Front brake calliper pistons (stainless steel)

[Patrick Slatts]

Advice sought for the following please;

I’ve had some stainless steel front calliper pistons made, to replace the corroded alloy originals. The surface finish on the new ss pistons have fine groves down their length, due to the cutting tool contact during manufacture. Ideally I guess the surface finish should either have been ground or a different cutting tool used. (They were made along with some other work as a part favour so I could go back if opinion suggests so).

Fortunately the outside diameter was very slightly oversize, which allowed me to smooth down the ridges using very fine emery paper with the pistons held and turned in a small lathe. The surface is now smooth and matches the diameter of the original pistons (40.8mm), but the groves are still present, though very reduced. A comparison could be the feel of the surface of a vinyl record (note; I don’t make a habit of drawing my finger nails across records , but I hope you get the idea).

Am I likely to encounter problems using these as they are?

I’m aware that I will need to use DOT5 synthetic fluid because of the stainless / alloy mix.
My understanding is that the pistons do not slide past the seals during the braking operation. They slide past when taking up wear in the pads.
All is not lost if unsuitable. A good friend gave me a callipers with pistons which are in pretty good condition, but I would still like these new one available if they are suitable.

Thanks for reading and any helpful advice forthcoming.
Chris I.

ss caliper (2).jpg (435.77 KiB) Viewed 6647 times

[P.I. Staker]

Sorry if this sounds a bit rude, but I hope Stevie Wonder turned those for nothing and gave you the material. I personally would not use them myself. Although in reality, they would probably work but there is the potential for them to weep. I would ask for you money back, or get them to remake them by someone who knows how to use a centre lathe.

[Seagreen]

I would be a bit wary about using those pistons with grooves still present. I have always understood the pistons to move a little when applying the brakes and then they retract once the lever is released, otherwise how does the brake system work if the piston does not slide past the seal, even if that slide movement is very small? If they do move or slide through the seal, there is a possibility of weeping and also of them not retracting properly and thus the brake pads will drag against the disc.
If you were to polish out the grooves would this result in the piston diameter becoming undersized - you don't state how oversize they are/were.
Adios,
Charles

[nickst4]

As implied in Patrick's original post, I suspect that the resilience of the seals permits the pistons to pressure the pads, and the rebound of the seals pulls the pistons back just enough for pad clearance, once all has bedded in.

Those S/S pistons do look very poor. I bought some from Powerhouse and they were properly ground to size, but they were nothing like as smooth as the anodised alloy pistons I took out of the Brembo calipers. With that particular bike, I've yet to find out how well the brakes work, as I'm sure the pads have not married-up to the discs yet.

Nick

[Vitesse]

Further thought. If you are switching to Dot 5 fluid be aware that you will have to have all new seals and rubber parts. Dot 5 does not mix with other fluids and the result can cause rubber components to swell. I've a friend whose Guzzi brakes locked solid for that reason.

[huub]

Just polish out the ridges.
I dont think you need to change the fluids,
I have been using homemade stainless brake pistons for years, never had corrosion issues.

https://drive.google.com/file/d/0Bz9ek_ ... p=drivesdk

[Patrick Slatts]

Thanks all for your helpful comments, and no offence taken P.I., any reference to the great Stevie Wonder is good. Why, he could even pick up one of these pistons and ...'feel the grove'!. ...I'll get 'me coat.

As mentioned, the need to sort replacements has been reduce at present. A friend has kindly given me a calliper with decent pistons within, which I plan to use. I would eventually like to make the doner caliper complete again for future use.

The little I've learnt about the operation is that when hydraulic pressure is applied, the seals flex with the movement of the pistons, rather than slide past them, and return when pressure is removed (or drawn back by the spring pressure in the master cylinder(?)). I'm lead to believe the pistons do however slide past the seals when taking up wear in the pads. Either way, the groves on these, despite being very fine, may cause problems with the action. Hence the doubt.

The pistons are now smoothed down to 40.8mm diameter, exactly that of the original alloy anodised. I'm reluctant to take further, even though we are talking small amounts.

Thanks for pointing out the DOT5 fluid compatibility with components exposed to the other types, I was aware of this but no harm in reminding me. Fortunately other than the master cylinder, all other components are either new or nos and are dry. (The caliper was bought from Harglo, Morini importers. They had a big sale of their spares stock in the 1980s and I went for this, only to find the pistons were corroded. Probably damp ingress but the calliper bores are fine).

Thanks again all. Thinking of your cooemnts and as i type the though comes that when it comes to brakes, I do not want any doubt at the back of my mind so essentially, these should not be used.

Cheers, Pat Slatts.