Question on Converting Front rotors from solid to vented "Caliper Spacers"

[keitho64]

I am in the middle of a front suspension rebuild and was thinking about converting my front rotors from solid to vented. I have the stock 911 T brakes with the 3" bolt spacing, I think they are the "M" calipers. The vented rotors are slightly wider than the solid rotors and the stock calipers will not bolt on.

I have a couple pairs of "M" calipers off an SC with the 3.5" spacing on the mounting. It appears as if the caliper spacers from them are not quiet the same.

There are three pictures I am attaching. The first is the "M" Caliper off the SC, the second is the spacer off the SC caliper and the third is the spacer from the SC setting on the 911T caliper. As you can see this won't fit.

So here is the question, can I get spacers to fit the M (stock 911 T calipers) calipers so I can use the vented rotors? I assume I will need slightly longer bolts for the caliper halves along with mounting pins for the pads. Is there such a part available? If so does anyone know where I can find them?










Thanks!

[Porboynz]

Maybe you have the thicker vented later model rotors that match the 3.5inch calipers. I think the vented rotors from a 70-77 will bolt right on and fit those standard looking M calipers with the 3 inch mounts. Posting the caliper thickness will confirm.

[Grady Clay]

Keitho64,

The 1969 911T has solid rotors and 3” ‘M’ calipers, same as the ’69 912.
With the ’70 model, there were also 3” ‘M’ calipers with the center spacer that allowed use of the vented rotor (same rotor as 911E and 911S with aluminum 3.5” calipers).

I would try and source a pair of 3” ‘M’ calipers with the spacer.
They were used from all 911s with iron calipers from ’70 all the way through the introduction of the ‘A’ caliper.

This was a common mod we did on ‘69 911Ts (and 912s) when almost new.

Be careful if you try to install a spacer or other part between calipers.
As I recall, the caliper body is machined differently for the seal rings.
As you discovered, the spacers differ.

Best,
Grady

[djpateman]

I agree with Grady. The parts needed for a caliper conversion would have to come from another caliper. Might as well get a another set of calipers; they are cheap and plentiful. I'm not certain if the hubs are different as well; I do know that there was a variety of hubs for different applications. Perhaps Grady can explain the ins and outs of the swap.

[keitho64]

Thank you all for the advice. I was able to source a pair of calipers from a 1971 911T and they have the correct width for the vented rotors as well as the 3" mounting flange. I still need to install the turbo tie rods but the new ball joints and bushings are done. When I am finished I will write up all my 'lessons learned' from this project. I must say 90% of all my answers have been found by searching Pelican!

Here is the before picture and then the current.










Thanks everyone!!

[Grady Clay]

keitho64,

Nice work but I see a couple of things that concern me.

First, and far most important, is you have the original ’69-’71 ‘pinch-bolt’ type attachment between the strut and ball joint.
I consider that a ‘safety defect’ that Porsche ‘let pass’. My opinion, but based on having seen many come apart while ‘in service’.
Porsche changed to the ‘wedge-pin’ type of attachment with the ’72 model.
The prior ‘pinch-bolt’ parts were removed from inventory.
If you needed a replacement strut, ball joint, etc. from Porsche, you needed to convert to the later ‘wedge-pin’ system.

On the other hand, there are still ‘pinch-bolt’ parts continuing in service.
I recommend that you maintain a close vigil that the connection between the ball joint pin and the strut maintains perfect integrity.
The bolt must also remain tight.




There are also a few important ball joint part situations:
The bolt is a ‘special’ strength grade 10K Porsche part number (red arrow 1).
There is a Porsche-part-number hardened flat washer under the bolt head in addition to a Schnorr safety washer.
The ball joint pin must have the 'half-moon’ semi-circular cut-out for the bolt (not the 'V' cut-out for the wedge pin.)
There is a very thin, large OD flat washer (‘shim’) between the ball joint rubber boot and the bottom of the strut (red arrow 2). (This prevents the ‘pinch-slit’ in the bottom of the strut from damaging the rubber boot.)


