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Author Topic: Caliper piston question  (Read 851 times)

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speedknot

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Caliper piston question
« on: May 04, 2009, 11:16:49 PM »
I just disassembled my front caliper on my 400F.  The piston wasn't too badly wedged in but I noticed when I cleaned everything up and attempted to reinstall it, the piston seemed to be very tight upon reinserting it into the piston cavity.  Its binding before the piston can get to the seal.  Is it possible that the first 1/8" of the piston cavity shrunk a bit?  I'm going to hone out the hole ever so slightly to see if this helps.  If any one else has any other ideas, throw them my way.  Thanks.
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Offline BlindJoe

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Re: Caliper piston question
« Reply #1 on: May 05, 2009, 12:18:44 AM »
put the caliper in the oven and the piston in the freezer for a few min and then give it a try
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Offline TwoTired

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Re: Caliper piston question
« Reply #2 on: May 05, 2009, 12:37:53 AM »
One of the things old brake fluid can do is form a hard crystaline surface coating on the aluminum.  I've seen this in master cylinder bores, too.  That's one of the reasons why brake cylinder hones exist.  The piston must be able to move freely in the bore without the seal.

Cheers,
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Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

speedknot

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Re: Caliper piston question
« Reply #3 on: May 05, 2009, 08:58:48 AM »
Quote from: TwoTired on May 05, 2009, 12:37:53 AM
One of the things old brake fluid can do is form a hard crystaline surface coating on the aluminum.  I've seen this in master cylinder bores, too.  That's one of the reasons why brake cylinder hones exist.  The piston must be able to move freely in the bore without the seal.

That's exactly what it was.  After doing a light hone of the cylinder opening the piston moved freely. 
After the piston was well seated in the opening I was faced with trying to get it back out.  When it was mounted on the bike I just pumped the brake till the cylinder fell out but being that everything is disassembled, I resorted to the air compressor method of unseating the cylinder.  Holy crap!  Even with light air pressure it shot across the room and lodged itself in the sheetrock wall.  That's another project now.  No one got hurt and it was a good laugh. ;D
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Online bryanj

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Re: Caliper piston question
« Reply #4 on: May 05, 2009, 03:33:54 PM »
Make sure the seal grove is super clean before re-assembly
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Semi Geriatric ex-Honda mechanic and MOT tester (UK version of annual inspection). Garage full of "projects" mostly 500/4 from pre 73 (no road tax in UK).

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speedknot

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Re: Caliper piston question
« Reply #5 on: May 05, 2009, 04:11:55 PM »
Quote from: bryanj on May 05, 2009, 03:33:54 PM
Make sure the seal grove is super clean before re-assembly

Good tip.  I actually took the small wire wheel for my dremel and cleaned out the seal seat.  A ton of crap came out of it.  A cruddy seal groove is usually the main cause of piston seize.  I just bought one of those phenolic caliper pistons from that guy on ebay.  It seemed to get some good reviews.
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