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Old 06-21-2014, 04:41 PM
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Brake Piston Dimensions

If someone could tell me the diameter and height of a front TJ phenolic caliper piston, I would be most appreciative. (To the nearest .001" if possible.)

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Old 06-21-2014, 09:38 PM   #2
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Tangofox. I did some searches but had a little trouble. Finally found replacement phenolic pistons at NAPAONLINE.COM and it shows the diameter of the piston to be 2.595". Hope this helps.

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Old 06-21-2014, 09:46 PM
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That helps a lot. Thanks very much.

I had searched as well, but didn't find anything.

I had just bought some Raybestos steel pistons when I noticed that the FSM warned against using steel pistons because their dimensions would not be correct. That did not make a lot of sense, if the pistons were made to fit the application. Anyway, the diameter measures to 2.595."

Maybe there is some part of the concept that I am missing. Anyone with insight is invited to provide same.
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Old 06-21-2014, 10:12 PM   #4
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Sounds like a smart move to me. I dislike phenolic pistons too because they swell and stick. Still fortunate enough to have the stockers on 00 but when the time comes, am going to metal. I know the metal transfers heat better and faster but recently flushed/changed my fluid to DOT4 so shouldn't be an issue. I know some say DOT4 absorbs moisture quicker but I will just start flushing every couple of years. Don't recall who it was but saw someone recommend rebuilding calipers with metal pistons a day or so ago.
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Old 06-21-2014, 10:38 PM
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Don't recall who it was but saw someone recommend rebuilding calipers with metal pistons a day or so ago.
Tha could have been me. I had done just that in other vehicles for the reasons that you stated, all with favorable results. Then, just today, I saw the caution in the FSM. I just cannot see why a steel piston would necessarily not fit, especially if it had been designed for that application.
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Old 06-22-2014, 09:01 AM   #6
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I agree with you. The only thing I thought of this morning is the rate of expansion on the steel is probably higher than the phenolic resin, however, I'm certain Raybestos would have taken this into account in their design. Keep us posted on how it goes. I may do the same on ours rather than buying reman calipers with steel pistons when the time comes.
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Old 06-22-2014, 10:06 AM
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I agree with you. The only thing I thought of this morning is the rate of expansion on the steel is probably higher than the phenolic resin...
I suspect that an expanded steel piston still fits better than a swolen phenolic one.

Investigating a little further, I see that the steel piston is the only option (piston purchased separately) that Raybestos offers for my application. And Omix-Ada also offers a caliper repair kit featuring a steel piston. Raybestos reman calipers, however, come with phenolic pistons.
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Old 06-22-2014, 05:08 PM   #8
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Agree. The only caliper failures I have ever seen is on Chrysler products with phenolic pistons. In the late 70s and early 80s, they were really bad. Never seen a steel piston caliper fail.

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