Calliper Piston Material question

[peterako]

Hello fellow Loti Lovers!

After two months of waiting for stainless steel pistons to be made for my
rear +2 callipers I gave Susan Miller a ring to see if she could supply
them.

(I'd normally have gone straight to Sue for these but it's a long and
convoluted story....)

Anyway, Sue said she only stocks the normal (?) steel pistons but would
check with her suppliers for stainless ones.

After checking with her brake experts/rebuilders (plural):

'They neither stock, nor use, stainless steel calliper pistons.
They say they often take out stainless pistons and replace them
when rebuilding callipers, because the stainless is too soft
and marks easily.'

I just wanted to get an opinion from the group as well.

Are stainless better/worse/indifferent to the standard?

Thanks in advance,
Peter 1973 +2S 130/5

[richardcox_lotus]

Peter

I don't know who Sue uses as her expert but my understanding is that Classicar Automotive in Cheshire (mentioned several times on this forum's brake threads) use Stainless as a matter of course.

Standing by to be contradicted !..

Richard

[garyeanderson]

hi peter

There is a search function at the top of the page. I submitted stainless piston and recieved 23 responces. this has been covered a few times. maybe the answer to your question is there, if not it may help narrow down your question to something that will get the responce you are looking for.
Gary

[tower of strength]

A mate of mine is a turner (lathe operative) and he made me a full set of stainless ones, using the best of the old ones as a pattern, only cost a drink!!

[bcmc33]

I've never really understood all the discussions on the desire to use stainless steel pistons. Perhaps someone can advise.
Standard pistons are made from a high grade of steel, hard chromed, ground and lapped.
Surely stainless steel pistons would have a comparatively soft seal surface and more prone to damage. What grade of stainless is used?

The surface finish is critical for the seal operation ? it would be interesting to know how this is achieved with the soft surface of stainless steel.

Brian Clarke
(1972 Sprint)

[Jason1]

Hi

As I understand it SS is better due to the fluid being hygroscopic and eventually rusting the piston.

As the piston rusts it expands and the pistons seize, so in theory the SS ones should last forever. I don't really see the hardness of the metal mattering as the seals should provide a gap so the piston never touches the bore?

Jason

[tower of strength]

Stainless steel is a hard material, try drilling a hole in your kitchen sink, its "well 'ard", chrome flakes as I understand it, not a problem if there is no air/water present but as already mentioned, brake fluid loves water, and this is what causes the corosion.

My mate has machined loads of stainless pistons for various cars we have built over the years, to date we havent had a problem.

Mark

[reb53]

Compared with hard chroming, stainless doesn't get a look in.
Compared with mild steel, stainless is , usually, harder and more difficult to work.
However I've found that the stainless I've worked with and come across in the past has a tendency to gall and "pick-up", making it unsuitable for sliding and rotating applications.

I recall making some mirror polished stainless shafts for the rear shockers that lasted about a week before scoring themselves useless.
They were replaced with hard chromed steel that lasted decades.

No doubt the makers of stainless pistons will say that it is all down to picking the correct grade of stainless for the application which may well be right.
I've come across quite a few " I've never had a problem with that" type tradesmen in the past. They're the ones that bog up cars without primer, weld stainless with mild steel MIG wire, and over tighten spark plugs.
The reason they never have a problem is because by the time the problem rears it's head the car has changed hands a few times and they never hear about it.

Ralph.

[patrics]

Hi All

There are quite a few potential problems with replacement pistons and with stainless in particular they are:

High rollbacks and knock backs giving pedal feel problems this is because the seal doesn?t ?nip? the piston to the same degree as the standard chrome plated finish.

High wear rate compared to standard.

Having said that I have used stainless pistons on my rear brakes as the piston size is small and so has less effect on pedal feel.

If you already have stainless on the front and haven?t notice a pedal feel problem then I wouldn?t worry about the wear. If they ever start leaking which I doubt then change them.


