914 Brake Caliper info, Whats the internal pin for? They stick Options

[Thorshammer]

Okay,


So I am rebuilding a set of calipers of a customers 914 FP racecar.

One front is a standard "M" caliper with 3 inch spacing and a center pin internal of the piston. This pin sticks out from the center of the caliper piston bore and the piston center has a rubber seal and the pin sticks into the piston.

Anyone know what this pin is for????

I also have another front caliper which has no center pin and the caliper piston is not cupped on the fluid side, so the second caliper has less fluid in it. I found a caliper in the customers spares kit which had a 411 XXX XXX part number on it. From my VW days I deduced this is probably a 411 VW caliper. Same piston diameter and 3.0 inch bolt spacing.

The rears are standard early front calipers from a 914-4.

Here are the questions:

What calipers will bolt onto a 3.0 inch bolt spacing that are two piston calipers??? for the front.

What calipers are the same bolt spacing as 914 front calipers, and are the same piston size?

Observations:

When rebuilding the standard 914 calipers (with the center pin) the caliper pistons move really hard, even with use, or new with Brembo caliper grease.

The "411" caliper pistons move very easily, and I mean this is a big difference. They also retract better, when on the car, you can actually turn the wheel much better than the other calipers.

I am at a loss for whether a piston design that has a cup on the fluid side (more fluid volume) will be better for heat dissapation, or if a piston design that has an almost flat back side (fluid side) will (less fluid) work better. Does anyone make a ceramic insert so the pads will not transfer heat to the pistons to the fluid as easily.

Obviously I am looking for a performance improvement, In two ways:

One piston retracts much better than the other, so less rolling resistance

One piston design that will keep the brakes cooler

This is my dilema.

Erik Madsen

[TimT]

QUOTE

One piston retracts much better than the other, so less rolling resistance

you really dont have to worry about this

usually runout from of the disc will knock the pads back.

I also have a set of M calipers with the wierd pin...this is a mystery to me as well

[Brett W]

I am with you on the pin, never coild figure out what it was for. My plan for my Production car was to take a set of stock calipers and heave the stock pistons. Machine a set of stainless pistons with a Phenolic insert to limit heat transfer or a set of Titanium ones to cut some weight. I figured there was at least two pounds to be removed from the stock calipers.

[ChrisFoley]

Inside the 914 front caliper pistons is a set of cone washers that exert friction on the pin. The cone washers keep the piston from moving freely and (I guess) help keep the pads a controlled distance from the rotor. As the pads wear the piston "ratchets" to a new position every now and then. You could just break off the pin but you will still have the hollow piston. I think an inverted piston would have slower heat transfer from the pads.
Calipers without such an internal mechanism just rely on the piston seal to keep the piston from moving too much.

[Eric_Shea]

I've been talking to Ate about these pins last week. I should have a definative reply back tomorrow AM.

The pin device is supposed to pull the piston back faster and pull the pad off the rotor.

In a racing situation I've been told by more than one person to remove the pin. In extreme temps it can actually bind. They (Ate) are discussing the option of removing them in a pair of S-Calipers we're working on now. Re-anodizing the calipers will ruin the steel pin. Most later calipers went with a dished piston for more cooling surface area and no pin (A-Caliper picture included... no pin).

Another bonus would be to remove the dampening device on the piston once the pin is gone. A little c-clip and out they come. You'll get more fluid volume up toward the dome of the piston.

I'll weight in tomorrow if I can get an answer back from Manfred at the factory.

On to your other questions:

You talk about M-Calipers (48mm pistons) and 411 calipers. The 411 caliper is probably a later 914 caliper. The M-Caliper was 911 issue only so, what struts and rotor offset are you looking at?

The 914 caliper is a 43mm piston. A later (1984 on) rear M-Caliper has a 42mm piston but no others in Porsche land that I know of come that close. Maybe DaveP will weigh in here on various VW units. Regardless, if they're going on teener struts you'll have offset issues.

