User 52121

What's everyone's thoughts on using aftermarket stainless steel pistons in a caliper rebuild vs. the stock aluminum ones?

Calipers in my car (987 Cayman S) need a rebuild. I haven't disassembled them yet to know if any of the pistons have pitting or need replacement - for now I'm assuming they will.

Found this: http://www.racingbrake.com/Save-10-o.../bp-402bsp.htm

Sounds interesting enough - stainless pistons absorb heat more slowly, etc.

Worthwhile? Anybody do it? Happy with the results?

jdistefa

Funny you should ask, I`m just in the process of ordering a set.

One of the other SPC guys uses them and likes....

certz

Jim, listen to Matt. He knows brakes and trannies. Wait, or is that he always brakes for a tranny. Damn, maybe it was his pen is always in a tranny?

Dave DE

One thing I've always wondered about with the SS pistons is the rate of thermal expansion. Steel does not expand as much as aluminum for a given rise in temp. I imagine this would open up the gap between the piston and caliper body a little vs an aluminum piston as temp goes up. I'd feel better if the people selling the piston had calculated this difference and could show this is not important.

jdistefa

Should I be offended or excited by this? So confusing.

certz

Brian C in Az

We know that the nominal diameter of the piston when cold will seal properly, so therefore any increase in piston diameter will seal as well.
What is your concern?
The piston bore gap will never be greater than when the caliper and pistons are cold.
As the brakes get hot, the pistons will expand and the bore will contract. Therefore, the gap will not open up as you suspect, rather is gets less as the assembly gets more hot.

Dave DE

My understanding is that the bore diameter will increase with temperature. Here is a link that might help, see section 'expanding holes':
http://physics.bu.edu/~duffy/py105/Temperature.html

User 52121

Yes, this. Expand enough to leak, weaken or fail the bore seal with extra clearance to the piston?

stownsen914

If the stainless pistons are specifically intended for the caliper you plan to put them in, I'd say they should be OK. You may not need to worry about expansion rates unless this isn't the case. If the aftermarket pistons are generic, also check the diameters carefully. If you calipers accept a certain size piston, say 35mm, the actual diameter may be a fraction of a mm over or under. Clearances are probably tight of so would need to be spot on.

Cory M

What braking issues are you having that you hope the stainless pistons will alleviate?

The thermal expansion of the stainless pistons is about 40% less than the stock aluminum (for the same temperature delta), if the stainless pistons do indeed run cooler the difference in expansion will be even greater.

User 52121

No hard issues yet - but planning ahead. It seems to be a well-known common issue that the brakes on the Cayman are generally lacking when it comes to track use, and overheat quickly. Since my calipers are due for a rebuild anyways, if I can get ahead of braking problems next season by doing SS pistons while I've got them apart... then why not? (And "why not" is what I'm trying to get answered here...)

ace37

I assume the calipers are aluminum then? Let's take a rough stab, back of the napkin style...

Assume the piston diameter is an inch when measured in a cold garage at 50F. Total guess.

Let's see what happens when the piston-caliper temperature is up at 600F. That means a temperature increase of +550F over that cold garage.

The thermal expansion of aluminum alloys is about 13 parts per million per degree F. I used 6061 as a reasonably typical alloy and representative value. For steels using 304 stainless it's about 9.6 parts per million per degree F. The expansion rates aren't constant with temperature but for this kind of estimate they're close enough. So...

For aluminum the radial piston thermal expansion would be:
alu_expansion = 1 inch * 550 degrees F * 13 ppm/F = 550 * 13 * 10^-6 inches = 0.00713 inches = 0.182 mm.

For steel the expansion would be:
stl_expansion = 1 inch * 550 degrees F * 9.6 ppm/F = 550 * 13 * 10^-6 inches = 0.00528 inches = 0.134 mm.

The gap due to the expansion / growth is the difference, so:
difference = alu_expansion - stl_expansion ~= 0.002 in = 0.05 mm
It might be twice that estimated number (difference = 0.10 mm) if both the piston is larger and the peak temperature is higher, but probably not more...

To make that number meaningful, the higher value of 0.10mm is about the average thickness of a hair for a person with asian genetics, and 0.05mm is considered "thin hair" for someone with european genetics.

I think we can conclude that the seal should be able to cover any thermal growth, and the question of how well they were manufactured in the first place is more important than the material change.


Sources for expansion coefficients:
https://psec.uchicago.edu/thermal_co...5517-90143.pdf
https://www.engineeringtoolbox.com/t...als-d_859.html

Source for hair thickness:
http://www.schwarzkopf.com/en/hair-c...ictionary.html

User 52121

Ok - going with the assumption the math above is correct....

Who's used these? Who's noticed any kind of improvement?

futurz

I bought the SS pistons from RB. Installed them myself. The OEM pistons were badly pitted and the coating was peeling off. No issues after about 4 days at the track. I paid extra to install Brembo seals and dust boots. Spending money on dust boots is a waste of money if all you ever do is track the car, which is what mine does. Mine were already cracking after the first day.
FWIW