Next:
Remove the seal race (green arrow) from the spindle.
There is an O-ring between the spindle and the race that should be replaced.
Inspect the spindle for damage (rust), particularly at the polished radius transition from the spindle shaft to the assembly (caliper mount part).
The seal race is a replaceable part if necessary.
When installing the new O-ring, grease the spindle under and behind the seal race.
The race can easily be installed by heating to ~220ºF and cooling the spindle.

You will have noticed how difficult it was to remove the seal race.
Before reinstalling the race, grind two ‘notches’ in the caliper mount part of the spindle assembly to clear the jaws of a 2-jaw puller.
This allows the subsequent use of a puller to remove the seal race. (Later struts have this.)
Paint your new ‘notches’ to prevent rust.

Next:
Remove the lever-arm (yellow arrow) from the sway bar.
Remove any (red) paint from the surface where the arm contacts the sway bar.
Use a flat-file to make these edges flat. You want tight ‘metal-to-metal’ contact.

When the arms are off, confirm that the sway bar is free to rotate by hand but not ‘sloppy’ in the bushing.
If too tight (hard to rotate even with an arm on), lube the rubber bushings with silicone lube.
Make sure the sway bar does not have radial or axial movement.
If radial movement, replace the rubber sway bar bushings (and lube).
If axial play, install a split piece of rubber hose (fuel or vent hose works well) on the sway bar with a worm-type hose clamp.
This can be between the rubber bushing and the arm or inside the chassis, behind the fuel tank. Do both sides.
Many of ‘front-end-clunks’ are from a loose sway bar.

Next:
I notice the boot retaining spring (brown arrow) is off the steering rack.
While replacing the tie-rods, inspect your steering rack for signs of dirt/water intrusion.
You can smear grease on the rack as you run it through travel extremes.
Rebuilding the rack is a ‘big deal’.

When you install the tie rods, ‘lock’ your steering wheel ‘straight forward’.
You will need to realign the front after all this.

Next:
Many (most?) recommend using OEM rubber brake hoses for street use.
They are perfectly suitable.

The disadvantage of using SS braid covered Teflon hoses (violet arrow) are the tube can be ‘kinked’ and not be observable.
Additionally, road dirt can get between the braid and the tube causing abrasive wear. Again not observable and not able to be completely cleaned.
They are ‘pretty’.

More subjective, the SS ‘race’ hoses make the bake hydraulic system more ‘firm’. This is good in a race application.
For the street this increases the propensity to ‘lock-up’ a tire when braking at the limit-of-traction.
You want a bit of ‘give’ to the system. (In fact I prefer the OEM rubber hoses on a race car.)

When assembling the brake pipes, slide the 'nut' away from the end and grease the outside of the pipe and the threads of the 'nut'.
When assembeling, this will leave grease between the pipe & nut and between the nut threads & the caliper threads to prevent rust.
Do not get grease inside with the brake fluid.


When reassembling, remember new wheel bearing seals.

Measure the run-out on your newly installed vented rotors.
Occasionally the mounting surface on the aluminum hub is damaged (corrosion) or contaminated (transferred rust or paint).
The surface of the hub should be bare aluminum and the mating surface of the rotor be high temperature paint.
Use new M8 bolts and steel lock-nuts (never Nylocs) with a plated flat washer to the aluminum.


If you decide to do away with your ‘pinch-bolt’ ball joint system, keep in mind you will need to replace the struts, ball joints and wedge pin hardware ($$$).
Another ‘good side’ is this allows choice of 3” or 3.5” calipers.
Starting with 1974 parts, there are self-centering hubs and a larger spindle.
You might consider a good-used front suspension from a late (low miles) Carrera with ‘A’ calipers and even thicker rotors.
Just another possibility.


