Front Brake caliper pistons

[skelteanema]

I have been repairing my brakes as they have been sticking on. I have pulled apart the left front brake calipers, and am repacing the pistons as there is some corrosion on one of the pistons. I have looked at the right one and the pistons seem fine. I ordered and have received a full set of pistons from SJ sportcars. Although they are a correct fit, they look somewhat different to those already on the car. Firstly the original were chrome plated, whereas the new ones are not but are a dark grey/blue steel (see pic). Secondly the area on the cup that contacts the brake pads is significantly smaller on the new piston that the old one. The questions are:
1. what are the new pistons made from and what is the corrosion protection? (are they high carbon, or something else)
2. Should I replace the right caliper pistons as well even though they seem fine and free moving?
3. Will mixing the different types of pistons have any significant impact on the brakes? I wouldn't have though so as it would be the diameter of the pistons that dictates the braking pressure, but i ain't no engineer.

Thanks,

[AussieJohn]

I had lots of sticking problems after rebuilding my calipers [with black pistons]; changed 3 sets of seals and couldn't stop pull-back problems; finally changed the pistons for chrome type and all is well. I believe the surface finish is the problem on the black ones.

[rgh0]

I dont know the material of the black pistons you show but Ii would only use polished stainless steel pistons these days.

The smaller the contact area between piston and pad the better to minimise heat transfer from the pad to the piston and then to the fluid. Racers often cut the contact area to make it castellated to reduce it further and to allow air to flow behind the pad into the piston back area to assist cooling.

regards
Rohan

[john.p.clegg]

"Racers often cut the contact area to make it castellated to reduce it further and to allow air to flow behind the pad into the piston back area to assist cooling." That sounds like a smashing idea but is there enough meat befor you get to the seals?

John

[rgh0]

you only cut the reduced thickness area where the piston contacts the pad so only a few mm deep and dont get into the seal area

cheers
Rohan

[Baggy2]

I used to have sticky piston problems particularly on the rear calipers but since using silicon fluid - about 20 years now - Ive not had a problem. I know there are those that wont agree but I am just reporting my experience.
Regarding the piston material I was told many years ago that Chromium plated pistons were less likely to sieze because chrome is a very different metal to iron , the major material in the caliper. This was a discussion I had when considering buying stainless steel pistons and I was told that as stainless is also a ferrous metal the was a higher chance of the caliper siezing because of microscopic 'welds' occuring between the two ferrous materials. It was a long time ago and I cant even remember who told me but it stuck in my mind because it did seem to have a certain logic to it.
Having said that I have used stainless pistons without problems so who knows???
All the best
Baggy

[rgh0]

"Galling" is the process where microscopic welds form when surfaces of similar material and hardness are in high load contact. The load between the cast iron cylinder wall and a piston is very low and the risk of galling non existent I believe for any normal piston materials including chrome plated steel or stainless steam.

The biggest problem is corrossion between the piston and piston bore. Chrome plating tends to fail overtime and the pistons then rust. Stainless pistons do not.

cheers
Rohan