Brake Pistons - stainless steel vs. hard chrome (original)

[Andre]

Brake Pistons - I have read some disconcerting opinions regarding stainless steel brake pistons. One is that SS might expand under heat and may thus lock up the pistons. Another is that it corrodes easy when exposed to salt. There are various types of SS, of which some are better suited for brake pistons than others. The aftermarket pistons I have seen are SS. The type of SS is never mentioned.

For me, the question is currently mute as I ordered an original rear piston set including seals from my Suzuki dealer for 69€. A steal considering that I would have paid 60€ for 2 stainless steel aftermarket pistons (incl. postage). But the question might come up for me soon when I overhaul the front brakes.

I assume that many of you have aftermarket SS pistons installed. What are your experiences and opinions?

[Blubber]

I am pretty sure there are even original aluminium cups.

Some adult will come along shortly

[Andre]

Checked, and yes there are aluminum pistons. And titanium, and phenolic, and theramic, and ...

Another concern is that SS-pistons are rough on the seals. Some buff these pistons before installing them.

[T 24]

You can make your brake pistons from stainless steel. 316L is normal stainless steel and it is OK material.
And you are correct in that heat expanding, first ones I made over 25 years ago stucked when they were hot.
After that I made them 0.1-0.2mm smaller than original pistons and they have worked OK over 20 years.
0.1-0.2mm is so little difference, that I havent had any problems with seals.
I havent heard or seen that corrosion problem.
You can also make threads in pistons, so you can easily pull them out when needed.

[Andre]

You Sir, are a blessed man being able to make your own, much improved, products!
Thank you for confirming the heat issue.

I am not surprised that you can't confirm the corrosion issue. 316L is not a "normal" stainless" steel. It contains up to 18% chromium, 14% nickel, and 3% molybdenum and is highly corrosion resistant. Therefore it is marine grade and also the preferred SS in medical devices. Not something most (if any) aftermarket producers would use.

[T 24]

OK. Correction.  316L is normal stailess steel in Finnish paper mill industry, where I used to work earlier.

[Blubber]

Stainless steel 316L ( mostly werkstof 1.4404 ) is also the preferred standard in pharmaceutical company's. Not just medical devices.

the complete machine parc ( vessels, drums, piping even frames ) is mostly 316L

How do i know this ? I work for one 

[Andre]

Yup, they can't tolerate corrosion. Interesting to me is that the surface of SS can heal itself through chemical reaction with oxygen. Some professor showed a little experiment to his class by putting a rubber band around a piece of SS, then put it in a sodium chloride solution. Took it out and let it sit for a couple days. When he took the rubber band off, corrosion was noted where the band sat. The non covered surface area was fine. I think this has application to our interest. Ride your bike on salted streets; when braking the piston comes out a ways, picks up salt water and then retracts into it's resting position. The seals act as the rubber band above, denying oxygen to the area covered. Letting the bike sit for a longer time without thorough cleaning of the piston, you get corrosion. The pistons will be the least of your problems as seen here:

[KiwiCol]

Now that's a dirty ole girl!! - best put a warning up or Barmy will see it & have a heart attack!

[northern]

I have one SS piston in rear brake caliper, which I get from Powerhouse... I need 1 but OEM's were sold by set.
Yes it fit, but I notice, my rear caliper is always warm (not hot). I reassemble it few times, but its always the same. Now, I read here, what SS is expanding more then OEM pistons, so it may be the reason - piston are made by same dimensions as OEM, and they became denser after the get warmer, and this is a loop... Not critical, but I do not like it...

I was thinking of getting OEM pistons, now I'm almost sure, I need to do it.

[Kiwifruit]

Now that's a dirty ole girl!! - best put a warning up or Barmy will see it & have a heart attack!

Lets hope its not too late Col !!

Nice to see you use it Andre 

[Andre]

I have one SS piston in rear brake caliper, which I get from Powerhouse... I need 1 but OEM's were sold by set.
Yes it fit, but I notice, my rear caliper is always warm (not hot). I reassemble it few times, but its always the same. Now, I read here, what SS is expanding more then OEM pistons, so it may be the reason - piston are made by same dimensions as OEM, and they became denser after the get warmer, and this is a loop... Not critical, but I do not like it...

I was thinking of getting OEM pistons, now I'm almost sure, I need to do it.

That is an interesting observation and may give the answer to a question I have not researched. What is the heat conductivity of SS vs. OEM pistons? Assuming OEM pistons are non-SS/hard plated chrome? Aluminum definitely conducts more heat than SS. See this video for a demonstration (https://www.youtube.com/watch?v=IUMMn0ZY1So). The heat from the piston gets first conducted to the break fluid before it can be felt at the caliper. If you have to brake a lot this can cause deterioration of braking power.

My mind is made up. I go for OEM. The original Suzuki rear piston set, which includes the seals, is less (by ca. 10€) than buying aftermarket SS pistons and seals.

[Andre]

Oh no, I don't want to cause Mr. Barmy a heart attack or worse, the permanent confinement in a mental hospital.

The bikes owner did a fairly good attempt in redeeming himself. The story can be found here:
Part 1 https://www.bikechatforums.com/viewtopic.php?t=173115
Part 2 https://www.bikechatforums.com/viewtopic.php?p=2181592