Best,
Grady

[keitho64]

WOW - now that is what I call Service and Advice, Thanks Grady!

First I will address the Ball Joint / Strut -
I have a set of Koni's to put on the car (with the 3.5" spacing) and the wedge bolts (all new) ... However I had new ball joints for the 69 which use the pinch bolt you see here. Since I cannot drive the car enough to fade the brakes I decided to stick with the stock struts and the 3" mounting for the calipers. My hope was to put the vented rotors on the car but if I have to stay with solid rotors for now that is fine with me.
The washers are on the pinch bolts, it is as it was when I removed it from the car. I did read all the threads about the pinch bolt vs. wedge pins and the car has been this way for the 10 years I have owned it. I will keep a close eye on this to make sure it is not a problem. I do not want this falling apart on me.

Seal Race -
I was not aware there was an o-ring behind it, thanks I will take care of that since I am in this far. I do have all new wheel bearings and seals to install.

Sway Bar -
The sway bar bushings are new and the sway bar does rotate but I cannot turn it by hand. With an adjustable wrench on the end, or by using the attaching lever, I can turn it. I will double check there is no paint where the sway bar and attaching lever meet.

Tie Rod / Steering Rack -
I will thoroughly inspect the inner part of the rack when the old boot and tie rods are out and grease them. I like the recommendation to lock the wheel straight when removing the tie rods.

The car does have a date with the alignment shop. In addition to what you see the control arm bushings and top bushings on the struts are all new.

Rotor - Hub Mounting -
I want to make sure I understand the suggestion on the mounting, is it hub > flat washer > rotor > lock washer > M8 bolt? I was sure on the bolt and lock washer but was not aware of a washer between the rotor and hub. Is that what you are referring to?

SS Hoses -
I have run both SS and rubber lines; in fact this car has had SS rear lines for a few years. I do prefer the firm pedal feel and I doubt I have enough of a 'feel' for when the brakes are about to lock up. This car is 90% street driven and and occasional autocross or DE event; which is at my limits, not the car.

I sincerely appreciate your insight into this project! You were a fantastic help when I was doing the 901 / 911 transmission swap and helping me sort out the different forks. I am happy to report it has been working great with the 901 and the 2.7 for a few years now.

[Grady Clay]

Good.
Keep an eye on the pinch bolts.
I would be more comfortable with you using the wedge-pin system parts you have.

Rotor - Hub Mounting -
I want to make sure I understand the suggestion on the mounting, is it hub > flat washer > rotor > lock washer > M8 bolt? I was sure on the bolt and lock washer but was not aware of a washer between the rotor and hub. Is that what you are referring to?

No, the head of the bolt goes on the inside of the rotor (toward the strut). No washer under the bolt head.

There is NOT a washer between the rotor and the hub.

Originally the rotors are painted on the surface that contacts the aluminum hub.
This high temperature paint prevents contact (galvanic) corrosion between the iron rotor and the aluminum hub.
The iron rotor surface should have paint. The aluminum surface of the hub should be bare metal, free of rust and old paint.

On the ‘outside’ of the hub (where the wheel mounts), there is a washer and nut.
Originally this was a split washer and plain nut.
Best is a plated (to prevent contact corrosion) flat washer and a steel lock nut (never a Nylock as the temperatures will melt the Nylon insert).
EDIT: Note that the hub is machine ‘spot-faced’ where the washer sits. Make sure the flat washer has small enough OD to not go over the edge of the spot-faced surface.
Make sure the bolt threads come through the nut by 1-2 threads (1.5 threads is ideal).
Torque the nuts (while securing the bolt head) to 23 Nm / ~17 ft-lbs.
The bolts are normal M8 (class 8.8) bolts.

SS Hoses -
I have run both SS and rubber lines; in fact this car has had SS rear lines for a few years. I do prefer the firm pedal feel and I doubt I have enough of a 'feel' for when the brakes are about to lock up. This car is 90% street driven and and occasional autocross or DE event; which is at my limits, not the car.

