Caliper Piston Scratched

redcoupe86 · 10-10-2007, 04:25 AM

I'm rebuilding my front calipers and I noticed that one of the pistons has a parallel scratch that runs the full length of the piston. Its very light, like someone took very sharp razor blade and put just enough force to leave a scratch, but it can be felt with the fingernail (but only barely).

I've scotch brited the piston but the scratch was still there. Then I polished the piston with my dremel to see if that would get it out - nope. However, I'm not sure I should have polished the piston b/c when I put brake fluid on the metal it didn't coat in an even sheen like the previous pistons, it beaded up like on a well waxed car. I'm not sure if its like a piston in the engine where it needs to be honed.

So some questions:

1. Will this scratch cause a leak or air to get into the system. Has anybody ever rebuilt pistons with very light scratches and came away OK.

2. do I need to remove the piston and use some 2000 grit sandpaper and wet sand a hone to it?

3. I'm I screwed and need to buy new pistons (if they even sell them) or rebuilt calipers?

diverdan · diverdan's Garage

I wouldn't worry too much about it about it. The piston just pushes the little rubber seal that seals with the surface of the cylinder. The only downside might be that the groove might pick up some dirt which could theoretically scratch the cylinder surface. Practically no worries. IMHO

Diverdan

aj88cab · aj88cab's Garage

Actually the piston moves against the seal which sits (stationary) in a groove in the cylinder. A scratched piston will eventually damage the seal. Replace the piston.

moneymanager · 10-10-2007, 06:42 AM

What a range of opinions. If the scratch is parallel to the piston's travel, as you suggest, you have very little to worry about. The worst thing that can happen here imho is that you might have to rebuild the caliper again in a few thousand miles.

hcoles · 10-10-2007, 06:53 AM

IMHO - buff (not in the parallel to travel direction) with 800 and give it a try with a new seal - probably be very ok - in the mean time maybe order another piston and put in the tool box incase there is an issue downstream. Put a few drops of bf or one of the brake piston lubes on the piston and seal before asm. May want to buff out the cylinder and see if you run into the debris. It is hard to get in there, I use a very fine scotch bright type material and then flush/clean. As others have mentioned in other posts... don't split the body of the caliper.

redcoupe86 · 10-10-2007, 07:51 AM

If I use sand paper, I assume I would wetsand the piston in a crosshatch pattern. Is 800 high enough grit?

My concern it that this very fine sratch would let brake fluid out or air in.

Porsche_monkey · 08:00 AM

What does a new piston cost, about $50 (for two)? Why bugger around and waste your time?

Just replace it, it's not worth the agravation if it leaks.

hcoles · 10-10-2007, 08:00 AM

if you want to use 1000 grit, that should work as well or even higher.
re. cross hatch - I tried to keep the pattern about 90 from the direction the fluid wants to try and get out. I think the way it works is that the piston slides on the rubber seal... when it starts to fetch up the seal grabs the piston, as the hydraulic pressure rises, and flexes for the final movement to apply hard braking. You may be too worried about this...

redcoupe86 · 10-10-2007, 08:53 AM

I only found some SS ones on pelican (a set of 4) for over $200.

Can you point me in the right direction for buying them individually?

Quote:

Originally Posted by PBH

What does a new piston cost, about $50 (for two)? Why bugger around and waste your time?

Just replace it, it's not worth the agravation if it leaks.

Gunter · Gunter's Garage

Have a feel inside the caliper to see what caused the scratch.
If it didn't leak before, do a fine polish and re-assemble.
The square seals are very forgiving.
Expect to bleed the system a few times all around before the pedal feels hard.

BertBeagle · 10-10-2007, 05:27 PM

Rule of thumb (nail) if you can feel a scratch then replace the piston. If this is an aluminum caliper do not go honing or sanding the caliper bores. If it is an aluminum piston you will damage it by sanding on it.

Without seeing the piston and caliper that is about it.

The seal is in a groove in the caliper piston bore. The piston moves in the bore (and seal) when the brake is applied. If there is a scratch deep enough to feel or through the anodize (on aluminum pistons) then it should be replaced.

