Stainless brake pistons - Anyone lived with them?

[Snod Blatter]

My CB250 needs some new brake pistons, the originals are pitted and nasty and will do new seals no good at all. Looking around I seem to have two choices, either OE style (coated mild steel, at a guess) pistons from David Silver or I can fit some aftermarket stainless pistons for slightly more money.

Other than the obvious "will it fit" gamble on aftermarket parts, stainless sounds like a stupid idea because they'll surely cause mad levels of galvanic corrosion when nestled inside a large lump of aluminium. And yet, a lot of people seem happy with them.. But I'm yet to see anyone report on how they fared after several years of salty all weather abuse.

So, has anyone tested stainless pistons long term?
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL

[stinkwheel]

Pure theory because I've never used them but they don't actually touch the alloy do they? So what completes the cell?

Would stainless be electrochemically any worse than the mild steel of the brake pad backing?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Snod Blatter]

As I understand it, the galvanic series states that the further apart two metals are in the series, such as this list, the more potential there is for corrosion to take place. As you can see, aluminium and steel are quite close to each other near the bottom of the list, while any kind of stainless steel is much further away.

But this is for in sea water, I guess where galvanic corrosion is of serious concern. Occasionally we may see salty water on the roads but it's not long term.. But how long does it take to be of concern to us?

And water full of caliper-related ions completes the cell. Is that close to sea water anyway in this case? I dunno.
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL

[Teflon-Mike]

I've had'em in the 750 for the last decade... I have not been an autoglym freak, whilst the ting lives outside, and I am only 'now' thinking of pulling them off and giving them a dab of coppa grease on the float pins... and wondering whether I dare look, 'cos they have only just, this last month started to get a bit 'sticky', since last strip and grease 3 years ago!
Slugs seem 'fine'.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[sickpup]

[Teflon-Mike]

[Polarbear]

[Bru]

Hi.

I fitted OEM pistons to my Honda, from DS, as part of a rebuild several years ago. The bike was not used much, over a couple of years, and one of the pistons subsequently began to corrode around the seal lip area. I had to replace the stuck piston again, but this time I went for stainless.

I have also had stainless pistons fitted to another bike, which has pretty much also stood for two years. The brakes are not seized.

I think that if the bike is used, and maintained, regularly then the pistons will last longer: a combination of scrubbing action and heat to keep moisture at bay (assuming the fluid is in reasonable condition).

My experience points me to stainless as a replacement, rather than OEM.

[I did find that the pistons I bought had machining which resulted in a rougher surface finish of the sealing area, compared to the fine-ground finish of the chromed originals. Still, no failure recorded to date.]

Aside from my musings, the issue of conductivity of brake fluid is quite an interesting subject. There are plenty of published documents online, should you wish to learn more.

[Snod Blatter]

[Robby]

I wouldn't worry too much about the galvanic corrosion. Even if it happens 5 times faster than normal, it means you pop out the pistons and clean behind the seals once every 2 years, instead of once every 10 years. If you're capable of changing pistons, you're capable of doing this.

The advantage of stainless pistons is that they won't rust. Chromed steel also shouldn't rust - OEM pistons last years - but the reality of pattern pistons is that they tend to be quite poorly made with very thin chrome and pit like a set of Chinese fork legs.

I do wonder how much the hardness of brake pistons matters. It would be interesting to try making a set out of bronze, simply because you could do it on a hobby lathe and get a really smooth finish.

[Snod Blatter]

I'm in there every 12-18 months cleaning out behind the seals. The dust seals on these Nissins are near useless. But then the Brembos on the front of the TRX and the Sumitomo on the rear are just as eager to gum up.. If only a manufacturer would dare have the imagination to use a TLS hydraulic drum

Considering Brembo and now Yamaha with their gold spots use pistons made of coated aluminium you'd have to guess that the hardness of the material isn't that important. Now, if someone in the aftermarket was making pistons like those I'd be all over them, they're lovely.
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1994 CB250, 1984 CBX250RS-E, 1989 K100RS, 1995 TRX850, 2016 Z250SL

[c_dug]

On a similar note (and possibly a minor hijack of the thread) is there any issue with the different expansion rates of various metals?

I have a load of metals at work that I could easily try, bronze, stainless, aluminium, brass, titanium etc... And the machinery to do so too, but presumably some metals would expand enough to seize in the caliper when hot? There isn't exactly miles of room between the calliper and piston!?
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I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[Islander]

I fitted a set to the Zephyr 750 about 6 years ago and they've been fine. The only slight problem I had when fitting them was the inner bore was too narrow for the inserts. That was simple to fix - I just ground the inserts down a bit.

I've seen no corrosion with them at all.

[sickpup]

[Teflon-Mike]

[stevo as b4]

Is it me being fucking simple here, or are people going far to deep into this and getting way too frothed up about brake caliper metallurgy and materials selection and design.

Surely your not all racing, or building ultra light custom built specials. OP seems an ok bloke, but is way too deep into discussing brake piston coatings, surface treatments, and materials properties, especially considering how often he says he's in there cleaning out his calipers and removing seals etc?

I just don't see why It really matters, it's not like your discussing how much cam shaft lift and overlap to use in a race engine build is it?

On my ZX6R I did 9500miles in 5years some wet some dry, but it never saw a winter or salt. I used to clean the calipers with brake cleaner and mini bottle brushes, but I never had to strip the allegedly awful Tokico 6pots, and with good sintered pads they were a brilliant brake tbh!

[sievuun]

Stainless steel brake pistons are absolute no problems in my experience. Some of mine are more than 30 years old, done more than 250.000km driving all year. The standard pistons may seize after first winter and can not go back in when they are corroded. Stainless steel items may seize too but after much longer service and then you just clean them and put them back in for 5 or 10 years more. I made mine myself, but buy them if you have the choice and want to keep your bike more than a year. I prefer to buy new rubber parts if available but the pistons will last the rest of your life.
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BMW: R50/5 R80GS R90/6+sidecar R90S R100RS R100RS+s.c K100RT+s.c K1100RS IFA: RT125/2 DKW: RT125/3 MZ: ES150/1 TS250/1+s.c ETZ250A ETZ250+s.c Kawasaki: GPZ750
Once had: R51/3, CB350four, CB350K4,XS1100 XS1100S+s.c,XT250, XV500, Z550, GT550triple, TS150/1, TS250, ETZ251 saxon tour

[Snod Blatter]

[smegballs]

I put stainless piston in back in 2014 with plenty of RRG. Pulled the caliper apart back in june just to check everything was ok and regrease the sliding pins.

There was a little bit of superficial muck that was literally just wiped off with a rag, but apart from that the only change in 2 years was the the RRG was now brown-rubber-grease.

It was all clean enough that I didn't even need to push the piston out far at all, so the dust seal had obviously done it's job well. So two years (including two winters) without attention and the whole thing was still perfectly fine.

Re: Pitting

https://i.imgur.com/yExEHASl.jpg
https://i.imgur.com/TG5vXG1l.jpg

My OEM piston looked like this when I took it out, pretty pitted IMO

[sickpup]

[kramdra]