What's allowed in FP? Tell me more...911 Struts? 914 struts? 43mm 914 calipers is a lot for the rears...

[Eric_Shea]

Here's some S-Caliper pins

[Eric_Shea]

Another...

[Eric_Shea]

The Reply from ATE:

Normally, the bolt was pressed in the housing. In the case of aluminium calipers, I suppose, that the bolt was connected with a plate and the plate was fixed in the housing. Please check your piston. In the piston there should be an equipment with a spring inside. This spring slides over the old. If the piston is knocked back towards the housing, because of a non planar rotor, the spring moves the piston back. You can test test the function by pressing the piston with the fingers into the housing. The
piston can be moved only a little distance and it will come back, if you
stop pressing.

The advantage of a caliper with this equipment is, what we call in German a "short pedal". The caliper should work without this bolt, because the seal has a similar function, but only over a distance of 1 mm. The point is, what is the clearance in the bearing and what rotor do you use. If you have less clearance in the bearing and a well machined rotor, you do not need this equipment. You must test this. We have no experiance, because since more than 30 years this mechanical knock is not used in new developments.

(right click on the pdf... maybe Andy can translate)

[Eric_Shea]

So... it sounds like Manfred's official explaination is more in line with what Chris has posted. I think others have been confused by the 'knock-back' description.

Instead of knocking the piston back off the rotor, it knocks a piston back toward the rotor offering shorter pedal travel.

Here's what I'm going to offer up:

For a race car, or a well maintained street car, I would eliminate them. I have heard 'multiple' stories of these failing leaving the pad against the rotor.

For my GT project, I'm running Stomski Racing pistons without the guts. Pins are in there but they do nothing. For My RS clone I have the full Monte, pins and knock-backs. Both have S-Calipers up front.

[Eric_Shea]

Hey Erik,

what ever became of the caliper/suspension thing?

E.

[Mueller]

QUOTE (Eric_Shea @ Mar 13 2006, 09:14 AM)

So... it sounds like Manfred's official explaination is more in line with what Chris has posted. I think others have been confused by the 'knock-back' description. Instead of knocking the piston back off the rotor, it knocks a piston back toward the rotor offering shorter pedal travel. Here's what I'm going to offer up: For a race car, or a well maintained street car, I would eliminate them. I have heard 'multiple' stories of these failing leaving the pad against the rotor. For my GT project, I'm running Stomski Racing pistons without the guts. Pins are in there but they do nothing. For My RS clone I have the full Monte, pins and knock-backs. Both have S-Calipers up front.

speaking of the knock-back pins, I was reading about the newest Cadillac 400hp sedan, during testing on the big track, there was enough flex in the front wheel hub assembly that the rotors would flex in one direction pushing the pads agains the pistons, this would move the piston back into the caliper too much, the result was a "long" pedal (and leaking fluid from the m/c res. if filled too high)..........

[Brett W]

QUOTE

speaking of the knock-back pins, I was reading about the newest Cadillac 400hp sedan, during testing on the big track, there was enough flex in the front wheel hub assembly that the rotors would flex in one direction pushing the pads agains the pistons, this would move the piston back into the caliper too much, the result was a "long" pedal (and leaking fluid from the m/c res. if filled too high)..........

Imagine that.... an american car company with a shitty product. Go figure (IMG:http://www.914world.com/bbs2/html/emoticons/mad.gif)

If you have a handle on your race car setup you should not have any noticable run out in the rotors. If your wheels bearings are setup properly and well maintained that shouldn't be a problem. If you look at the AP calipers, Wilwood calipers, or Brembo calipers they don't use such band-aids. They have a very small seal that handles all pad retraction.

[davep]

My understanding of the pin is that it acts with the piston seal to retract the piston once the hydraulic pressure is removed to provide venting clearance and reduce drag.

My friend with the 914/6 GT says they used a slightly loose front wheel bearing setting, and the flex in racing caused the pistons to be pushed farther back than may have been desireable. To counter this they used the 23mm master cylinder, braced, to move sufficient fluid in one stoke to properly apply the brakes. This was with the 908 calipers, but they are little different from the 911S aluminum caliper.