If you are still not sure take the piston and caliper to a trusted mechanic.

hcoles · 10-10-2007, 05:32 PM

good points I assumed steel piston and caliper.. that's what I have...
redcoupe... can you post a close up of the scratch? maybe we can get some idea of the depth...

BertBeagle · 10-10-2007, 05:43 PM

If the caliper is cast iron and the piston stainless or plated steel - the same rules apply although you can polish a stainless piston with scotch brite. However if it has a scratch deep enough to feel it needs replacing. Yes you can sand a steel piston to remove a light scratch but then you do not have a round piston. It may seal but I would not put it in my car or a customers caliper.

Quote:

Originally Posted by hcoles

good points I assumed steel piston and caliper.. that's what I have...
redcoupe... can you post a close up of the scratch? maybe we can get some idea of the depth...

Early_S_Man · 10-10-2007, 07:23 PM

The OEM pistons are hard chrome plated, and from your comment [and obvious concern] about brake fluid not 'wetting' the piston surface along the scratch ... it sounds like the plating may have come off during your buffing operation. It probably won't give you any leaking problem near-term, but how long are you willing to drive the car with 'worries' in the background whenever you step on the brake pedal???

The piston was probably scratched during disassembly for a previous rebuild by a grain of sand imbedded at the chamfer of the caliper bore. Compressed air can move that piston rather rapidly!

This problem illustrates why silicone grease should always be used during caliper rebuilds! It does not run or migrate once installed. A 'bead' of silicone grease at the bore chamfer/piston interface acts as a barrier against entry of dirt or sand, even if the rubber dust shield is cracked or torn away!

BertBeagle · 10-11-2007, 03:25 AM

Quote:

Originally Posted by Early_S_Man

The OEM pistons are hard chrome plated, and from your comment [and obvious concern] about brake fluid not 'wetting' the piston surface along the scratch ... it sounds like the plating may have come off during your buffing operation. It probably won't give you any leaking problem near-term, but how long are you willing to drive the car with 'worries' in the background whenever you step on the brake pedal???

The piston was probably scratched during disassembly for a previous rebuild by a grain of sand imbedded at the chamfer of the caliper bore. Compressed air can move that piston rather rapidly!

This problem illustrates why silicone grease should always be used during caliper rebuilds! It does not run or migrate once installed. A 'bead' of silicone grease at the bore chamfer/piston interface acts as a barrier against entry of dirt or sand, even if the rubber dust shield is cracked or torn away!

This is good advise. Use only silicone grease designed for brake system rubber seals - that is a silicone grease that does not cause the rubber to swell. Brake rubber seals are made of EPDM or EP rubber.

Porsche_monkey · 10-11-2007, 03:33 AM

Quote:

Originally Posted by redcoupe86

I only found some SS ones on pelican (a set of 4) for over $200.

Can you point me in the right direction for buying them individually?

I must have mis-read the web-page, because I can't find it now

Buy a used caliper.

redcoupe86 · 10-11-2007, 04:29 AM

Where do you all suggest buying a used caliper from. Does PP have them. I only see new.

Quote:

Originally Posted by PBH

I must have mis-read the web-page, because I can't find it now

Buy a used caliper.

hcoles · 10-11-2007, 04:42 AM

along these lines.... has anyone made a tool to reach in.. expand and "grab" the piston to turn slightly to line them up or work back and forth to get the piston out without using fluid/air compression? I guess this winter I'll take out my pistons and see how the buffed off or buffed chrome is doing... it sure didn't look like there was a plating on there last time I had them out.

hcoles · 10-11-2007, 04:44 AM

re. used caliper... an idea(s) put out a WTB on Pelican used parts forum and maybe call Parts Heaven Hayward CA. I think they are in Hayward. Google it and it should pop up.

Porsche_monkey · 10-11-2007, 04:44 AM

Quote:

Originally Posted by redcoupe86

Where do you all suggest buying a used caliper from. Does PP have them. I only see new.

Try the 'Used Parts Classifieds'.... post a 'Wanted to Buy'.