Titanium Brake Caliper Pistons for early SWB Race Cars

[chris_seven]

Appendix K Period F Race cars have to use Solid Discs and brake temperatures can become quite exciting at times.

We have been rebuilding early calipers with Stainless Steel pistons for some time as this material does give some reduction in heat transfer coefficients compared to a standard piston.

The optimum piston material would be a Grade 5 6AL4V Titanium Alloy as this material has an even lower heat transfer than stainless steel and about the same coefficient of expansion as a standard steel piston so there is no risk of seal roll back and pad knock off.

AP Racing offer Titanium Pistons as their 'premium' product.

I am just about to order a small batch of these pistons. They will be machined to suit calipers with the Anti-Knock off pins and I will make 48mm for the front and 38mm for the rear.

Tey will be balck carbide coated for wear resistance.

They won't be cheap at around $60 each but they will offer the ultimate in thermal barrier terms and shoudl be useful for race cars.

If anyone is interested let me know and I can increase the batch size.

No payment up front is needed and I expect them to take 4-5 weeks before they are available.

[jdilger]

I would like a set for my 71 911S. Please send payment information.

[chris_seven]

1967 MY with the introduction of the 911S. All earlier cars had solid discs as per Homologation Form 183.

I think that the 911T used solid discs as standard up to 1969 unless ventilated brakes were ordered as an upgrade.

[Bert]

Appendix K Period F Race cars have to use Solid Discs and brake temperatures can become quite exciting at times.

We have been rebuilding early calipers with Stainless Steel pistons for some time as this material does give some reduction in heat transfer coefficients compared to a standard piston.

The optimum piston material would be a Grade 5 6AL4V Titanium Alloy as this material has an even lower heat transfer than stainless steel and about the same coefficient of expansion as a standard steel piston so there is no risk of seal roll back and pad knock off.

AP Racing offer Titanium Pistons as their 'premium' product.

I am just about to order a small batch of these pistons. They will be machined to suit calipers with the Anti-Knock off pins and I will make 48mm for the front and 38mm for the rear.

Tey will be balck carbide coated for wear resistance.

They won't be cheap at around $60 each but they will offer the ultimate in thermal barrier terms and shoudl be useful for race cars.

If anyone is interested let me know and I can increase the batch size.

No payment up front is needed and I expect them to take 4-5 weeks before they are available.

Hi Chris,

You note Ti pistons for SWB cars. Will the 48 mm pistons fit the aluminum S caliper that were installed on 1969 - 73 cars?

Will there be a weight difference between the Ti and stainless steel pistons?

Thanks,

[chris_seven]

Bert,

The 'damper' pistons fitted to the SWB cars are shorter than the pistons in the 'S' calipers but we are going to make this design as well and eventually pistons for SC calipers.

The density of Titanium is around 55% of Stainless Steel so they will be just over half the weight.

Thermal Conductivity - W/(m.K)

Steel 58

Austenitic Stainless Steel 16

6AL4V Titanium 6.7

Aluminium 6061/6082 180

Some production cars are now using polymers and I had thought of trying Torlon whch has a thermal conductivity of about 0.4 and a low density but I think this might just be a step too far without significant testing.

Titanium is quite well established.

[jdilger]

Chris,
I am interested in a set to fit the SC calipers.

[Bert]

Chris,

I am interested in one set of Ti pistons for "S" Aluminum Calipers. I am also interested in a set of Valve Spring Retainers, if you make another batch.

Thanks,

Bert

Bert,

The 'damper' pistons fitted to the SWB cars are shorter than the pistons in the 'S' calipers but we are going to make this design as well and eventually pistons for SC calipers.

The density of Titanium is around 55% of Stainless Steel so they will be just over half the weight.

Thermal Conductivity - W/(m.K)

Steel 58

Austenitic Stainless Steel 16

6AL4V Titanium 6.7

Aluminium 6061/6082 180

Some production cars are now using polymers and I had thought of trying Torlon whch has a thermal conductivity of about 0.4 and a low density but I think this might just be a step too far without significant testing.

Titanium is quite well established.

[66S]

Could an aluminum piston be a better choice for an S caliper?

[chris_seven]

There is a great deal of debate on this subject and differing views.

I am not a fan of Aluminium Pistons in brake calipers for two simple reasons:

The radial compression of a seal is quite important as if affects the way in which the piston retracts after braking.

In road car terms I suppose that it is better to minimise pad contact but in race car terms too much stored energy in the seal can cause pad knock off and we always fit springs (about 3lbs force) behind the pistons to eliminate any chance of knock off. You can also buy small check valves to do this hydraulically.

If the seal and caliper have been deigned to suit an aluminium piston this is unlikely to be much of an issue but I have seen comments made that heavy brake application with cold brakes can cause rapid seal wear with alloy pistons due to the different compression.

If you swap a steel piston for an aluminium piston and make them size for size then the radial compression in the seal will increase and the static modulus of the rubber will change and more energy will be stored in the seal, increasing the chance of pad knock off. You could make the aluminium piston a little smaller but I wouldn't like to do this without some detailed testing and durability work.

Coefficient of expansion are as follows:

6061/6082 Aluminium Alloy 22 ppm/degK

Typical Medium Carbon Steel 10.5 ppm/degK

6AL4V Titanium 9 ppm/degK

The difference between Steel and Titanium is sufficiently small that it can be neglected for this application.

The second issue is thermal conductivity and hence heat build up in the brake fluid.

The higher the condictivity the faster the brake fluid heats up and I think this is a bad thing.

Titanium is almost 30 times better than Aluminium with regard to thermal conductivity.

Modern calipers tend to have very thick pads compared to older designs and this must help but the use of Titanium Thermal Barrier plates between the pistons and pads is becoming increasingly common.

Discussions with companies such as AP Racing are interesting.There was a time when the majority of AP calipers had Stainless Steel Pistons but this has now changed and many systems are supplied with Aluminim componenst but all have an option of a Stainless Steel piston. When you discuss suitability the Technical guys alwaysrecommend the upgrade for anything other then Hillclimb or Autocross cars.

There is some case for Aluminium pistons in Aluminium calipers reducing the possibility of Bi-Metallic Corrosion but this only becomes an issue in poorly manitained vehicles when the brake fluid has become wet and should have been changed or the car has stood for years.

Titanium and Aluminium have a greater potential for Bimetallic corrosion than Steel and Aluminium and this could be an issue if 'raw' Titanium was used.

Titanium is also not ideal when used in a 'rubbing seal' applicaton and the Titanium carbide Coating we are going to apply has two benefits.

The first is that it greatly enhances wear resistance to approcahing that of a tool steel and the second is that it also enhances corrosion resistance and eliminates bi-metallic corrosion and should be betetr then either stell or stainless steel in this respect.

Soory to bang on but apart from cost it is a great material for caliper pistons.

[chris_seven]

Titanium S Pistons just came back from being coated with TiCN.

The thermal barrier caliper plate are also ready and we have 911S . M Type and L Type (SWB rear) finished in 0.5 and 1.0mm thickness .

We also made the P84 at the same time as well as some plates to blank early and SC inlet ports