911 rear rotor interchangeability?

3liter914-6 · 05-13-2004, 12:34 PM

Ok, so I converted my 914-6 to mostly 911 rear suspension. Everything but the trailing arms, and have run into a bit of a problem. I was going to use the 911 e-brake, but haven't been able to get a cable made to use it yet. No biggie I thought, I'll just use the stock rear calipers to pass inspection, then switch back over when I can get a change to have custom cables made. NOPE.

The stock rear rotor fouls on the 911 drum brake assy. So I grabbed the '69 911 rotor that went with the 911 rear susp. I pirated the parts off of. Well, that won't work because it's vented and too wide for the caliper.

. So I fired up the PET, and '68 isn't vented, does anyone know how thick these are? I know I've seen webpages that list the thickness of various Porsche rotors, just can't seem to find them now. I assume the '68's have the beveled inside edge to clear the backing plate that the '69s have and my 914-6 rotors don't?

Thanks for any help you can give,

Adam

3liter914-6 · 05-13-2004, 12:50 PM

Duh. Oh yeah, a '68 rotor will bolt up to my '69+ hub, right?

3liter914-6 · 05-13-2004, 02:45 PM

bump

Grady Clay · 05-13-2004, 02:53 PM

Adam,

First off, what exactly is the collection of parts you have?

I see ’68 vented? rotors and ’68 calipers to fit? 914-6 OE rear brakes? Parking brake assembly from what?

Best,
Grady

3liter914-6 · 05-13-2004, 03:15 PM

Hi Grady,

The setup was taken off a set of '69 911 trailing arms with vented rotors. I've swapped the e-brake drum setup onto my 914-6 trailing arms, I'm using the hubs/stub axles/bearing retainers off of the '69 trailing arms.

To pass my state's inspection I need an e-brake. I was going to have custom cables made to actuate the e-brake assy., but haven't been able to find a place able to make the cable. I figured that since I still have perfectly good 914 calipers (with integrated e-brake) and the cables still installed, I would just install those and use the stock caliper to pass inspection. However the 914 rotor is flat on the back so it fouls on the 911 e-brake backing plate. I then grabbed the '69 rotor, which is notched on the back to clear, however it's vented and none of the calipers I have will fit over it.

Looks like '68 was fitted with non-vented rotors, so I'm hoping these will fit. They're $115 a piece a the FLAPS, so I'd like to make sure they'll work before I lay down the cash. So basically I'm wondering will the '68 rotor bolt up to the '69 hub and clear the bearing retainer/backing plate, and how does its width compare to the 914 rear rotor?

Grady Clay · 05-13-2004, 03:44 PM

Adam,

OK, let me see if I understand.

You have a complete set of rear brakes form a ’69 911. Tht includes vented rotors, calipers to match, parking brake assembly with 911 cables, hubs, stub axles, and bits & pieces.

You have the OE 914-6 brakes.

I understand the 914-6 rotor not going back on with the 911 parking brake assembly in place. I understand the 914-6 caliper not fitting the 911 vented rotor.

I don’t understand the issue of ’68 parts.

Best,
Grady

3liter914-6 · 05-13-2004, 03:51 PM

Quote:

Originally posted by Grady Clay
Adam,

I don’t understand the issue of ’68 parts.

Best,
Grady

I think you have most of it. I appreciate your trying to understand this.

I need a rotor that's thin enough to fit inside the calipers that I have, but won't foul on the backing plate. Looks like 911s had solid rotors until '68 so I thought this might be a good candidate. I just need to know if they have the right clearancing for the backing plate, and if they're thin enough to fit in the caliper.

Thanks,

Adam

3liter914-6 · 05-13-2004, 04:07 PM

Ok, found out the '68 rotor is the right thickness, now I just need to make sure it'll bolt up to a '69 hub

Grady Clay · 05-13-2004, 04:27 PM

Adam,

I just went back and studied your posts. Pardon me for being so dense.

It appears you are contemplating buying a pair of ’68 non-vented rotors as an interim solution. Don’t, they probably won’t fit the parking brake. The parking brake changed ’68 to ’69. I think the ’68 was wider among other. There is also the issue of the axial location of the rotor and positioning of the caliper.

To get your car back on the road with a parking brake, the best solution is to take the 911 parking brake assembly back off and reinstall the 914 wheel bearing retainer. Then all your 914-6 stuff (rotor & caliper) will fit and you can use the 914-6 parking brake.

It sounds like you have most of the right parts. The ’69 vented brakes will work great and you can have new cables fabricated using some of the parts you have with modifications. The steel internal cable is easy. The difficult part is the Bowden tube housing surrounding the cable. On the 914-6 system the tube is affixed to the trailing arm and the cable simply pulls on the lever on the caliper. On the 911 system, the cable pulls on part of the parking brake while the un-restrained Bowden tube pushes on another part.

Porsche calls these parts that the cable and Bowden tube operate on “mechanical expanders.” In the illustration, the cable operates by pulling on the one pictured, and the Bowden tube pushes on another one behind the one shown.
"

"
(C) 1970 Dr. Ing. h.c.F. Porsche A.-G.

Best,
Grady

3liter914-6 · 05-13-2004, 04:50 PM

Darn it all. I would just yank the bearing retainer off, but I just replaced the rear bearings and it wasn't a good experience. I think if the '68 rotor won't work, I'll just get a pair of 911 rear calipers and figure out a way to jury-rig the cables so they'll work for inspection.

