Educate me about brake caliper piston machining (SS vs Alu)

Hey all,
First of all, I'm in remote West Africa, no liability laws here, I know we normally wouldn't make parts for brakes because of liability, etc. Doesn't apply here, no need to COA your reply. (Unless you see an actual safety problem, then I'm all ears.)

My 2011 Chevy Avalanche has, each of the last couple of rainy seasons, developed a problem with sticky brake caliper pistons. Last year when it happened it was the passenger rear, and I took the caliper apart, replaced the seal/boot, and hit the plastic piston with some 600 grit sandpaper on the lathe until the fit was looser (stiff but easily movable by hand). It worked fine, but I just noticed that side has chewed up its pads way too early, and upon taking it apart, found once again that the piston is very stiff in the caliper. Couldn't budge it by hand even with the caliper disconnected.

Our climate here has a strange mix of crazy heat and humidity during the rainy season, followed by quite dry after, and so I'm suspicious that maybe whatever kind of plastic this is, is swelling in the humidity maybe? (Why doesn't it do that against the humidity of the brake fluid, I don't know??) Anyway, I want a permanent fix this time, rather than sanding it down to size only to have it swell again next year.

So, I'd like to turn a new pair of pistons (single-piston calipers time two sides).

The caliper piston seems like it was originally 2". As it is right now, the end that's deepest in the bore mics at 2.001 and the part that's regularly engaged with the seal, closer to the pads is 2.007". I stuck a 2" punch (as a gage pin, lol) in, and it's a nice snug fit against the cylinder seal, so I think the piston nominal OD should be 2.000"?

I have both SS (scrap yard, unsure of the grade) or 6061 available for the project. I'd prefer to make it out of AL if that's strong enough, just because of how much easier it is to work with. A quick scan online showed many sports cars using AL brake caliper pistons, so I figure it must be done? Also, either SS or 6061 are going to be a lot tougher than the plastic this part is replacing. The caliper body appears to be cast AL, so I figure AL also has the advantage of similar thermal expansion characteristics as the body it's being used in. Any objections to the choice of 6061 instead of mystery SS?

Lastly, what do I need to know about important geometry on the part? Should it be a straight cylinder, or should there be a little taper like my plastic part currently has? (No idea if it did from the factory or not.) The current parts has a big fat counterbore in the end that contacts the pad, but the pad doesn't need it (the part of the pad it contacts is flat). Reproduce this counterbore in the metal part, or just leave it solid?

Thanks a lot for any informed opinions!
(The parts will be made on a manual lathe, in case that influences anyone's advice.)

I would worry about fretting with an aluminum piston against an aluminum caliper, but I don't know if it really happens or if it's just a theoretical problem.

I recall my father welding up broken truck axles in West Africa. Just get a replacement? Out in the bush? You've got to be kidding.

TGTool said:

I would worry about fretting with an aluminum piston against an aluminum caliper, but I don't know if it really happens or if it's just a theoretical problem.

I recall my father welding up broken truck axles in West Africa. Just get a replacement? Out in the bush? You've got to be kidding.

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he has to go somewhere at times and were ever that is there is amazon they will send it . i use to work for a guy that did euro automotive machining they would take gullwing 300 heads and grind out arias that you could almost stick your fist in to and weld it back up . but why i ask ? his reply . were are you going to get a replacement ? then there were the one that came in with the fix my burned out light bulb mentality . to that he would say NEW WILL FIX IT ! and in this case NEW WILL FIX IT ! after all you buy gas , you have food , shoes . your reading this on your phone so if you can get all that other stuff you can get a two bit new caliper

thermite said:

Going in the wrong direction...

You'll not "get there from here" with sliding (single piston) calipers by f**king with the pistons. Among other factors, they have to be well ANODIZED to even work for very long if both sides are Aluminium. Aluminium in cast-iron won't gall as easily, but still need anodizing for both protection from wear and from corrosion binding.

Try to NOT mess with the pistons. Period.

BTDTGTTS, many makers.