If you have the choice, use the rubber hoses – you will like them.

If you feel you need a firmer pedal, look elsewhere.
Most obvious is brake bleeding (& technique).
There can be defects in the caliper mechinsm that ‘retracts’ the pads.
This can be a sign of an imperfect master cylinder.

The physics around ‘too firm brakes’ is:
Consider the 360º rotation of the tire and brake rotor.
The tire ‘grip’ is not exactly 100% the same all the way around:
There are temperature differences (it just sat in the sun).
There was a grain of sand or pavement patch on the pavement.

On the brake rotor, there are always slight differences of the pad ‘grip’ on the rotor.
There are very slight differences in thickness of the rotor.
There is the place where the rotor last stopped (hot) under the pads.

So…
When you are at the limit of traction braking (and you are there more often than you think), the tire-rotor assembly will have a slight propensity to have more stopping (brake) while having less ‘grip’ (tire) at some point in the rotation of the assembly against the pads and road.
When this happens, the tire tends to slip (skid) on the pavement.

As you know, this is greatly exaserbated if there is a turning L-to-R or R-to-L difference in weight bias on the front tires.

With a slightly more ‘compliant’ hydraulic system, the imperfections in grip and braking pass without slip.
This is only just prior to the transition to your ‘locking up a brake’.

That said, there is ‘too compliant’ which is uncomfortable (feels like you won’t stop) or dangerous (run out of pedal travel).

Our Porsches have some of the best braking in the world.
I like to keep them the absolute best possible.

Best,
Grady

[keitho64]

OK so here is the latest, every source that said they had the wider calipers for the vented rotors has fallen through so I am going to install the solid rotors on for now. As fate would have it another road block came up. The piston design in the calipers I have appears to have a return spring on the piston where it fits into the bore of the caliper. Three of the pistons look like this:



I assume this is the correct way as I can see the snap ring at the base of the piston.



The fourth one looks like this:




I tried to pull this off but do not want to force it. I read through my manual but there are no pictures of the inside of the calipers. So here are my questions:

Does anyone know how to get the 'spring' assembly out of the caliper? From what I can see on the other three, there is a snap ring in the head of the piston but I cannot see how this is stuck inside the bore on the caliper.

Does this part come with a new piston?

Are new pistons available for this style caliper?

Do I need to find another caliper?

Anyone have a pair of calipers off a T with vented rotors they want to sell?

Sorry for all the questions but I want to get the car back on the road, Spring is in the Air!

Thanks everyone.

[Porboynz]

The wider spacer M calipers really should be easy to come by in the US, they are easy enough to find here in NZ and we have way less wrecked Porsches. If you end up reassembling the calipers you have, I do recommend new seals between the caliper halves. After 40 years the originals will likely not reseal after being disturbed as I found out when I did mine.

I would try using mole grips on that return spring assembly stuck in that caliper. Protect it with tape or a rag and twist at the same time as you apply pressure to pull it out. Later model caliper pistons did away with that assembly. Its interesting that its retained with a spring circlip, in the later M caliper the retainer is peened in place, at least they are on my 1972 era calipers.

[keitho64]

OK here is the latest, I am sticking with the solid rotors for now until I can find the right set of calipers with the 3" mounting and space for the vented rotors.

I was able to get the calipers rebuilt and here are a few pictures of the progress.

Everything clean and ready to go together.


All clean inside.


Pistons in, aligned and new seals on the caliper halves.


Ready to install.


With the exception of the dirty hub and old rotor everything else is new.


Now to bleed the brakes and get her aligned. Summer fun is about to begin!

[keitho64]

Got the brakes bled, did my best to get the alignment close enough to get to a shop and took her out for a ride tonight. It felt good with all the new suspension parts. I still have some air in the lines since the pedal will get firmer as you pump it but almost done!