Brake calipers.

That's a good one - better put that in your signature line.

I see no reason as tolerances are tolerances and how did the factory selectively fit disc brake pistons. There is a fair bit of tolerance in the caliper body anyway. I have not come across any oversize spare part pistons which you could buy. Unlike wheel cylinder seals which have over sizes.

rm

I guess it's one of the those instructions largely aimed at the uninitiated. Do you think that if the "instructions" are liberally peppered with this sort of thing it's enough to put most Saturday morning mechanics off?

That aside, I think common sense comes into play - it's a bit like making sure cylinder head bolts (if used again?) go back into the holes they were stressed in, but I doubt whether the car would mind if you mixed up the bolts holding the handbrake assembly.

Also, "if the pistons show marks of any kind even minor scratches they must be discarded". Hmmmm, little bit of dressing with wet n dry, eyeballing and plenty of cleaning with brake fluid seems to do the trick, even with a substantial "nick" in the metal. Mind you, new seals help!

(I hasten to add that last bit is gleaned from working on my own cars - I would never advocate "pragmatic mechanics" on brakes for other people ;-))

-- Ken Davidson DocDelete

overhauling

you fit new

A printing error IMO. Brake bore and piston clearances are fairly generous as fits go. But the machined sizes are quite accurate and consistant, so swapping pistons makes very little difference to individual piston bore clearance. Mike.

Depends if they're stainless or those accursed plated ones.

-- Die Gotterspammerung - Junkmail of the Gods

Good point - I've only come across ones that don't shed their finish when dressed up so I'm presuming they're s/s all the way through. Plated? I suppose it depends on how thick the plating is, though I guess the term "plating" implies a maximum thickness of only a few microns?

So, when scratched / marked / sanded they'll shed material?

-- Ken Davidson DocDelete

Once you get the smallest of rust pits, the rust travels beneath the surface and their reliable lifetime is minimal.

Try a Range Rover. You get a dozen of the damned things to keep an eye on 8-(

-- Die Gotterspammerung - Junkmail of the Gods

I doubt many Saturday morning mechanics would buy a factory manual, and in any case it's very definitely aimed at the pro workshop.

Crikey - I've never ever tried to keep cylinder head bolts in their original holes. I will usually measure them against a new one for stretching, though.

Mine are chrome plated, so scrap as they rust when nicked.

Hardly - an editorial one possibly.

Yes - the seal takes up the clearance. If it was a tight fit, any dirt would cause it to seize, and in the case of mine there are no dust covers.

My thoughts too.

On Wed, 19 Nov 2003 09:58:12 -0000, "DocDelete" wrote: ... Hmmmm, little bit of dressing with wet n dry, eyeballing and

ah, but for how long?

How about 8 years and counting! Must admit though the nick in the piston is at the "plunger" end so very unlikely to be called into contact with seal itself - just made sure to deburr it. Yes, it's a case of being sensible, using the noddle and not taking unnecessary risks.

-- Ken Davidson DocDelete

What the hell am I on? Plunger? = caliper....

-- Ken Davidson DocDelete

I would imagine for the same reason that you would put a piston of the engine back in the same pot if you were freshening up with a new set of rings for instance. So that any wear on the two components match.

MrCheerful

No not the same - different set of circumstances. Closer bore tolerance, piston on rod, wear pattern etc just to name a couple.

"ops"

when overhauling

you fit new

language. *

piston of the

new set of

match.

There is no comparison between the 2 types of piston. Engine pistons have a totally different function to those in a braking system. Mike.

No. A tolereance is arrived at after deciding the minimum and maximum clearance between 2 or more components. So a maximum dia brake piston, will still be an acceptable fit in a minimum dia bore, and vice versa.

The clearance between a brake piston and it's bore is around 1 thou, or 2 thou on dia. Quite a sloppy fit. With such a wide clearance, there's no reason for having matched fits. Even a thou either way on dia, would make little difference to it's operation, but I've no doubt that the tolerances actually used are too tight to allow that much variation.

Mike.

They both convert a fluid pressure on one side into a force in a solid object on the other. The only difference is that brake pistons don't move as much.

Ian

That's a very common misconception. It's quite possible to meet tolerances on two mating parts and still have to use them in matched pairs. The specs might (to put it crudely) say: Bore: 1" +/- 10 thou, Piston 0.980" +/- 10 thou, Clearance on diameter: 20 thou +/- 5 thou.

I was once at the receiving end of a problem caused by precisely the misunderstanding above: several thousand meters of ABS tubing in various sizes, all mostly within spec ... except for the bit saying "and each size must fit into the one above", which someone forgot to read, assuming that the individual tolerances were all that mattered.

To give a classic car f'rexample: Citroen DS suspension components were initial matched pairs: it was only aftera few years that manufacturing standards got so high (and for these things they have to be pretty amazingly good) that everything became interchangeable.

Ian

made

meet

matched

You should read the thread before replying. The clearance on brake pistons is such, that a matched fit is not req'd. If a matched fit were needed. A note to that effect would be on the drg.

The specs might (to put it crudely) say: Bore: 1" +/- 10 thou,

precisely the

in

saying

forgot to

mattered.

IOW the parts did not conform to the dwg tolerance.

I repeat. If a part or parts are made to tolerance it should fit. Unless of course the draughtsman has made a mistake. Mike.