Grady Clay · 05-13-2004, 05:09 PM

Adam,

You can change the 911 parking brake assembly and the little 914 wheel bearing retainer back and forth without disturbing your new wheel bearings. Its just 4 bolts.

Another solution for a parking brake might be to plumb in an extra single master cylinder into the rear brake hydraulic circuit. I think I have seen these commercially available for hot rods. Sort of a “J.C. Whitney Solution.” Someone chime in here.

Am I correct, you have two issues? First you need a parking brake so you can have the 914-6 inspected. Second, you want to properly install a 911 parking brake system. Am I missing anything?

Best,
Grady

3liter914-6 · 05-13-2004, 06:03 PM

Quote:

Originally posted by Grady Clay
Adam,
You can change the 911 parking brake assembly and the little 914 wheel bearing retainer back and forth without disturbing your new wheel bearings. Its just 4 bolts.

Am I correct, you have two issues? First you need a parking brake so you can have the 914-6 inspected. Second, you want to properly install a 911 parking brake system. Am I missing anything?

Best,
Grady

Hi Grady,

You're not missing anything. That's pretty much it.

I've already got the hubs in, so I think removing those to replace the bearing retainers would toast the bearing, right?

Thanks,

Adam

Grady Clay · 05-13-2004, 06:31 PM

Adam,

Yes, you can drive the hub out of the bearing without damaging it. Use a very heavy soft “dead blow” hammer to hit the drift tool. If the outboard inner race comes off with the hub, leave it seated on the hub. Installing the hub I prefer to use the stub axle and nut to draw the pieces together while rotating. Hub in freezer or dry ice helps also. Since all the parts are new, lubed, and clean it should go easily.

However:

At this point I would continue with the conversion to the 911 parts and figure a temporary way to have the parking brake operate on the rear hydraulic circuit (JC Whitney solution.)

Best,
Grady

Zeke · 05-13-2004, 07:50 PM

Quote:

Originally posted by Grady Clay

Another solution for a parking brake might be to plumb in an extra single master cylinder into the rear brake hydraulic circuit. I think I have seen these commercially available for hot rods. Sort of a “J.C. Whitney Solution.” Someone chime in here.

There is a hydraulic parking brake kit available. I've seen it on a 914 with Wilwood brakes. It might pass inspection, but while this 914 was at the track, the brake let go and the car rolled into a trailer. I can't recommend the thing.

3liter914-6 · 08:39 PM

Quote:

Originally posted by Zeke
There is a hydraulic parking brake kit available. I've seen it on a 914 with Wilwood brakes. It might pass inspection, but while this 914 was at the track, the brake let go and the car rolled into a trailer. I can't recommend the thing.

Line locks work well for their intended purpose, but bleed off hydraulic pressure after a while so aren't to be used as a parking brake.

In PA, your e-brake cannot be hydraulic. That doesn't mean I can't slip it by some mechanic who doesn't have the rule book memorized.

I think I'll just take the time to do it properly, or semi properly. I actually welded a support piece to the trailing arm, and I have the about 6" from the end of the cable and the springs that push out on the e-brake shoes. I think maybe I can kludge up an adaptor that will let me mate the cables together at least enough so I have some sort of e-brake for inspection.

We'll see I guess. I appreciate all the input and advice.

thanks,

Adam

EDIT:

Just mic'd out the rotors that came with the trailing arms I bought. Figured they were junk, but they're close to 20mm's so it's just some calipers I need now. Yay.

Grady Clay · 05-14-2004, 07:50 AM

Adam,

Cool, you have good vented rotors. Will a little Scotch Brite and elbow grease clean them up?
The spec new is 20.0 – 0.2 mm and the wear limit is 18.0 mm if worn symmetrically.

Do I read correctly – you didn’t get calipers with your 911 trailing arms?
If you get used calipers, be prepared to rebuild them.

Now on to the fun stuff:

I have done this conversion several times, never the same twice.
The difficult issue is the Bowden tube.
It must fit the 914 chassis at the front, be the correct length,
and adapt to the 911 pull-push system at the parking brake assembly.

You must make a new internal steel cable with the appropriate end connections.
Almost any good aircraft cable shop in the country can fabricate
those given the ends to compare and you spec the length.
You must build the Bowden tube first as the cable can’t be inserted after it is fabricated.

The most suitable Boden tube I made was a bastard combination
of both the 914 tube and the 911 tube.
I cut both and made an adapter to connect the two.
Length is critical and the adjustment at the 911 end can only do so much.

If I were to do it again, I would use the entire 911 Bowden tube
and make an adjustable adaptor to the cut 914 tube.
This would give more adjustment range and put the adaptor
alongside the front of the engine.

Remember the tube can’t be rigidly mounted to the engine or heat exchangers.
It must have those fabric straps to allow movement.

Where have you made a mount for the push tube?
Have you provisions to use the plastic bushing that goes around the push tube?
Send us some photos.

Again, please give me an inventory of all the parts you have.

What are you doing for axles? 914-6? ’69 911? Other?
What is the engine? 3.0SC CIS? Other?
What transmission? 914-6? 915? Other?
What is your intended use? Nice street car? Daily? Track use?

Here are some illustrations to help clarify our discussion.
"

"
(C) 1966 Dr. Ing. h.c.F. Porsche A. G.
"

"
(C) 1966 Dr. Ing. h.c.F. Porsche A. G.

"

"
(C) 2004 Dr. Ing. h. c. F. Porsche AG

Please someone else who has receintly done this chime in here.

Best,
Grady