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Hello,
Anyone got any advise or opinions on a decent quality set of caliper pistons?
I liked the look of this but it lacks the raised section on the rear.
http://www.ebay.co.uk/itm/Classic-Mini- ... 20c2d9ad1b
Thanks

the trouble with alloy pistons in calipers is that the heat soaks into the brake fluid and you loose the pedal when you use the brakes hard


this is the EXACT reason that the alloy calipers we sell run stainless steel pistons as we found out (or rather a customer did) that the alloy pistons that where fitted to them at first where just not safe

in racing use its been found that even the lightweight stainless ones can cause problems

Here in the US, SCCA a couple of years back allowed FF to remove LD19 and LD20 calipers and replace with an Alloy caliper. For a short time there were Cooper 7.5 style calipers for sale all over the place. I picked up as many sets as I could afford at the time. Most had light SS pistons and the rest alloy pistons. Most I have rebuilt with MSC stainless pistons and sold on. I have kept a couple of sets with the alloy pistons. My plan is to run a set on my new auto cross / you may call it sprinting car. Once I pick a class I may go with alloy calipers. The other set may end up on a vintage racer. If they cause a problem, I always carry a set of loaded calipers in my spares.

I guess my point is I don't know why serious Formula guys would give up safety for .5 lb. a little less tha .25 kilo. unsprung per corner. Steve (CTR)

Rich is right. Under no circumstances fit alloy pistons in your caliper..

Thanks gents, I'll go stainless.

I agree stainless for a street car. There seems to be two of you who have very strong opinions about alloy. You fail to provide any reasoning for this. Have you tested them, had them fail on you? I have had people telling me things about minis since the mid 60s and I still like to figure it out for myself. Steve (CTR)

They transfer heat better, boiling your brake fluid on a race car. Road cars: they are softer, and wear, sticking and leaking...

Basic metallurgy says ally and steel don't mix,try taking a head of a Suzuki gt750

I too like to fit stainless, but also at times (at least locally) have found the price of them a bit difficult to justify. The stock Chrome too do last very well and shouldn't be overlooked.

In the case of the alloy ones, I do seem to recall there are special seals that should be used with them. I really don't think they'd be justified in a street car.

CooperTune wrote: You fail to provide any reasoning for this. Have you tested them, had them fail on you? I have had people telling me things about minis since the mid 60s and I still like to figure it out for myself. Steve (CTR)

steve, my reasoning is sound, its the experiences of some very well known & successful racers who used the minispares alloy calipers that where fitted with the alloy pistons when they first came out over 15 years ago who discovered (often to their cost) that the alloy pistons where just not man enough to cope with the heat soak from hard brake applications - once stainless pistons where fitted the problem was solved to the point where its never been an issue since - even with some of the newer carbon metallic pads. it was because of this input from people who where using the calipers at the top end of their use that minispares very quickly deleted the alloy piston idea and have never looked at as a product since.

ive even heard of historic racers now struggling to manage heat soak in cast iron cooper s calipers when using these modern thin stainless pistons - some of them are having far thicker pistons made to give the brake fluid more chance to recover - the friction from carbon pads really does give the brakes a good work out!


I guess if your using alloy pistons and have never lost the pedal as a result, well I guess your just not driving hard enough......

It's nice to see a variety of opinions!

Normally race cars are "open-wheel" and have ducting to help cool the caliper, not just the disc. This also applies to racing saloons as well. So helping to cool the caliper.

I can't really see the point of putting alloy pistons in cast iron calipers: the weight saving is not that much (if you want to save weight, fit alloy calipers!), and as stated the thermal conductivity of the alloy is much greater than the cast iron, so you'll feed any heat into the fluid before you feed it to the cast iron body -hence boiling fluid.
(You do of course want the unsprung mass -the wheel, upright disc and caliper "+half the wishbones"- to be as low as possible.)

When large amounts of heat were being fed into the pistons, we (I used to work for AP Racing) used to fit insulation blocks in the pistons.
And later, titanium pistons as they have good insulation properties, and are light, ( but "galling" or pick-up is a problem in sliding/moving applications, so you need to think about plating/anodising them).

Minis are always a problem for brake cooling, in a racing application, as there is not much room to duct air.

The hard anodised pistons we used were ground after anodising, as the surface was quite rough from the process (the friction to the seal is important for correct function). I think I can say that the pistons advertised have not been ground (the outer diameter would be a slightly lighter colour), so I wouldn't recommend their use.

So I would recommend, as most suggest, to stick with steel, or stainless steel.

I have the early Minispares 4 pots with alloy pistons.
I had to replace them, they were badly corroded.

Hey Andy 1071, thanks for sharing info about the subject. My first car ( which I still own ) was and is a 1930 Model A Ford. It came with mechanical brakes which I converted to hydraulic after cleaning out a few ditches. My first mini was a 1960 850 with single leading brakes. Cleaned out a few ditches with that one also. My last mini auto cross / sprinting car was 911 lbs/ 415 kilo with no driver. I fitted it with 11 inch disc and Wilwood four pot calipers. Could just about pull the rear wheel off the ground under braking. I realize the issues with 7.5 brakes and a fast car. I agree alloy caliper and piston does not sound like a good idea. The cast calipers I have with alloy pistons are off FFs and are used. I'll strip a couple down and take some pictures.

Rich, thanks for your input and humor. Before I hung up my helmet as owner and driver of our vintage team we won the Jefferson 500 two years in a row running on a lap by myself both years, in a one hour on a 3 plus mile road course. Maybe if your brakes fail you, you are slowing down to much. A few years back we had Nick Swift fly over and drive one of our cars in the Gold cup at VIR. He was able to lead every lap for the weekend and win the feature race overall. Doing the best we can with what we have. Steve (CTR)