Brake calipers - metal vs phenolic pistons

[newbury]

Caught some road trash into a brake caliper and it seems to have torn up a piston.
Looking at Autozone, NAPA, RockAuto, Fordparts they either list phenolic or don't say. O'Reilly's lists metal and phenolic pistons.
Is there some reason that most have gone to phenolic?

 

[dstig1]

Cheaper. Simply that. Phenolic is a thermoset plastic that can take pretty serious heat, but if you are using the brakes hard (any serious road racing or towing) I would go with metal. FWIW

 

[newbury]

Thanks, that is what I thought. It's only $10 more.

 

[mitch becton]

worked in shop one time where we did only automotive light truck brakes anything with phenolic piston only got steel we rebuilt them in house saw hundreds with phenolic piston where piston got really hot and stuck out killing pads rotor and more and as stated before just a part of package the cheapen up vehicle will do same with steel but not near as often

 

[Bird]

I have no idea how long the manufacturers have been using "plastic" pistons, but I know it was more than 40 years ago when I was surprised to learn that the dual piston calipers on our police sedans had plastic pistons. Now police sedans sometimes saw some very severe service and I don't recall there ever being any more problem with those pistons than with the old metal ones.

Now of course it's possible that the newer ones are not as good as the old plastic, but generally it appears to me that the reverse would be more likely. Most modern plastics are better than the older ones.

 

[sld]

The advantage of the plastic is that there is little to no expansion with heat. Steel expands and contracts quite a bit relatively.

 

[dstig1]

The advantage of the plastic is that there is little to no expansion with heat. Steel expands and contracts quite a bit relatively.

Bzzzt. Wow that is so backwards, I cannot get my head in the game. Plastics expand MUCH more than any metal. As a class, and in any instance I have ever seen. OK, here's the quickie web search on Phenolic vs Steel: Bakelite (aka Phenloic with fillers)- 22, Steel 12. That is almost twice the expansion coefficient for phenolic vs steel. And if you go down to "phenolic without fillers" you get 60-80 or 5-7x steel.

Coefficients of Linear Thermal Expansion

 

[newbury]

worked in shop one time where we did only automotive light truck brakes anything with phenolic piston only got steel we rebuilt them in house saw hundreds with phenolic piston where piston got really hot and stuck out killing pads rotor and more and as stated before just a part of package the cheapen up vehicle will do same with steel but not near as often

Did some of them look like this?

/edit - I figure I need a little bondo and some duct tape.

 

[crazyal]

The advantage of the plastic is that there is little to no expansion with heat. Steel expands and contracts quite a bit relatively.

I'm not sure if I would be worried about expansion of a steel piston in a steel caliper.

This reminds me of the "heavy duty" u-joints that no longer need a grease fitting. I doubt that they removed the fitting because u-joints last longer than they did in the past. Plastic is much cheaper to make. A steel piston requires machine work to get the clearances needed to fit in the caliper correctly. Machining plastic is much easier.