Eric, I'd love to hear of your conversations with ATE.

[Eric_Shea]

QUOTE

it acts with the piston seal to retract the piston once the hydraulic pressure is removed to provide venting clearance and reduce drag

Just the opposite... that's what I thought too.

QUOTE

Eric, I'd love to hear of your conversations with ATE

It's posted above with a pdf:

QUOTE

The Reply from ATE: Normally, the bolt was pressed in the housing. In the case of aluminium calipers, I suppose, that the bolt was connected with a plate and the plate was fixed in the housing. Please check your piston. In the piston there should be an equipment with a spring inside. This spring slides over the old. If the piston is knocked back towards the housing, because of a non planar rotor, the spring moves the piston back. You can test test the function by pressing the piston with the fingers into the housing. The piston can be moved only a little distance and it will come back, if you stop pressing. The advantage of a caliper with this equipment is, what we call in German a "short pedal". The caliper should work without this bolt, because the seal has a similar function, but only over a distance of 1 mm. The point is, what is the clearance in the bearing and what rotor do you use. If you have less clearance in the bearing and a well machined rotor, you do not need this equipment. You must test this. We have no experiance, because since more than 30 years this mechanical knock is not used in new developments.

[Thorshammer]

Thanks for everyones input.

Here is what I am trying to do:

Choose the best piston design for heat dissapation

Get the pads to not cause drag on the rotors. This is friction, and I don't need it. I have 7/10 front tilton Master cylinders and 5/8 rear cylinders. This gives me good fluid volume so pedal travel is not an issue. With the 23mm cylinder I doubt this is an issue either.

As for the struts I am using on this customers car, they are 911 struts with a 3.0 inch bolt spacing. I have noticed the VW 411 caliper has the same bolt pattern, but without the center pin, and the caliper piston is shaped differently.


Thanks to all that responded.

Erik


PS: Brent I wish I could use Brembos or Wilwoods. Would that be nice.

[Eric_Shea]

QUOTE

911 struts with a 3.0 inch bolt spacing. I have noticed the VW 411 caliper has the same bolt pattern, but without the center pin, and the caliper piston is shaped differently

This would be a vented rotor and M-Calipers then. I would imagine the 'late' M-Caliper with the dished pistons is what you're looking for.

This would be without the knock-back pin (which can bind and actually pushes the piston toward the rotor). BMW pistons are also dished out but I believe they're about 1/6th of an inch taller so you may have problems with those.

I have a set of A-Pistons out and I could check those against some M-Pistons. They may work but they would have to be installed in an M-Caliper to get the 3" bolt spacing. Any M-Caliper could be converted by simply breaking off the pin.

What's Groot using...? He has my old struts on his car and they're the same 3" M-strut.

[Thorshammer]

A small Change,

I am using a 7/10 in the rear and 5/8 master cylinder in the front.

Erik

[hedfurst]

I believe that most modern calipers have the "cup" of the piston towards the brake pad, I think that it would decrease heat transfer to the brake fluid. I have seen rebuilt front 914 calipers that had the pin broken off and the pistons reversed, imitating the modern caliper piston placement. Seemed to work OK.

[ChrisFoley]

Stainless steel caliper pistons with the cup facing out would be the cat's ass, (bees knees, etc.) IMO. (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[Brett W]

Check out the design of the Wilwood high temp pistons and heat shields. They do a thin stainless cup with an insert that allows a stainless cup to be screwed inside the piston to keep heat at bay.

(IMG:http://www.wilwood.com/Products/007-ThermlockPistons/THERMLOCK-T2-PISTON-EXP.jpg)

(IMG:http://www.wilwood.com/Products/007-ThermlockPistons/THERMLOCK-PISTON-EXPLODED.jpg)

(IMG:http://www.wilwood.com/Products/007-ThermlockPistons/t1Assembled_200.jpg)