New rear brake pistoins

[chrisonord]

I have finally got my rear brake caliper back in one piece, and will get the thing fitted tomorrow. I could not get anywhere with anyone to get hold some OE pistons, so I got a pair made from stainless steel. The originals are anodised aluminium, and score quite easy, these will probably out last the bike, and for 30 quid for the pair, they will do for me.





Yes, I know the caliper is still dirty but it is corroded in, and a hell of a lot better than it was believe me
Chris.
Bugger it won't let me sort my spelling mistake out on the tittle.

[nerald]

Tittie ?

[chrisonord]

ferfuksake, I have only had one JD tonight.

[Precis]

There's a reason those pistons were made of aluminium....
Your caliper bodies will now get damaged before the pistons, so you'd do well to keep the fluid very clean.

[chrisonord]

I have seen stainless steel pistons for sale for other calipers, and one of the ebay shops I contacted was selling a replacement piston kit to fit a different caliper, and they were stainless too. I can't see what problem having stainless pistons could cause, if anything they should not heat up as much as an aluminium one, and not boil the fluid, something the rear calipers suffer from on the capo. another thing, the stainless is a hardened one so would be less prone to scoring too.
chris.

[Precis]

Early Guzzi caliper pistons were hard-chromed, but they rusted (and often seized) if any water got into the fluid, so factory replacement (and retro-compatible) pistons have been anodised aluminium for several years; I put those into a few of our older bikes and have had no issues since.
Stainless is much harder - so if anything's going to get damaged, it will be the caliper body, rather than the sacrificial (and cheaper) piston.
Have you considered the differential in expansion rates when the calipers get hot?
And I have never suffered from boiling fluid on either Caponord, despite riding with load and in high ambient temperatures - do you have enough free-play on the rear master cylinder?

[the_toe_cutter]

Ive had plain stainless pistons in calipers before.
Dont get the issue tbh, im not an engineer or brake technician but i cant understand the concept of having a piston made from a scrificial weaker material than the caliper body they slide in and out in one plain... And stainless ones shouldnt rust or pit esp if they are 318.

[chrisonord]

Talking to a friend today who is well into his performance cars and owns his own company selling stuff to tune and customise minis, he said brake pistons on motorbikes are only aluminium to keep down the production costs. Performance car brake pistons are made from Stainless steel. He knows his stuff, as he has not only made minis go fast, but has also got a Mk1 cortina with a 400 BHP TVR V8 under the bonnet, and has had several porches and blown, jaguars.
Chris.

[Precis]

Oh but cars are so boring - I drive a big V8 most days - dunno how many hundred horsepower - probably about 400 - lots, too many, makes the thing go sideways, chews out rear tyres - but I don't pay for 'em, so who cares?
I had a racing Mini years ago - the suspension only woke up above 140km/h and with all the interior cladding ripped out and only one seat & a roll-cage, it was like driving around in a drum. Good fun though, and it won on the track and in hill-climbs too.
I raced in the 944 Turbo Cup, packing dry ice around the turbo on the grid and driving with the heater on to try & keep the damn thing from boiling; I had a 911 Carerra on the street, probably the worst car I ever owned, and yes, my Jaguar was blown too - as in 'blown up' one conrod made 4 different holes in the block! Luckily a Nissan 'Sly-line" engine fitted, was faster, used less fuel, had more power and didn't leak. Most people just stuck in a Chev V8, but I liked the feel of the straight 6. Most fun I had in a race-car was in a Sports 2000 - tin-tops are just transport, they're not racing cars.
Loved my Silver Shadow though: Ruiniously Magnificent, is how I described it.

As for "brake pistons on motorbikes are only aluminium to keep down the production costs", I say Phooey - it's much more likely to be to keep weight down - why else would they be using aluminium or titanium in GP bikes?

Certainly in your climate, stainless makes some sense over steel - now I think about it, the only calipers I had that rusting problem with, were among the half dozen or so bikes that I had in the UK. However, I don't know if stainless expands more than steel or the aluminium of the caliper when it heats up (which your back brakes obviously do). If it does expand more (any engineers with the relevant info at their fingertips? - I can't be bothered researching it) given the much smaller area available for heat disappation, I expect it might seize the pistons. If it expands less - the caliper might leak. But if your mate with the hopped-up cars says it's good, it must be so.
And as for "sacrificial" - any grit that gets between the caliper's bore and the new stainless piston will almost certainly score the caliper, as t's softer.

So, if you're happy that your fluid's squeaky clean AND the brakes into which these stainless pistons are inserted will not overheat (or stainless expands at very nearly the same rate as aluminium), you're Golden.

Let us all know how you go.

[BigSteve]

Well Chris.....we will have to try to get your breaks hot when going on one of Blu's rideouts..

Well done for trialing out this new part for us!!