Mass-market, built-to-a -price disk brakes rely on distortion of the seal for their VERY TINY required amount of retraction force. Slight distortion as pressure moves the piston, bounce-back when pressure is removed.

But it isn't a LOT of force!

The merest amount clear, or at least low drag, and they are happy.
Fail at it? "Excessive wear" is the only other option to offset drag.

Which can cook the pad and distort the rotor from heat. Even set a tire afire in extremis.

Got that Tee-shirt, too!

Pistons BOTH sides are usually good with this tiny retraction force for long, long, years.

Sliding calipers stick in the sliding part!

Usually a pair of dowel pins, optionally covered with elastomeric bellows, see also dovetailed slides.

Your workable fix is to REGULARLY free-up dirt, corrosion, thickened lube if any, in that area. "Dry" can be better than any lube of any kind. Lock-Eze graphite might suit.

Stock pistons - or their seals, actually - can then do what they are meant to do.

Clearencing the PISTONS, OTOH can actually degrade the distortion to the seal that acts as a limited-travel retraction spring.

And you will be worse-off, not better.

Both of my PRESENT vehicles (Jaguar XJ8-L, Range-Rover Sport HSI Deluxe) have the option of fitting Brembo double-puck, NON sliding units. But EVERYTHING must be changed, mounting on out.

You don't even want to know the cost!

So I pull routine maintenance on mine... around 2 to 4 times as often as I need to change pads and rotors.


THIS is what I suggest. Stock pistons and calipers.

Then get your head in there more often so as to get yourself dirty but the sliding caliper areas clean.

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The slides are a thing that need to be cleaned and regressed here every year. That's not this problem, the slides were still moving very freely when I pulled the caliper today, having just been lubed a few months ago. The excessive pad wear has occurred 100% since then, with the slides moving freely the whole time.

This may be one of those scenarios where a tiny bit of rotor axial run-out is needed to kick the pads back from the rotor.

Agree with those here that suggest you just replace the caliper with new.......

Some notes if you are determined to make new.
6061 is too soft for brake pistons. 2024 minimum.
Needs good thick hard anodize on the OD if they are to survive at all....Especially in an aluminum caliper.
Allow for buildup in base size before coating. Your 2.00 inch finished size looks good, measure the caliper bore to be sure.
Retain the counter bore at the pad interface...this is there to make sure the OD of the piston does the work and keeps the pad from rocking.


There are a number of products that are made to be packed inside the dust boot to prevent moisture and dirt from getting to the pistons. (Believe Castrol makes one)
Might be a good addition to the brakes no matter which way you go.....

Good luck.
Cheers Ross

The counter bore on the pad end of the piston is to limit heat transfer ,less contact area. As others have said the only retracting force is that provided by the torsion of the deforming of the seal ,so all of the fits need to be on spec. for things to work correctly. As I see it given your location SS would be the material to use but will need close to a mirror finish on the OD to prevent wear on the housing. If you could get cast iron calipers I think most of you issues would go away. Too many parts on modern vehicle's are made with aluminum for the weight savings to improve fuel mileage that would better be steel or cast iron!

I don't know what causes it, but back in the day (early 70's) Mopar went to non-metallic pistons. They were NOTORIOUS for sticking..........

I recall my father welding up broken truck axles in West Africa. Just get a replacement? Out in the bush? You've got to be kidding.

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1yesca said:

he has to go somewhere at times and were ever that is there is amazon they will send it .

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Umm, this would have been in the 1950's. Jeff Bezos was not even a possibility at that point so Amazon was in another universe, far, far away.

Is it possible the caliper cylinder has a stainless liner? That will impact piston material choices.
Bill D

The groove the piston seal sits in has straight sides but the bottom of the groove has a 5 to 7 degree taper in it. The reason for the taper at the bottom of the groove is to cause the seal to act as a spring of sorts to pull the piston back into the bore when the pressure is released. This is called piston rollback in the industry. Insufficient piston rollback keeps the brake pads pressed against the rotor and wears the pads out quickly. There must be a tight fit between the piston and seal, not only for fluid retention, but rollback. Pistons are straight sided and the ones we use in manufacture are typically chrome plated steel or phenolic. Pressure testing brake calipers requires that the caliper be able to hold 5000psi for 5 seconds without pressure loss or leaking. 5k psi is way over working pressure which can be as high as 500psi in an emergency braking situation.

