", no oil or grease under the bolt head to give false torques, using 8.8 bolts as they are, what torque do you recommend? :huh:

The following are handbook data. The bolt tension corresponds with 65% proof load.

For property class 8.8 bolts (proof load stress = 660 MPa):

Bolt, Tension, Torque

M5, 5.4 kN, 5 Nm

M6, 7.6 kN, 9 Nm

M8, 13.8 kN, 22 Nm

M10, 21.9 kN, 44 Nm

M12, 31.8 kN, 77 Nm

M16, 59.2 kN, 190 Nm

For propert class 10.9 bolts:

Bolt, Tension, Torque

M5, 7.67 kN, 8 Nm

M6, 10.86 kN, 13 Nm

M8, 19.76 kN, 32 Nm

M10, 31.27 kN, 63 Nm

M12, 45.5 kN, 109 Nm

M16, 84.5 kN, 270 Nm

The handbook did not have proof loads or torques for metric fine thread bolts, but comparing UNF to UNC bolts, the 65% proof loads and torques are approx 13% higher for the fine thread bolts.

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I can't see any problem spliting them as long as the person assembling them is compitant to do it (as you are).

I can't see anything wrong with this either. I could add that to avoid distortion, undo the bolts progressively one turn at a time, like when removing cylinder heads but I won't, since the seal relies on squeezing the hell out of the small o-ring and not wholly on the flat faces.

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Nice one Les, just like I have been doing it for 20 years, I never knew you were not supposed to split them so I always just did it. I have never had any problem with the calipers as a result.

Might I remind everyone that almost any liquid left in the system apart from brake fluid will B8gg*r things up. Also, red brake grease is made for the job of lubricating seals when fitting. It also helps to stop corrosion if lightly smeared on rubber and steel bits. Dont put it on the pads or discs though :-)

Geoff

PS I have never been able to get replacement "o" rings for between the calipers, I re-use the old ones, it seems OK if they are in good condition.

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I never understood the instruction not to split calipers.

It's not as if there's any sealant or anything in between.

Good emphasis on the health warnings - if you can't tighten bolts properly or fix easy stuff on your truck, don't try this!

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The following are handbook data. The bolt tension corresponds with 65% proof load.

For property class 8.8 bolts (proof load stress = 660 MPa):

Bolt, Tension, Torque

M5, 5.4 kN, 5 Nm

M6, 7.6 kN, 9 Nm

M8, 13.8 kN, 22 Nm

M10, 21.9 kN, 44 Nm

M12, 31.8 kN, 77 Nm

M16, 59.2 kN, 190 Nm

For propert class 10.9 bolts:

Bolt, Tension, Torque

M5, 7.67 kN, 8 Nm

M6, 10.86 kN, 13 Nm

M8, 19.76 kN, 32 Nm

M10, 31.27 kN, 63 Nm

M12, 45.5 kN, 109 Nm

M16, 84.5 kN, 270 Nm

The handbook did not have proof loads or torques for metric fine thread bolts, but comparing UNF to UNC bolts, the 65% proof loads and torques are approx 13% higher for the fine thread bolts.

and now with lb/ft cause I'm old fashioned like that ;)

For property class 8.8 bolts (proof load stress = 660 MPa):

Bolt Tension, Torque

M5 5.4 kN, 5 Nm 3.7 lb/ft

M6 7.6 kN 9 Nm 6.6 lb/ft

M8 13.8 kN, 22 Nm 16.2 lb/ft

M10 21.9 kN 44 Nm 32.5 lb/ft

M12 31.8 kN 77 Nm 56.8 lb/ft

M16 59.2 kN 190 Nm 140.1 lb/ft

For propert class 10.9 bolts:

Bolt, Tension, Torque

M5 7.67 kN 8 Nm 5.9 lb/ft

M6 10.86 kN 13 Nm 9.6 lb/ft

M8 19.76 kN 32 Nm 23.6 lb/ft

M10 31.27 kN 63 Nm 46.5 lb/ft

M12 45.5 kN 109 Nm 80.4 lb/ft

M16 84.5 kN 270 Nm 199.1 lb/ft

Seems I've been over-torqueing my 8.8 M8's, M10's and M12's :blink:

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Les,

I don't think there's any problem with your method. I had to do exactly the same thing with the calipers on our Camper van. I had no choice in spiltting the calipers and rebuilding them, as new replacement calipers are not available any more. Luckily the rebuild kit we bought came complete with new o rings. We've covered many miles since the rebuild, and not had any braking problems.

Dale.

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...

Seems I've been over-torqueing my 8.8 M8's, M10's and M12's :blink:

In most cases I would not worry about tensioning a little higher than those handbook values. They are for 65% proof load and another 10% won't hurt for the usual metal to metal joints.

In my experience most bolt failures are due to under tensioning, or loss of pre-load from loosening. I am not including severe overloads due to accidents or from some other failure.

Sometimes the applied loads exceed the design values (due to modifications etc). In these cases it is necessary to increase the bolt pre-load, or change the design if the loads are too high.

It is vital that the pre-load is greater than the applied bolt tensile load and the joint faces are always in tight metal to metal contact to avoid fatigue failure of bolts.

A problem with small dia (M12 and smaller) screws and bolts, is that they can fail due to torsional stress during tightening.

The torsional stress (from tightening) mostly dissapears afterwards, and is not considered accumulative with the direct tensile stress from the applied bolt load.

BTW, the ISO standard for nuts was revised (nut thickness), so that the bolt will nearly always (>90%) break before the thread strips. The Canadians on the committee did a lot of the research for this.

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