Question about brake caliper seal size (Page 1/1)
Patrick OCT 22, 01:52 AM

Although I'm working on my Subie, this question probably pertains to any caliper rebuild, including Fieros. So please bear with me.

First, an admission. I've been working on cars for 50 years, but I've never rebuilt a brake caliper. Ever.

I discovered that the left rear caliper piston of my '98 JDM Subaru Impreza STi was starting to bind due to the rubber boot being torn and the protruding edge of the piston becoming corroded. With the car being JDM (Japanese Domestic Market), it's always a challenge trying to find parts... but that's a rant for a different forum. Anyway, I got what I believe to be the correct caliper rebuild kit. The rubber boot is correct, but I wasn't sure about the internal seal. The OD of the piston is 38mm, whereas the ID of the seal is at least 39mm. If anything, I thought the ID of the seal should be slightly smaller than the OD of the piston.

Is it usual for the replacement seal to be larger than the caliper piston? Does the seal get compressed somehow when it's installed in the bore? Or do I simply have the wrong seal?

Please help a "newb" out!
theogre OCT 22, 02:58 AM
Assuming the main seal is in the caliper before piston...
Main seal is a bit compress by the caliper and might be right size when installed. Then piston compress it a bit more to seal w/o binding.

Plus many "books" have errors even if parts are right or not like Fiero Rad cap in most stores w/o current Stant cap in their computers.

Try using Japanese auto site to check the size. If site defaults to Japanese, (Some use your IP to set language) Look careful for language option because many have English or multiple options. Maybe somewhere near top or bottom. Or can try Google Translate.

------------------
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(Jurassic Park)

The Ogre's Fiero Cave

Patrick OCT 22, 03:59 PM

quote
Originally posted by theogre:

Assuming the main seal is in the caliper before piston...
Main seal is a bit compress by the caliper and might be right size when installed. Then piston compress it a bit more to seal w/o binding.



Thanks Ogre. Yes, I'm referring to the internal seal that needs to be positioned within the groove in the bore of the caliper before the piston is inserted.

I'm beginning to suspect that the ID of the new seal is indeed supposed to be slightly larger than the OD of the piston, and that the cylinder itself prevents the seal from being loose on the piston when everything is assembled. It's just that because I had never done this before, it seemed bizarre that the seal was a loose fit on the piston before installation.

Because I wasn't sure what to do yesterday, I ended up just leaving the original internal seal in place, cleaning up the bit of corrosion on the outer perimeter/edge of the piston, and reassembling the caliper with only the addition of the new rubber boot. I'll keep an eye on that caliper for awhile to see if anything untoward is occurring.

This is a picture which shows the rip in the original rubber boot. The opening allowed water to enter, and to eventually start to corrode the now exposed internal edge of the piston.




During my research, I learned what moves the piston back when the brakes are released. It's actually the internal rubber seal... as the diagram below illustrates. I had no idea!
pmbrunelle OCT 22, 06:16 PM
Older Corvettes didn't have the rectangular-section seals we take today for granted.

With what they had, the pads dragged more, reducing fuel economy slightly.
Iain NOV 02, 09:09 AM

quote
Originally posted by Patrick:


Although I'm working on my Subie, this question probably pertains to any caliper rebuild, including Fieros. So please bear with me.

First, an admission. I've been working on cars for 50 years, but I've never rebuilt a brake caliper. Ever.

I discovered that the left rear caliper piston of my '98 JDM Subaru Impreza STi was starting to bind due to the rubber boot being torn and the protruding edge of the piston becoming corroded. With the car being JDM (Japanese Domestic Market), it's always a challenge trying to find parts... but that's a rant for a different forum. Anyway, I got what I believe to be the correct caliper rebuild kit. The rubber boot is correct, but I wasn't sure about the internal seal. The OD of the piston is 38mm, whereas the ID of the seal is at least 39mm. If anything, I thought the ID of the seal should be slightly smaller than the OD of the piston.

Is it usual for the replacement seal to be larger than the caliper piston? Does the seal get compressed somehow when it's installed in the bore? Or do I simply have the wrong seal?

Please help a "newb" out!



Patrick, if you have problems in the future it may be worth checking these people:
http://biggred.co.uk/
I've used them a lot, very helpful, and their parts fit perfectly.
They're on ebay.co.uk with the same username, can't give you a link as ebay is blocked on this computer.
Patrick NOV 02, 03:18 PM

quote
Originally posted by Iain:

Bigg Red

They're on ebay.co.uk with the same username...



Interestingly enough, according to their eBay site, they won't ship to Canada. Strange. Might still be a good site to poke around in though. Thanks Ian.
DimeMachine NOV 02, 04:29 PM
[QUOTE]Originally posted by Patrick:


Although I'm working on my Subie, this question probably pertains to any caliper rebuild, including Fieros. So please bear with me.

First, an admission. I've been working on cars for 50 years, but I've never rebuilt a brake caliper. Ever.


Thanks for your confession. I have been working on cars for 36 years and have never rebuilt a brake caliper. I was wondering if I was the only one... lol

Cheers!

Dime

[This message has been edited by DimeMachine (edited 11-02-2020).]

Patrick NOV 02, 11:31 PM

quote
Originally posted by DimeMachine:

I have been working on cars for 36 years and have never rebuilt a brake caliper. I was wondering if I was the only one... lol



Well damn, you're not much help!

Iain NOV 06, 09:21 AM
Wonder if it's an ebay Global Shipping thing.

I've sent lots of Suzuki bits to people in Canada in the past, never had any problems.

If I can ever help let me know.

regards
iain