Holescreek said:

The groove the piston seal sits in has straight sides but the bottom of the groove has a 5 to 7 degree taper in it. The reason for the taper at the bottom of the groove is to cause the seal to act as a spring of sorts to pull the piston back into the bore when the pressure is released. This is called piston rollback in the industry. Insufficient piston rollback keeps the brake pads pressed against the rotor and wears the pads out quickly. There must be a tight fit between the piston and seal, not only for fluid retention, but rollback. Pistons are straight sided and the ones we use in manufacture are typically chrome plated steel or phenolic. Pressure testing brake calipers requires that the caliper be able to hold 5000psi for 5 seconds without pressure loss or leaking. 5k psi is way over working pressure which can be as high as 500psi in an emergency braking situation.

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Interesting! I knew about seal distortion (with seal in bore) causing the retraction of the piston, but I wasn't aware of the angle at the bottom of the seal groove. Is that a common feature in all SiB calipers? And presumably the angle is oriented to make the seal tighter at the front (towards the disk) side? Or is it the other way?

And for that matter, how do the seal on piston calipers retract? If anything, deformation of the seal should make things worse, right?

Did your company make complete caliper assemblies, or just the pistons?

Don't forget hoses sometimes fail and act like a check valve. My only solution to your problem is to replace caliper Assembly.

Thank you all for your input. You've convinced me that I shouldn't be trying for a shop-made permanent solution.

I put the pistons on the lathe and using 120/280/600 grit sandpaper brought them back down a few thou. I ended up at exactly 2" dia at the mid point of the pistons, and a few thou under at the "deep in the cylinder" end. A test drive shows no unusual heat on either wheel and strong braking. That will get me through the year until I'm in back in the States and can buy a commercial solution. The caliper bodies themselves seem fine (unsleeved bore, btw), so I think I just need a "performance" brake piston made out of metal, so it won't swell like the phenolic ones seem to do. I'll be watching the pad life carefully in the meantime to make sure they're not being worn prematurely by not retracting.

Much appreciated! If anyone knows offhand of someone making such pistons in anodized alu or ss, I'd be interested in a referral.

Thanks!

TGTool said:

Umm, this would have been in the 1950's. Jeff Bezos was not even a possibility at that point so Amazon was in another universe, far, far away.

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ya but we are talking NOW not THEN if he orders it they will send it . after all Alaska has its bush pilots . and i am sure were he is at there's some rag a tee man that mad maxes around bring thing's to them that want to stay put and if that's the case why have a car for after all that may make one want to drive somewhere to get some thing like a new brake caliper

FWIW, I've had a terrible problem with sticking pistons on several different makes of cars. I think it's climate related and the only fix is just buy a new/rebuilt caliper and piston. It's all for nothing if you don't completely flush the brake fluid while you're at it.

I'd go with SS over alu. Not just for the wear resistance but also to reduce heat transfer. Different context, but mtb disk caliper pistons are commonly made out of alu which causes alot of problems with fluid boiling on long descents. I once had complete brake failure once which was no fun at all. Better brakes (eg. Shimano) use ceramic pistons to get around that problem.

SS pistons are done in British cars all the time, it is not rocket science

If the seal is in the caliper and runs on the piston, making ss pistons is a proven solution.

Look up Girling stainless pistons if you doubt.

Aluminum, no way, heat up and seize in a heartbeat.


35 years in with SS brake pistons I turned on a 6 inch atl@s lathe

Not clear is the seal groove in the cylinder or is it on the piston? If it is in the cylinder then making a piston becomes much easier.
Bill D.

Bill D said:

Not clear is the seal groove in the cylinder or is it on the piston? If it is in the cylinder then making a piston becomes much easier.
Bill D.

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Seal in caliper - almost all are made this way now. OP would have mentioned if there was any issues with the seal groove if it was in the piston.