ATE 36mm pistons, again

[AlfromNH]

I've done some searching on this, and I'm in a similar position a few folks have been.

My caliper pistons are corroded and to my eye, unusable. New replacements are unavailable. I'm working on a drawing in Sketchup that I might shop around to machine shops. I also have a line on a used ATE caliper.

I'm afraid any caliper of the same age is likely to have the same problem. I'm not fully confident I could give a machinist all the pertinent info he would need to make a set of pistons, and this being my brakes I don't want to leave something to chance.

I'm wondering if my best plan might be to find a Brembo caliper on eBay. An '81 - 84 would bolt right on, no mods necessary?

[Barry]

Yes Brembo calipers would bolt right on and you would have more chance of finding them with decent pistons.

Depending how bad the corrosion is would the pistons polish up ? The only part of the piston needing a perfect finish is the where it contacts the seal. So if there is pitting on the inner part of the piston then they have had it. Corrosion usually starts on the outer section where the piston is exposed beyond the seal. If caught in time and polished out this of no real consequence. Basically you have to push the pistons in far enough to accommodate new pads and see if the pitted areas are going to touch the seals.
  

[Luca]

I cleaned up my pistons using penetrating oil and a copper scrubbing pad.  There was still some pitting around them, but not where they contact the seals.

There are only a couple measurements for the piston.  The most critical is the 36mm bore size.  After that it's just a matter of cutting out the groove/shoulder at the top of the piston and drilling out a hole in the center (that center hole shouldn't be critical, maybe not even necessary)

Any competent machinist will be able to make a piston for you.  Clean the rust off of one of yours and bring it with you so he can take some measurements.

If you are really nervous about having just any old machinist do the job, go to an engine rebuilding shop that does industrial work.  These are the folks who rebuild cranks and cams worth more than a pack of pristine airheads.

[AlfromNH]

Here's a shot of one of my pistons.


This particular spot looks like it might never reach the seal, but there's more there.

I won't be riding this bike until Spring, so I think I'll just keep an eye out for a Brembo caliper in the meantime.

[Julio A.]

Brembo pistons are resistant to corrosion. Here's a side by side comparison of a 36mm Brembo and ATE piston. I think they might be even interchangeable since they measure the same.

The Brembo piston is the grey colored one.

[Luca]

The main concern between going between brembo and ate pistons is the groove cut for the dust seal.  The ATE groove looks bigger to accommodate the snap ring that holds the seal in place.

Keep in mind Afrom that you might need to rebuild a brembo caliper that's been sitting on a shelf for years.  Also, the Brembo caliper takes a different pad than the ate, so keep that in mind when the time comes to get those.

[nhmaf]

Despite being chromed for intended longevity - the ATEs always seem to suffer the effects of aging much worse - I guess all it really takes is for a tiny little hole in the chrome to open up and then let it some brake fluid, which then starts undermining the surrounding chrome plating from below.

There are some who have turned pistons from stainless steel - though I think that they have been for Brembos - in theory it could be done for either.

I agree with Luca - though the overal diameters are the same, the  groove for the seals are different, and I believe they take a different seal, so aren't really interchangeable

[AlfromNH]

Seems like it makes the most sense to find a Brembo caliper. I need pads and lines anyway.  I suspect I can probably buy a used Brembo and rebuild it for about what it would cost to restore this unit.

[Tony Smith]

There are some who have turned pistons from stainless steel - though I think that they have been for Brembos - in theory it could be done for either.

Well, you could just put the Brembo seal in as well.

BTW, I posted recently on turning up new ATE pistons from a piece of stainless steel shafting.

The thing about disc brake calipers is that the piston to bore clearances are not too critical critical. if it doesn't leak when assembled, its fine. When I turned up the ATE pistons I actually asked someone I know who is in the brake rebuilding business how much clearance was OK, his reply was that a "sloppy fit" didn't matter, just as long as the seal didn't leak it would be fine. He also laughed at me for replacing my heavily corroded ATE pistons - he would have polished them and reused.

If I recall correctly the difference between the ATE pads and Brembo pads is the overall "thickness" - the ATE requiring the "thinner" pad. back int eh day aty least one aftermarket pad manufacturer used to make pads that would fit either the ATE or Brembo calipers.

[Luca]

I'm with Tony on the rebuild, but mostly because I prefer the look of the ATE caliper, and I like to save good parts.

Furthermore, though I don't have experience with both, but I'm willing to bet the ATE caliper finish is more resilient.  I had to weld a big grade 8 nut onto one of my caliper bolts (that hold the caliper together) to remove it.  The heat didn't bother a thing on the ATE.  Not sure if the black painted Brembos would fare as well.

You could also consider re-chroming the ATE pistons.

IIRC, you have the holes for calipers on both fork sliders.  If you are considering going to a dual disc setup, you might consider that in deciding your brand of caliper as well.

@Tony

1) I think the ATE and Brembo pads have a different shape on the backing plate.  I'm pretty sure  the ears on the pad are different to accommodate different anti-rattle springs.  Julio A. has a thread on this in the FAQ section.  The R65 Brembo/ATE calipers are each unique to the R65.  Are you confusing them with the calipers on the big brothers?

2) Your post on turning new pistons was my inspiration to do the same when the need arises

3) While I have broken the rules with what is "acceptable" on vehicular repair, I find that it is best left to the person with wrench in hand to determine what will fly.  If in question, listen to the cautious voice.  Though your friend might have laughed at you for replacing your cruddy pistons, a clean set of pistons could prolong the life of a new piston seals.  Some folks aren't willing to chance the fact that a rough piston could prematurely wear through a seal and lead to a sudden failure of the braking system.

[nhmaf]

Luca and Tony makes some good points about the ATE calipers.   But, in my experience, it seems to be easier to find Brembo caliper parts and pads, as BMW continued to use the Brembos through the 80s, so you've got a long stretch of time of deployment across the product line working for you.

Personally, I'd keep my eyes peeled for both/either - finding another ATE caliper set to match on the other side, or a dual set of Brembos.    I wouldn't mismatch ATE/Brembo from left to right - they would still work of course, but you wouldn't want to create a parts-frankenstein anymore than necessary!

[AlfromNH]

Geez, you guys make me second-guess every decision I make!

An eBay seller is sending me a complete ATE caliper, as well as mc and lines for me to check and buy if they look good.

If that doesn't work out I'll watch eBay, etc for either an ATE or Brembo caliper.

I'm not looking to add a 2nd disc at this point, but it is worth considering that as part of the equation. Brembo would be my first choice in that case.

[Ed Miller]

Somewhere on this forum I posted a link to a guy in the U.S. who makes replacement pistons from stainless steel.  They weren't that expensive, about 30 bucks each I think.  Of course I would need 4 of them....