If you want to buy good quality standard pistons or other brake bits then go to:

Performance Braking Ltd
Singleton Court
Wonastow Road
Monmouth
NP25 5JA
Tel (+44) 01600 713117

This is where most of the ?Girling? parts you see for sale come from including the 26R calipers.
You could also try BG Developments 01527 873716

Regards
Steve

[cliveyboy]

Trying not to repeat what has already been stated.
Changing to stainless steel can cause as many problems as it solves.
The big factor is what grade of stainless to use and what material to match it up with. As Ralph mentioned stainless steel has a tendancy to "pick up". For the non technical this means it reacts with the mating material and tends to seize up. This is almost guaranteed if you use the same grade of stainless steel for both parts. Usually two different grades of stainless are used or one part is coated. The best way is to use totally different materials. In engines you can use stainless steel valves but they are put into bronze valve guides.
In a brake cylinder I would not expect too much trouble using stainless steel pistons as the rubber seal tends to hold the piston central in the bore and you will get a certain amount of lubrication from the brake fluid. The one thing you want to avoid though is having the bore of the cylinder stainless as well. If you must have both parts stainless then one of the parts should be plated.

Clive

[AussieJohn]

Be wary of standard replacement pistons as well. I recently bought a front set from a well known lotus supplier and managed to ruin the seal when fitting the piston, found that there was hardly any radius on the back of the piston. I ground and polished a bigger radius and managed to fit them ok but couldnt bleed the system properly. traced to one piston not being pushed out of the caliper, it needed an even bigger radius to allow the fluid behind the piston, these pistons caused me a few bl**dy headaches, so the message is Check the radius in the pistons before fitting. cheers, John.

[Dave-M]

I think it's time for a reality check here!
Chrome on mild steel is used for cost reasons not because it is vastly superior to Stainless but it's much cheaper.
Chrome is porus and will corrode over time Stainless will not.
Why should Stainless gall or pick up? The pistons only move about 15 to 20 thousandths of an inch per brake application. This is hardly going to cause an issue.with pick up or galling,
Wear rate is to all intents and purposes non existant because as said in a previous post the piston is supported centrally by the seal and is lubricated by the surrounding brake fluid.
If your brake system is clean (internally) there will be no issues with scratches or scoring.
If you do not change your brake fluid at the recommended intervals you deserve rusty chromed pistons and all the associated problems which, incidentally you will not get with stainless pistons.
I suspect some brake reconditioners are anti stainless for financial reasons rather than performance reasons, but that's just my opinion.
Just as a final aside regarding piston hardness, it is not uncommon in some weight critical applications, to use aluminium alloy pistons with great success.
Classicar Automotive recommend chrome pistons and from the comment attributed to Sue Millers Brake supplier I suspect they are Classicar Automotive as well. The comment is exactly the same as Classicar told me in a phone conversation.
Get the Stainless pistons in. They are fit and forget.
Regards
Dave

[Keith Scarfe]

I think it's time for a reality check here!

So is the comment about pad knockback using stainless true?
I get a low but firm pedal on the first press then higher on the second. I have bled gallons of fluid through and cannot get any air out. I am thinking it is this problem mentioned where the stainless pistons retract further into the caliper after application meaning the first press next time gives a low pedal. Does this sound correct? Will this problem be made better using chromed steel pistons?
Cheers.
Keith.

[Dave-M]

[quote="patrics"]High rollbacks and knock backs giving pedal feel problems this is because the seal doesn?t ?nip? the piston to the same degree as the standard chrome plated finish.[/quote]

Surely if the seal does not nip the piston the problem will be the opposite and the brakes will drag. The piston retracts by using the flex in the rubber seal, ie. no nip (your word) no retraction.

Pad knock back is caused by the disc not running true between the pads.
usual causes are, wheel bearings or distorted discs. 0.004" out of true is the max recommended (from memory). Not much is it !!

For the life of me I cannot see how the difference in piston material can cause differing retraction rates.

I have used stainless on quite a few different calipers and never noticed this knockback problem at all.

Keith,
By way of a test you could try the following :- With the car on level ground (engine running, Handbrake off), apply the footbrake twice and you will feel the long/short pedal movement you described.
Wait a couple of minuites and repeat the test. If you still feel the long/short pedal effect, it cannot be knockback as nothing has moved to knock back the pads except the outside possibility of excessive wheel bearing wear which I am sure a man of your calibre would not tolerate.

Regards
Dave

[Keith Scarfe]

Thanks Dave. Yes I have checked all that. Wheel bearings good, runout within. Yes I was interested to read the comments about stainless pistons causing this but couldn't understand it myself. As it does this whilst stationary I must assume it is air in there, but I cant find it or get it out. It is ok, livable with, but a nice high pedal would be better. I'll just have to live with it.