Brake pistons: steel or aluminium?

[dieselnutjob]

I need to overhaul the front brake calipers on the Jag (seals and pistons) as there is evidence of partial seizing, and the parts are cheap anyway.

 

For a few quid more they sell aluminium pistons instead of steel. There doesn't appear to be a chrome option.

 

I heard that aluminium transmits more heat into the fluid but as I'm not using it for track days would it matter?

[Dead_E23]

Given that brake pipes don't have fins, I'm not sure I understand why that would be an advantage.

 

I wonder if they would be less prone to corrosion?

[gordonbennet]

I'd want the same material as the caliper is made of , eg steel pistons in steel calipers.

 

If proper brake grease is used properly when the brakes are serviced (does anyone else does that anymore?), and salt regularly cleaned off during the winter then corrosion shouldn't be a problem.

[Tamworthbay]

Are stainless steel ones available? That is always my material of choice and never had a problem once they are fitted.

[Guest Hooli]

I'd go with the same as the original, they lasted well enough the first time.

[dollywobbler]

2CV has ally pistons. I've never ever had a sticky piston issue. Pretty much every British car I've owned has had sticky pistons at some point, because steel corrodes (though why British ones seem especially bad, I'm not sure. Mini, Range Rover and Jag all have/had the same issue).

[garethj]

As said, you probably want the same material as the calipers.  Stainless steel isn't necessarily the best answer as it'll make aluminium corrode even faster.

[xkjagnz]

Ideally should be the same material as the caliper as differing heat expansion could cause binding (aluminium expands more than steel so loss of clearance as it gets hot)

[dieselnutjob]

that makes sense to me.  a magnet sticks to the caliper so steel it is then.

 

thanks

[cros]

There appears to be some sort of consultation going on. The MD of JLR has just tweeted that he's happy to see the difference between old, polluting diesels and new, clean ones has been acknowledged. I'm like, 'hold on a minute. It's modern diesels that are the problem due to horrendous NOx!' He was then banging on about low carbon emissions yadda yadda, when that's not really what's causing concern at the moment.

Synthetic fluid has kept my pistons going in and out freely for years irrespective of what they're made from.

Lots of people disagree, often garages who mend seized brakes. I've only used it after replacing all rubbers and have never had a problem since I went over to it on a Meriden Triumph in 1992. My Sherpa coupe has had it in since 2005, brakes are looked at before each MOT but cylinders haven't needed work since then. It's used through the winter and I've not cleaned the salt off even though I should have. I have no connection with Mr Silicon Brake Fluid- it's pricey but not as costly as a new calliper.

[lisbon_road]

That point about differential expansion I think isn't valid as there is significant clearance in brakes, as it's taken up by the rubber seals. 

[Tamworthbay]

As said, you probably want the same material as the calipers.  Stainless steel isn't necessarily the best answer as it'll make aluminium corrode even faster.

Only if it's in contact, which in a caliper it shouldn't be.

[lisbon_road]

Er, are we a bit confused here?  The caliper is presumably cast steel and the pistons are steel or aluminium, with stainless being avaible for some other cars.  Nowhere are we looking at combining aluminium and stainless?  Or did I miss something.

 

(I agree with Tamworth, who is saying much the same as I was....)

[Dave_Q]

That point about differential expansion I think isn't valid as there is significant clearance in brakes, as it's taken up by the rubber seals. 

 

Thermal expansion is measured in µm/m/°C and is about 15 for most steels and 20-25 for aluminium.

 

Brakes could be what, 500°C?

 

So a steel piston of say 50mm diameter would expand by 375 microns (0.375mm) by 500°C, the aluminium one by approx 0.55mm.

 

Probably not enough to cause a clearance issue.

[martc]

As discussed above - stick with the same metal. The expansion due to heat, as my learned colleague Dave Q says, is probably not a problem.

 

I'd be worried about corrosion, in particular the setting up of an electrochemical cell due to the presence of salty water in the winter and the different potentials of iron and aluminium - see the back of an old Land Rover for a practical example.

[Snipes]

I'd want the same material as the caliper is made of , eg steel pistons in steel calipers.

 

If proper brake grease is used properly when the brakes are serviced (does anyone else does that anymore?), and salt regularly cleaned off during the winter then corrosion shouldn't be a problem.

Do they do steel calipers? I've only ever seen cast iron or alloy.

[Tamworthbay]

As discussed above - stick with the same metal. The expansion due to heat, as my learned colleague Dave Q says, is probably not a problem.

 

I'd be worried about corrosion, in particular the setting up of an electrochemical cell due to the presence of salty water in the winter and the different potentials of iron and aluminium - see the back of an old Land Rover for a practical example.

It's not a practical example as they are not in contact. If it was such a problem they wouldn't use steel bolts to build and attach alloy caliper s which they do. A piston will have zero contact with the caliper body unless there is a serious problem.

[gordonbennet]

Do they do steel calipers? I've only ever seen cast iron or alloy.

OK cast iron, sorry was using steel as term to differentiate it from alloy.

 

I'd prefer steel pistons in cast iron calipers, and i dislike alloy calipers intently, even where steel sliders have been greased reasonably they can still corrode up badly and jam solid.

Almost never had to renew a cast iron caliper on a car in my ownership, the few which have been unsalvageable were alloy.